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在Android全部经常使用的原生控件其中。使用方法最复杂的应该就是ListView了，它专门用于处理那种内容元素许多。手机屏幕无法展示出全部内容的情况。
ListView能够使用列表的形式来展示内容，超出屏幕部分的内容仅仅须要通过手指滑动就能够移动到屏幕内了。
另外ListView另一个很奇妙的功能，我相信大家应该都体验过，即使在ListView中载入很许多的数据。比方达到成百上千条甚至许多其它，ListView都不会发生OOM或者崩溃，而且随着我们手指滑动来浏览许多其它数据时，程序所占用的内存居然都不会跟着增长。
那么ListView是怎么实现这么奇妙的功能的呢？当初我就抱着学习的心态花了很长时间把ListView的源代码通读了一遍，基本了解了它的工作原理，在感叹Google大神能够写出如此精妙代码的同一时候我也有所敬畏，由于ListView的代码量比較大，复杂度也很高，很难用文字表达清晰。于是我就放弃了把它写成一篇博客的想法。那么如今回忆起来这件事我已经肠子都悔青了。由于没过几个月时间我就把当初梳理清晰的源代码又忘的一干二净。于是如今我又又一次定下心来再次把ListView的源代码重读了一遍，那么这次我一定要把它写成一篇博客，分享给大家的同一时候也当成我自己的笔记吧。
首先我们先来看一下ListView的继承结构，例如以下图所看到的：
能够看到，ListView的继承结构还是相当复杂的，它是直接继承自的AbsListView。而AbsListView有两个子实现类，一个是ListView。另一个就是GridView。因此我们从这一点就能够猜出来。ListView和GridView在工作原理和实现上都是有许多共同点的。然后AbsListView又继承自AdapterView。AdapterView继承自ViewGroup，后面就是我们所熟知的了。先把ListView的继承结构了解一下。待会儿有助于我们更加清晰地分析代码。
Adapter的作用
Adapter相信大家都不会陌生。我们平时使用ListView的时候一定都会用到它。
那么话说回来大家有没有细致想过。为什么须要Adapter这个东西呢？总感觉正由于有了Adapter，ListView的使用变得要比其它控件复杂得多。那么这里我们就先来学习一下Adapter究竟起到了什么样的一个作用。
事实上说究竟，控件就是为了交互和展示数据用的。仅仅只是ListView更加特殊。它是为了展示许多许多数据用的，可是ListView仅仅承担交互和展示工作而已，至于这些数据来自哪里，ListView是不关心的。因此，我们能设想到的最基本的ListView工作模式就是要有一个ListView控件和一个数据源。
只是假设真的让ListView和数据源直接打交道的话。那ListView所要做的适配工作就很繁杂了。
由于数据源这个概念太模糊了，我们仅仅知道它包括了许多数据而已。至于这个数据源究竟是什么样类型。并没有严格的定义，有可能是数组，也有可能是集合，甚至有可能是数据库表中查询出来的游标。
所以说假设ListView真的去为每一种数据源都进行适配操作的话，一是扩展性会比較差，内置了几种适配就仅仅有几种适配，不能动态进行加入。二是超出了它本身应该负责的工作范围，不再是仅仅承担交互和展示工作就能够了，这样ListView就会变得比較臃肿。
那么显然Android开发团队是不会同意这样的事情发生的。于是就有了Adapter这样一个机制的出现。
顾名思义，Adapter是适配器的意思。它在ListView和数据源之间起到了一个桥梁的作用，ListView并不会直接和数据源打交道，而是会借助Adapter这个桥梁来去訪问真正的数据源。与之前不同的是，Adapter的接口都是统一的。因此ListView不用再去操心不论什么适配方面的问题。
而Adapter又是一个接口(interface)，它能够去实现各种各样的子类，每一个子类都能通过自己的逻辑来去完毕特定的功能，以及与特定数据源的适配操作，比方说ArrayAdapter能够用于数组和List类型的数据源适配。SimpleCursorAdapter能够用于游标类型的数据源适配，这样就很巧妙地把数据源适配困难的问题解决掉了。而且还拥有相当不错的扩展性。简单的原理示意图例如以下所看到的：
当然Adapter的作用不仅仅仅仅有数据源适配这一点，另一个很很重要的方法也须要我们在Adapter其中去重写。就是getView()方法，这个在以下的文章中还会详细讲到。
RecycleBin机制
那么在開始分析ListView的源代码之前。另一个东西是我们提前须要了解的。就是RecycleBin机制，这个机制也是ListView能够实现成百上千条数据都不会OOM最重要的一个原因。事实上RecycleBin的代码并不多。仅仅有300行左右，它是写在AbsListView中的一个内部类。所以全部继承自AbsListView的子类。也就是ListView和GridView，都能够使用这个机制。那我们来看一下RecycleBin中的主要代码，例如以下所看到的：
/**
* The RecycleBin facilitates reuse of views across layouts. The RecycleBin
* has two levels of storage: ActiveViews and ScrapViews. ActiveViews are
* those views which were onscreen at the start of a layout. By
* construction, they are displaying current information. At the end of
* layout, all views in ActiveViews are demoted to ScrapViews. ScrapViews
* are old views that could potentially be used by the adapter to avoid
* allocating views unnecessarily.
*
* @see android.widget.AbsListView#setRecyclerListener(android.widget.AbsListView.RecyclerListener)
* @see android.widget.AbsListView.RecyclerListener
*/
class RecycleBin {
private RecyclerListener mRecyclerListener;
/**
* The position of the first view stored in mActiveViews.
*/
private int mFirstActivePosition;
/**
* Views that were on screen at the start of layout. This array is
* populated at the start of layout, and at the end of layout all view
* in mActiveViews are moved to mScrapViews. Views in mActiveViews
* represent a contiguous range of Views, with position of the first
* view store in mFirstActivePosition.
*/
private View[] mActiveViews = new View[0];
/**
* Unsorted views that can be used by the adapter as a convert view.
*/
private ArrayList<View>[] mScrapViews;
private int mViewTypeCount;
private ArrayList<View> mCurrentScrap;
/**
* Fill ActiveViews with all of the children of the AbsListView.
*
* @param childCount
* The minimum number of views mActiveViews should hold
* @param firstActivePosition
* The position of the first view that will be stored in
* mActiveViews
*/
void fillActiveViews(int childCount, int firstActivePosition) {
if (mActiveViews.length < childCount) {
mActiveViews = new View[childCount];
}
mFirstActivePosition = firstActivePosition;
final View[] activeViews = mActiveViews;
for (int i = 0; i < childCount; i++) {
View child = getChildAt(i);
AbsListView.LayoutParams lp = (AbsListView.LayoutParams) child.getLayoutParams();
// Don't put header or footer views into the scrap heap
if (lp != null && lp.viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
// Note: We do place AdapterView.ITEM_VIEW_TYPE_IGNORE in
// active views.
// However, we will NOT place them into scrap views.
activeViews[i] = child;
}
}
}
/**
* Get the view corresponding to the specified position. The view will
* be removed from mActiveViews if it is found.
*
* @param position
* The position to look up in mActiveViews
* @return The view if it is found, null otherwise
*/
View getActiveView(int position) {
int index = position - mFirstActivePosition;
final View[] activeViews = mActiveViews;
if (index >= 0 && index < activeViews.length) {
final View match = activeViews[index];
activeViews[index] = null;
return match;
}
return null;
}
/**
* Put a view into the ScapViews list. These views are unordered.
*
* @param scrap
* The view to add
*/
void addScrapView(View scrap) {
AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();
if (lp == null) {
return;
}
// Don't put header or footer views or views that should be ignored
// into the scrap heap
int viewType = lp.viewType;
if (!shouldRecycleViewType(viewType)) {
if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
removeDetachedView(scrap, false);
}
return;
}
if (mViewTypeCount == 1) {
dispatchFinishTemporaryDetach(scrap);
mCurrentScrap.add(scrap);
} else {
dispatchFinishTemporaryDetach(scrap);
mScrapViews[viewType].add(scrap);
}
if (mRecyclerListener != null) {
mRecyclerListener.onMovedToScrapHeap(scrap);
}
}
/**
* @return A view from the ScrapViews collection. These are unordered.
*/
View getScrapView(int position) {
ArrayList<View> scrapViews;
if (mViewTypeCount == 1) {
scrapViews = mCurrentScrap;
int size = scrapViews.size();
if (size > 0) {
return scrapViews.remove(size - 1);
} else {
return null;
}
} else {
int whichScrap = mAdapter.getItemViewType(position);
if (whichScrap >= 0 && whichScrap < mScrapViews.length) {
scrapViews = mScrapViews[whichScrap];
int size = scrapViews.size();
if (size > 0) {
return scrapViews.remove(size - 1);
}
}
}
return null;
}
public void setViewTypeCount(int viewTypeCount) {
if (viewTypeCount < 1) {
throw new IllegalArgumentException("Can't have a viewTypeCount < 1");
}
// noinspection unchecked
ArrayList<View>[] scrapViews = new ArrayList[viewTypeCount];
for (int i = 0; i < viewTypeCount; i++) {
scrapViews[i] = new ArrayList<View>();
}
mViewTypeCount = viewTypeCount;
mCurrentScrap = scrapViews[0];
mScrapViews = scrapViews;
}
}这里的RecycleBin代码并不全，我仅仅是把最基本的几个方法提了出来。
那么我们先来对这几个方法进行简单解读，这对后面分析ListView的工作原理将会有很大的帮助。
- fillActiveViews() 这种方法接收两个參数，第一个參数表示要存储的view的数量，第二个參数表示ListView中第一个可见元素的position值。
RecycleBin其中使用mActiveViews这个数组来存储View，调用这种方法后就会依据传入的參数来将ListView中的指定元素存储到mActiveViews数组其中。
- getActiveView() 这种方法和fillActiveViews()是相应的，用于从mActiveViews数组其中获取数据。该方法接收一个position參数。表示元素在ListView其中的位置，方法内部会自己主动将position值转换成mActiveViews数组相应的下标值。
须要注意的是，mActiveViews其中所存储的View，一旦被获取了之后就会从mActiveViews其中移除，下次获取相同位置的View将会返回null，也就是说mActiveViews不能被反复利用。
- addScrapView() 用于将一个废弃的View进行缓存，该方法接收一个View參数，当有某个View确定要废弃掉的时候(比方滚动出了屏幕)，就应该调用这种方法来对View进行缓存，RecycleBin其中使用mScrapViews和mCurrentScrap这两个List来存储废弃View。
- getScrapView 用于从废弃缓存中取出一个View，这些废弃缓存中的View是没有顺序可言的。因此getScrapView()方法中的算法也很easy，就是直接从mCurrentScrap其中获取尾部的一个scrap view进行返回。
- setViewTypeCount() 我们都知道Adapter其中能够重写一个getViewTypeCount()来表示ListView中有几种类型的数据项，而setViewTypeCount()方法的作用就是为每种类型的数据项都单独启用一个RecycleBin缓存机制。实际上，getViewTypeCount()方法通常情况下使用的并非许多。所以我们仅仅要知道RecycleBin其中有这样一个功能即可了。
了解了RecycleBin中的主要方法以及它们的用处之后。以下就能够開始来分析ListView的工作原理了。这里我将还是依照曾经分析源代码的方式来进行，即跟着主线运行流程来逐步阅读并点到即止，不然的话要是把ListView全部的代码都贴出来，那么本篇文章将会很长很长了。
第一次Layout
无论怎么说，ListView即使再特殊终于还是继承自View的。因此它的运行流程还将会依照View的规则来运行，对于这方面不太熟悉的朋友能够參考我之前写的 Android视图绘制流程全然解析。带你一步步深入了解View(二) 。
View的运行流程无非就分为三步，onMeasure()用于測量View的大小，onLayout()用于确定View的布局，onDraw()用于将View绘制到界面上。而在ListView其中，onMeasure()并没有什么特殊的地方。由于它终归是一个View，占用的空间最多而且通常也就是整个屏幕。onDraw()在ListView其中也没有什么意义。由于ListView本身并不负责绘制，而是由ListView其中的子元素来进行绘制的。那么ListView大部分的奇妙功能事实上都是在onLayout()方法中进行的了。因此我们本篇文章也是主要分析的这种方法里的内容。
假设你到ListView源代码中去找一找。你会发现ListView中是没有onLayout()这种方法的，这是由于这种方法是在ListView的父类AbsListView中实现的，代码例如以下所看到的：
/**
* Subclasses should NOT override this method but {@link #layoutChildren()}
* instead.
*/
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
super.onLayout(changed, l, t, r, b);
mInLayout = true;
if (changed) {
int childCount = getChildCount();
for (int i = 0; i < childCount; i++) {
getChildAt(i).forceLayout();
}
mRecycler.markChildrenDirty();
}
layoutChildren();
mInLayout = false;
}能够看到，onLayout()方法中并没有做什么复杂的逻辑操作，主要就是一个推断，假设ListView的大小或者位置发生了变化，那么changed变量就会变成true。此时会要求全部的子布局都强制进行重绘。除此之外倒没有什么难理解的地方了，只是我们注意到。在第16行调用了layoutChildren()这种方法，从方法名上我们就能够猜出这种方法是用来进行子元素布局的。只是进入到这种方法其中你会发现这是个空方法。没有一行代码。这当然是能够理解的了，由于子元素的布局应该是由详细的实现类来负责完毕的。而不是由父类完毕。那么进入ListView的layoutChildren()方法。代码例如以下所看到的：@Override
protected void layoutChildren() {
final boolean blockLayoutRequests = mBlockLayoutRequests;
if (!blockLayoutRequests) {
mBlockLayoutRequests = true;
} else {
return;
}
try {
super.layoutChildren();
invalidate();
if (mAdapter == null) {
resetList();
invokeOnItemScrollListener();
return;
}
int childrenTop = mListPadding.top;
int childrenBottom = getBottom() - getTop() - mListPadding.bottom;
int childCount = getChildCount();
int index = 0;
int delta = 0;
View sel;
View oldSel = null;
View oldFirst = null;
View newSel = null;
View focusLayoutRestoreView = null;
// Remember stuff we will need down below
switch (mLayoutMode) {
case LAYOUT_SET_SELECTION:
index = mNextSelectedPosition - mFirstPosition;
if (index >= 0 && index < childCount) {
newSel = getChildAt(index);
}
break;
case LAYOUT_FORCE_TOP:
case LAYOUT_FORCE_BOTTOM:
case LAYOUT_SPECIFIC:
case LAYOUT_SYNC:
break;
case LAYOUT_MOVE_SELECTION:
default:
// Remember the previously selected view
index = mSelectedPosition - mFirstPosition;
if (index >= 0 && index < childCount) {
oldSel = getChildAt(index);
}
// Remember the previous first child
oldFirst = getChildAt(0);
if (mNextSelectedPosition >= 0) {
delta = mNextSelectedPosition - mSelectedPosition;
}
// Caution: newSel might be null
newSel = getChildAt(index + delta);
}
boolean dataChanged = mDataChanged;
if (dataChanged) {
handleDataChanged();
}
// Handle the empty set by removing all views that are visible
// and calling it a day
if (mItemCount == 0) {
resetList();
invokeOnItemScrollListener();
return;
} else if (mItemCount != mAdapter.getCount()) {
throw new IllegalStateException("The content of the adapter has changed but "
+ "ListView did not receive a notification. Make sure the content of "
+ "your adapter is not modified from a background thread, but only "
+ "from the UI thread. [in ListView(" + getId() + ", " + getClass()
+ ") with Adapter(" + mAdapter.getClass() + ")]");
}
setSelectedPositionInt(mNextSelectedPosition);
// Pull all children into the RecycleBin.
// These views will be reused if possible
final int firstPosition = mFirstPosition;
final RecycleBin recycleBin = mRecycler;
// reset the focus restoration
View focusLayoutRestoreDirectChild = null;
// Don't put header or footer views into the Recycler. Those are
// already cached in mHeaderViews;
if (dataChanged) {
for (int i = 0; i < childCount; i++) {
recycleBin.addScrapView(getChildAt(i));
if (ViewDebug.TRACE_RECYCLER) {
ViewDebug.trace(getChildAt(i),
ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP, index, i);
}
}
} else {
recycleBin.fillActiveViews(childCount, firstPosition);
}
// take focus back to us temporarily to avoid the eventual
// call to clear focus when removing the focused child below
// from messing things up when ViewRoot assigns focus back
// to someone else
final View focusedChild = getFocusedChild();
if (focusedChild != null) {
// TODO: in some cases focusedChild.getParent() == null
// we can remember the focused view to restore after relayout if the
// data hasn't changed, or if the focused position is a header or footer
if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)) {
focusLayoutRestoreDirectChild = focusedChild;
// remember the specific view that had focus
focusLayoutRestoreView = findFocus();
if (focusLayoutRestoreView != null) {
// tell it we are going to mess with it
focusLayoutRestoreView.onStartTemporaryDetach();
}
}
requestFocus();
}
// Clear out old views
detachAllViewsFromParent();
switch (mLayoutMode) {
case LAYOUT_SET_SELECTION:
if (newSel != null) {
sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
} else {
sel = fillFromMiddle(childrenTop, childrenBottom);
}
break;
case LAYOUT_SYNC:
sel = fillSpecific(mSyncPosition, mSpecificTop);
break;
case LAYOUT_FORCE_BOTTOM:
sel = fillUp(mItemCount - 1, childrenBottom);
adjustViewsUpOrDown();
break;
case LAYOUT_FORCE_TOP:
mFirstPosition = 0;
sel = fillFromTop(childrenTop);
adjustViewsUpOrDown();
break;
case LAYOUT_SPECIFIC:
sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);
break;
case LAYOUT_MOVE_SELECTION:
sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
break;
default:
if (childCount == 0) {
if (!mStackFromBottom) {
final int position = lookForSelectablePosition(0, true);
setSelectedPositionInt(position);
sel = fillFromTop(childrenTop);
} else {
final int position = lookForSelectablePosition(mItemCount - 1, false);
setSelectedPositionInt(position);
sel = fillUp(mItemCount - 1, childrenBottom);
}
} else {
if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
sel = fillSpecific(mSelectedPosition,
oldSel == null ? childrenTop : oldSel.getTop());
} else if (mFirstPosition < mItemCount) {
sel = fillSpecific(mFirstPosition,
oldFirst == null ? childrenTop : oldFirst.getTop());
} else {
sel = fillSpecific(0, childrenTop);
}
}
break;
}
// Flush any cached views that did not get reused above
recycleBin.scrapActiveViews();
if (sel != null) {
// the current selected item should get focus if items
// are focusable
if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {
final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&
focusLayoutRestoreView.requestFocus()) || sel.requestFocus();
if (!focusWasTaken) {
// selected item didn't take focus, fine, but still want
// to make sure something else outside of the selected view
// has focus
final View focused = getFocusedChild();
if (focused != null) {
focused.clearFocus();
}
positionSelector(sel);
} else {
sel.setSelected(false);
mSelectorRect.setEmpty();
}
} else {
positionSelector(sel);
}
mSelectedTop = sel.getTop();
} else {
if (mTouchMode > TOUCH_MODE_DOWN && mTouchMode < TOUCH_MODE_SCROLL) {
View child = getChildAt(mMotionPosition - mFirstPosition);
if (child != null) positionSelector(child);
} else {
mSelectedTop = 0;
mSelectorRect.setEmpty();
}
// even if there is not selected position, we may need to restore
// focus (i.e. something focusable in touch mode)
if (hasFocus() && focusLayoutRestoreView != null) {
focusLayoutRestoreView.requestFocus();
}
}
// tell focus view we are done mucking with it, if it is still in
// our view hierarchy.
if (focusLayoutRestoreView != null
&& focusLayoutRestoreView.getWindowToken() != null) {
focusLayoutRestoreView.onFinishTemporaryDetach();
}
mLayoutMode = LAYOUT_NORMAL;
mDataChanged = false;
mNeedSync = false;
setNextSelectedPositionInt(mSelectedPosition);
updateScrollIndicators();
if (mItemCount > 0) {
checkSelectionChanged();
}
invokeOnItemScrollListener();
} finally {
if (!blockLayoutRequests) {
mBlockLayoutRequests = false;
}
}
}这段代码比較长。我们挑重点的看。
首先能够确定的是。ListView其中眼下还没有不论什么子View，数据都还是由Adapter管理的，并没有展示到界面上，因此第19行getChildCount()方法得到的值肯定是0。接着在第81行会依据dataChanged这个布尔型的值来推断运行逻辑。dataChanged仅仅有在数据源发生改变的情况下才会变成true，其它情况都是false，因此这里会进入到第90行的运行逻辑。调用RecycleBin的fillActiveViews()方法。按理来说，调用fillActiveViews()方法是为了将ListView的子View进行缓存的。可是眼下ListView中还没有不论什么的子View。因此这一行临时还起不了不论什么作用。
接下来在第114行会依据mLayoutMode的值来决定布局模式，默认情况下都是普通模式LAYOUT_NORMAL，因此会进入到第140行的default语句其中。而以下又会紧接着进行两次if推断，childCount眼下是等于0的，而且默认的布局顺序是从上往下，因此会进入到第145行的fillFromTop()方法，我们跟进去瞧一瞧：
/**
* Fills the list from top to bottom, starting with mFirstPosition
*
* @param nextTop The location where the top of the first item should be
* drawn
*
* @return The view that is currently selected
*/
private View fillFromTop(int nextTop) {
mFirstPosition = Math.min(mFirstPosition, mSelectedPosition);
mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);
if (mFirstPosition < 0) {
mFirstPosition = 0;
}
return fillDown(mFirstPosition, nextTop);
}从这种方法的凝视中能够看出。它所负责的主要任务就是从mFirstPosition開始，自顶至底去填充ListView。而这种方法本身并没有什么逻辑。就是推断了一下mFirstPosition值的合法性，然后调用fillDown()方法。那么我们就有理由能够推測，填充ListView的操作是在fillDown()方法中完毕的。进入fillDown()方法。代码例如以下所看到的：/**
* Fills the list from pos down to the end of the list view.
*
* @param pos The first position to put in the list
*
* @param nextTop The location where the top of the item associated with pos
* should be drawn
*
* @return The view that is currently selected, if it happens to be in the
* range that we draw.
*/
private View fillDown(int pos, int nextTop) {
View selectedView = null;
int end = (getBottom() - getTop()) - mListPadding.bottom;
while (nextTop < end && pos < mItemCount) {
// is this the selected item?
boolean selected = pos == mSelectedPosition;
View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);
nextTop = child.getBottom() + mDividerHeight;
if (selected) {
selectedView = child;
}
pos++;
}
return selectedView;
}能够看到，这里使用了一个while循环来运行反复逻辑，一開始nextTop的值是第一个子元素顶部距离整个ListView顶部的像素值，pos则是刚刚传入的mFirstPosition的值。而end是ListView底部减去顶部所得的像素值，mItemCount则是Adapter中的元素数量。因此一開始的情况下nextTop必然是小于end值的，而且pos也是小于mItemCount值的。那么每运行一次while循环，pos的值都会加1，而且nextTop也会添加。当nextTop大于等于end时，也就是子元素已经超出当前屏幕了，或者pos大于等于mItemCount时。也就是全部Adapter中的元素都被遍历结束了，就会跳出while循环。
那么while循环其中又做了什么事情呢？值得让人留意的就是第18行调用的makeAndAddView()方法，进入到这种方法其中，代码例如以下所看到的：
/** * Obtain the view and add it to our list of children. The view can be made * fresh, converted from an unused view, or used as is if it was in the * recycle bin. * * @param position Logical position in the list * @param y Top or bottom edge of the view to add * @param flow If flow is true, align top edge to y. If false, align bottom * edge to y. * @param childrenLeft Left edge where children should be positioned * @param selected Is this position selected?这里在第19行尝试从RecycleBin其中高速获取一个active view，只是很遗憾的是眼下RecycleBin其中还没有缓存不论什么的View，所以这里得到的值肯定是null。* @return View that was added */ private View makeAndAddView(int position, int y, boolean flow, int childrenLeft, boolean selected) { View child; if (!mDataChanged) { // Try to use an exsiting view for this position child = mRecycler.getActiveView(position); if (child != null) { // Found it -- we're using an existing child // This just needs to be positioned setupChild(child, position, y, flow, childrenLeft, selected, true); return child; } } // Make a new view for this position, or convert an unused view if possible child = obtainView(position, mIsScrap); // This needs to be positioned and measured setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]); return child; }
那么取得了null之后就会继续向下运行，到第28行会调用obtainView()方法来再次尝试获取一个View。这次的obtainView()方法是能够保证一定返回一个View的，于是以下立马将获取到的View传入到了setupChild()方法其中。那么obtainView()内部究竟是怎么工作的呢？我们先进入到这种方法里面看一下：
/**
* Get a view and have it show the data associated with the specified
* position. This is called when we have already discovered that the view is
* not available for reuse in the recycle bin. The only choices left are
* converting an old view or making a new one.
*
* @param position
* The position to display
* @param isScrap
* Array of at least 1 boolean, the first entry will become true
* if the returned view was taken from the scrap heap, false if
* otherwise.
*
* @return A view displaying the data associated with the specified position
*/
View obtainView(int position, boolean[] isScrap) {
isScrap[0] = false;
View scrapView;
scrapView = mRecycler.getScrapView(position);
View child;
if (scrapView != null) {
child = mAdapter.getView(position, scrapView, this);
if (child != scrapView) {
mRecycler.addScrapView(scrapView);
if (mCacheColorHint != 0) {
child.setDrawingCacheBackgroundColor(mCacheColorHint);
}
} else {
isScrap[0] = true;
dispatchFinishTemporaryDetach(child);
}
} else {
child = mAdapter.getView(position, null, this);
if (mCacheColorHint != 0) {
child.setDrawingCacheBackgroundColor(mCacheColorHint);
}
}
return child;
}obtainView()方法中的代码并不多。但却包括了很很重要的逻辑。不夸张的说。整个ListView中最重要的内容可能就在这种方法里了。那么我们还是依照运行流程来看，在第19行代码中调用了RecycleBin的getScrapView()方法来尝试获取一个废弃缓存中的View，相同的道理，这里肯定是获取不到的，getScrapView()方法会返回一个null。这时该怎么办呢？没有关系，代码会运行到第33行，调用mAdapter的getView()方法来去获取一个View。那么mAdapter是什么呢？当然就是当前ListView关联的适配器了。而getView()方法又是什么呢？还用说吗，这个就是我们平时使用ListView时最最经常重写的一个方法了，这里getView()方法中传入了三个參数，各自是position，null和this。那么我们平时写ListView的Adapter时，getView()方法一般会怎么写呢？这里我举个简单的样例：
@Override
public View getView(int position, View convertView, ViewGroup parent) {
Fruit fruit = getItem(position);
View view;
if (convertView == null) {
view = LayoutInflater.from(getContext()).inflate(resourceId, null);
} else {
view = convertView;
}
ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);
fruitImage.setImageResource(fruit.getImageId());
fruitName.setText(fruit.getName());
return view;
}getView()方法接受的三个參数，第一个參数position代表当前子元素的的位置。我们能够通过详细的位置来获取与其相关的数据。第二个參数convertView，刚才传入的是null。说明没有convertView能够利用，因此我们会调用LayoutInflater的inflate()方法来去载入一个布局。接下来会对这个view进行一些属性和值的设定。最后将view返回。
那么这个View也会作为obtainView()的结果进行返回，并终于传入到setupChild()方法其中。事实上也就是说，第一次layout过程其中。全部的子View都是调用LayoutInflater的inflate()方法载入出来的，这样就会相对照较耗时。可是不用操心。后面就不会再有这样的情况了，那么我们继续往下看：
/** * Add a view as a child and make sure it is measured (if necessary) and * positioned properly. * * @param child The view to add * @param position The position of this child * @param y The y position relative to which this view will be positioned * @param flowDown If true, align top edge to y. If false, align bottom * edge to y. * @param childrenLeft Left edge where children should be positioned * @param selected Is this position selected?setupChild()方法其中的代码尽管比較多，可是我们仅仅看核心代码的话就很easy了。刚才调用obtainView()方法获取到的子元素View，这里在第40行调用了addViewInLayout()方法将它加入到了ListView其中。那么依据fillDown()方法中的while循环，会让子元素View将整个ListView控件填满然后就跳出。也就是说即使我们的Adapter中有一千条数据，ListView也仅仅会载入第一屏的数据，剩下的数据反正眼下在屏幕上也看不到，所以不会去做多余的载入工作。这样就能够保证ListView中的内容能够迅速展示到屏幕上。* @param recycled Has this view been pulled from the recycle bin?
If so it * does not need to be remeasured. */ private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft, boolean selected, boolean recycled) { final boolean isSelected = selected && shouldShowSelector(); final boolean updateChildSelected = isSelected != child.isSelected(); final int mode = mTouchMode; final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL && mMotionPosition == position; final boolean updateChildPressed = isPressed != child.isPressed(); final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested(); // Respect layout params that are already in the view. Otherwise make some up... // noinspection unchecked AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams(); if (p == null) { p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT, 0); } p.viewType = mAdapter.getItemViewType(position); if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) { attachViewToParent(child, flowDown ? -1 : 0, p); } else { p.forceAdd = false; if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { p.recycledHeaderFooter = true; } addViewInLayout(child, flowDown ?
-1 : 0, p, true); } if (updateChildSelected) { child.setSelected(isSelected); } if (updateChildPressed) { child.setPressed(isPressed); } if (needToMeasure) { int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec, mListPadding.left + mListPadding.right, p.width); int lpHeight = p.height; int childHeightSpec; if (lpHeight > 0) { childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY); } else { childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED); } child.measure(childWidthSpec, childHeightSpec); } else { cleanupLayoutState(child); } final int w = child.getMeasuredWidth(); final int h = child.getMeasuredHeight(); final int childTop = flowDown ? y : y - h; if (needToMeasure) { final int childRight = childrenLeft + w; final int childBottom = childTop + h; child.layout(childrenLeft, childTop, childRight, childBottom); } else { child.offsetLeftAndRight(childrenLeft - child.getLeft()); child.offsetTopAndBottom(childTop - child.getTop()); } if (mCachingStarted && !child.isDrawingCacheEnabled()) { child.setDrawingCacheEnabled(true); } }
那么到此为止，第一次Layout过程结束。
第二次Layout
尽管我在源代码中并没有找出详细的原因。但假设你自己做一下实验的话就会发现，即使是一个再简单的View，在展示到界面上之前都会经历至少两次onMeasure()和两次onLayout()的过程。事实上这仅仅是一个很小的细节。平时对我们影响并不大。由于无论是onMeasure()或者onLayout()几次。反正都是运行的相同的逻辑，我们并不须要进行过多关心。可是在ListView中情况就不一样了。由于这就意味着layoutChildren()过程会运行两次，而这个过程其中涉及到向ListView中加入子元素，假设相同的逻辑运行两遍的话，那么ListView中就会存在一份反复的数据了。因此ListView在layoutChildren()过程其中做了第二次Layout的逻辑处理。很巧妙地攻克了这个问题。以下我们就来分析一下第二次Layout的过程。
事实上第二次Layout和第一次Layout的基本流程是差点儿相同的，那么我们还是从layoutChildren()方法開始看起：
@Override
protected void layoutChildren() {
final boolean blockLayoutRequests = mBlockLayoutRequests;
if (!blockLayoutRequests) {
mBlockLayoutRequests = true;
} else {
return;
}
try {
super.layoutChildren();
invalidate();
if (mAdapter == null) {
resetList();
invokeOnItemScrollListener();
return;
}
int childrenTop = mListPadding.top;
int childrenBottom = getBottom() - getTop() - mListPadding.bottom;
int childCount = getChildCount();
int index = 0;
int delta = 0;
View sel;
View oldSel = null;
View oldFirst = null;
View newSel = null;
View focusLayoutRestoreView = null;
// Remember stuff we will need down below
switch (mLayoutMode) {
case LAYOUT_SET_SELECTION:
index = mNextSelectedPosition - mFirstPosition;
if (index >= 0 && index < childCount) {
newSel = getChildAt(index);
}
break;
case LAYOUT_FORCE_TOP:
case LAYOUT_FORCE_BOTTOM:
case LAYOUT_SPECIFIC:
case LAYOUT_SYNC:
break;
case LAYOUT_MOVE_SELECTION:
default:
// Remember the previously selected view
index = mSelectedPosition - mFirstPosition;
if (index >= 0 && index < childCount) {
oldSel = getChildAt(index);
}
// Remember the previous first child
oldFirst = getChildAt(0);
if (mNextSelectedPosition >= 0) {
delta = mNextSelectedPosition - mSelectedPosition;
}
// Caution: newSel might be null
newSel = getChildAt(index + delta);
}
boolean dataChanged = mDataChanged;
if (dataChanged) {
handleDataChanged();
}
// Handle the empty set by removing all views that are visible
// and calling it a day
if (mItemCount == 0) {
resetList();
invokeOnItemScrollListener();
return;
} else if (mItemCount != mAdapter.getCount()) {
throw new IllegalStateException("The content of the adapter has changed but "
+ "ListView did not receive a notification. Make sure the content of "
+ "your adapter is not modified from a background thread, but only "
+ "from the UI thread. [in ListView(" + getId() + ", " + getClass()
+ ") with Adapter(" + mAdapter.getClass() + ")]");
}
setSelectedPositionInt(mNextSelectedPosition);
// Pull all children into the RecycleBin.
// These views will be reused if possible
final int firstPosition = mFirstPosition;
final RecycleBin recycleBin = mRecycler;
// reset the focus restoration
View focusLayoutRestoreDirectChild = null;
// Don't put header or footer views into the Recycler. Those are
// already cached in mHeaderViews;
if (dataChanged) {
for (int i = 0; i < childCount; i++) {
recycleBin.addScrapView(getChildAt(i));
if (ViewDebug.TRACE_RECYCLER) {
ViewDebug.trace(getChildAt(i),
ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP, index, i);
}
}
} else {
recycleBin.fillActiveViews(childCount, firstPosition);
}
// take focus back to us temporarily to avoid the eventual
// call to clear focus when removing the focused child below
// from messing things up when ViewRoot assigns focus back
// to someone else
final View focusedChild = getFocusedChild();
if (focusedChild != null) {
// TODO: in some cases focusedChild.getParent() == null
// we can remember the focused view to restore after relayout if the
// data hasn't changed, or if the focused position is a header or footer
if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)) {
focusLayoutRestoreDirectChild = focusedChild;
// remember the specific view that had focus
focusLayoutRestoreView = findFocus();
if (focusLayoutRestoreView != null) {
// tell it we are going to mess with it
focusLayoutRestoreView.onStartTemporaryDetach();
}
}
requestFocus();
}
// Clear out old views
detachAllViewsFromParent();
switch (mLayoutMode) {
case LAYOUT_SET_SELECTION:
if (newSel != null) {
sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
} else {
sel = fillFromMiddle(childrenTop, childrenBottom);
}
break;
case LAYOUT_SYNC:
sel = fillSpecific(mSyncPosition, mSpecificTop);
break;
case LAYOUT_FORCE_BOTTOM:
sel = fillUp(mItemCount - 1, childrenBottom);
adjustViewsUpOrDown();
break;
case LAYOUT_FORCE_TOP:
mFirstPosition = 0;
sel = fillFromTop(childrenTop);
adjustViewsUpOrDown();
break;
case LAYOUT_SPECIFIC:
sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);
break;
case LAYOUT_MOVE_SELECTION:
sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
break;
default:
if (childCount == 0) {
if (!mStackFromBottom) {
final int position = lookForSelectablePosition(0, true);
setSelectedPositionInt(position);
sel = fillFromTop(childrenTop);
} else {
final int position = lookForSelectablePosition(mItemCount - 1, false);
setSelectedPositionInt(position);
sel = fillUp(mItemCount - 1, childrenBottom);
}
} else {
if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
sel = fillSpecific(mSelectedPosition,
oldSel == null ? childrenTop : oldSel.getTop());
} else if (mFirstPosition < mItemCount) {
sel = fillSpecific(mFirstPosition,
oldFirst == null ? childrenTop : oldFirst.getTop());
} else {
sel = fillSpecific(0, childrenTop);
}
}
break;
}
// Flush any cached views that did not get reused above
recycleBin.scrapActiveViews();
if (sel != null) {
// the current selected item should get focus if items
// are focusable
if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {
final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&
focusLayoutRestoreView.requestFocus()) || sel.requestFocus();
if (!focusWasTaken) {
// selected item didn't take focus, fine, but still want
// to make sure something else outside of the selected view
// has focus
final View focused = getFocusedChild();
if (focused != null) {
focused.clearFocus();
}
positionSelector(sel);
} else {
sel.setSelected(false);
mSelectorRect.setEmpty();
}
} else {
positionSelector(sel);
}
mSelectedTop = sel.getTop();
} else {
if (mTouchMode > TOUCH_MODE_DOWN && mTouchMode < TOUCH_MODE_SCROLL) {
View child = getChildAt(mMotionPosition - mFirstPosition);
if (child != null) positionSelector(child);
} else {
mSelectedTop = 0;
mSelectorRect.setEmpty();
}
// even if there is not selected position, we may need to restore
// focus (i.e. something focusable in touch mode)
if (hasFocus() && focusLayoutRestoreView != null) {
focusLayoutRestoreView.requestFocus();
}
}
// tell focus view we are done mucking with it, if it is still in
// our view hierarchy.
if (focusLayoutRestoreView != null
&& focusLayoutRestoreView.getWindowToken() != null) {
focusLayoutRestoreView.onFinishTemporaryDetach();
}
mLayoutMode = LAYOUT_NORMAL;
mDataChanged = false;
mNeedSync = false;
setNextSelectedPositionInt(mSelectedPosition);
updateScrollIndicators();
if (mItemCount > 0) {
checkSelectionChanged();
}
invokeOnItemScrollListener();
} finally {
if (!blockLayoutRequests) {
mBlockLayoutRequests = false;
}
}
}
相同还是在第19行，调用getChildCount()方法来获取子View的数量。仅仅只是如今得到的值不会再是0了。而是ListView中一屏能够显示的子View数量，由于我们刚刚在第一次Layout过程其中向ListView加入了这么多的子View。以下在第90行调用了RecycleBin的fillActiveViews()方法，这次效果可就不一样了，由于眼下ListView中已经有子View了，这样全部的子View都会被缓存到RecycleBin的mActiveViews数组其中，后面将会用到它们。
接下来将会是很很重要的一个操作，在第113行调用了detachAllViewsFromParent()方法。这种方法会将全部ListView其中的子View全部清除掉，从而保证第二次Layout过程不会产生一份反复的数据。那有的朋友可能会问了。这样把已经载入好的View又清除掉，待会还要再又一次载入一遍，这不是严重影响效率吗？不用操心。还记得我们刚刚调用了RecycleBin的fillActiveViews()方法来缓存子View吗。待会儿将会直接使用这些缓存好的View来进行载入，而并不会又一次运行一遍inflate过程。因此效率方面并不会有什么明显的影响。
那么我们接着看。在第141行的推断逻辑其中，由于不再等于0了。因此会进入到else语句其中。
而else语句中又有三个逻辑推断。第一个逻辑推断不成立，由于默认情况下我们没有选中不论什么子元素，mSelectedPosition应该等于-1。第二个逻辑推断一般是成立的。由于mFirstPosition的值一開始是等于0的。仅仅要adapter中的数据大于0条件就成立。那么进入到fillSpecific()方法其中，代码例如以下所看到的：
/**
* Put a specific item at a specific location on the screen and then build
* up and down from there.
*
* @param position The reference view to use as the starting point
* @param top Pixel offset from the top of this view to the top of the
* reference view.
*
* @return The selected view, or null if the selected view is outside the
* visible area.
*/
private View fillSpecific(int position, int top) {
boolean tempIsSelected = position == mSelectedPosition;
View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected);
// Possibly changed again in fillUp if we add rows above this one.
mFirstPosition = position;
View above;
View below;
final int dividerHeight = mDividerHeight;
if (!mStackFromBottom) {
above = fillUp(position - 1, temp.getTop() - dividerHeight);
// This will correct for the top of the first view not touching the top of the list
adjustViewsUpOrDown();
below = fillDown(position + 1, temp.getBottom() + dividerHeight);
int childCount = getChildCount();
if (childCount > 0) {
correctTooHigh(childCount);
}
} else {
below = fillDown(position + 1, temp.getBottom() + dividerHeight);
// This will correct for the bottom of the last view not touching the bottom of the list
adjustViewsUpOrDown();
above = fillUp(position - 1, temp.getTop() - dividerHeight);
int childCount = getChildCount();
if (childCount > 0) {
correctTooLow(childCount);
}
}
if (tempIsSelected) {
return temp;
} else if (above != null) {
return above;
} else {
return below;
}
}fillSpecific()这算是一个新方法了。只是事实上它和fillUp()、fillDown()方法功能也是差点儿相同的。基本的差别在于，fillSpecific()方法会优先将指定位置的子View先载入到屏幕上，然后再载入该子View往上以及往下的其它子View。
那么由于这里我们传入的position就是第一个子View的位置。于是fillSpecific()方法的作用就基本上和fillDown()方法是差点儿相同的了，这里我们就不去关注太多它的细节。而是将精力放在makeAndAddView()方法上面。再次回到makeAndAddView()方法，代码例如以下所看到的：
/**
* Obtain the view and add it to our list of children. The view can be made
* fresh, converted from an unused view, or used as is if it was in the
* recycle bin.
*
* @param position Logical position in the list
* @param y Top or bottom edge of the view to add
* @param flow If flow is true, align top edge to y. If false, align bottom
* edge to y.
* @param childrenLeft Left edge where children should be positioned
* @param selected Is this position selected?
* @return View that was added
*/
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
boolean selected) {
View child;
if (!mDataChanged) {
// Try to use an exsiting view for this position
child = mRecycler.getActiveView(position);
if (child != null) {
// Found it -- we're using an existing child
// This just needs to be positioned
setupChild(child, position, y, flow, childrenLeft, selected, true);
return child;
}
}
// Make a new view for this position, or convert an unused view if possible
child = obtainView(position, mIsScrap);
// This needs to be positioned and measured
setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
return child;
}仍然还是在第19行尝试从RecycleBin其中获取Active View，然而这次就一定能够获取到了，由于前面我们调用了RecycleBin的fillActiveViews()方法来缓存子View。那么既然如此，就不会再进入到第28行的obtainView()方法，而是会直接进入setupChild()方法其中。这样也省去了许多时间，由于假设在obtainView()方法中又要去infalte布局的话，那么ListView的初始载入效率就大大减少了。
注意在第23行。setupChild()方法的最后一个參数传入的是true，这个參数表明当前的View是之前被回收过的，那么我们再次回到setupChild()方法其中：
/**
* Add a view as a child and make sure it is measured (if necessary) and
* positioned properly.
*
* @param child The view to add
* @param position The position of this child
* @param y The y position relative to which this view will be positioned
* @param flowDown If true, align top edge to y. If false, align bottom
* edge to y.
* @param childrenLeft Left edge where children should be positioned
* @param selected Is this position selected?
* @param recycled Has this view been pulled from the recycle bin? If so it
* does not need to be remeasured.
*/
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
boolean selected, boolean recycled) {
final boolean isSelected = selected && shouldShowSelector();
final boolean updateChildSelected = isSelected != child.isSelected();
final int mode = mTouchMode;
final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&
mMotionPosition == position;
final boolean updateChildPressed = isPressed != child.isPressed();
final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();
// Respect layout params that are already in the view. Otherwise make some up...
// noinspection unchecked
AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
if (p == null) {
p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
ViewGroup.LayoutParams.WRAP_CONTENT, 0);
}
p.viewType = mAdapter.getItemViewType(position);
if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&
p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
attachViewToParent(child, flowDown ? -1 : 0, p);
} else {
p.forceAdd = false;
if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
p.recycledHeaderFooter = true;
}
addViewInLayout(child, flowDown ? -1 : 0, p, true);
}
if (updateChildSelected) {
child.setSelected(isSelected);
}
if (updateChildPressed) {
child.setPressed(isPressed);
}
if (needToMeasure) {
int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
mListPadding.left + mListPadding.right, p.width);
int lpHeight = p.height;
int childHeightSpec;
if (lpHeight > 0) {
childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
} else {
childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
}
child.measure(childWidthSpec, childHeightSpec);
} else {
cleanupLayoutState(child);
}
final int w = child.getMeasuredWidth();
final int h = child.getMeasuredHeight();
final int childTop = flowDown ? y : y - h;
if (needToMeasure) {
final int childRight = childrenLeft + w;
final int childBottom = childTop + h;
child.layout(childrenLeft, childTop, childRight, childBottom);
} else {
child.offsetLeftAndRight(childrenLeft - child.getLeft());
child.offsetTopAndBottom(childTop - child.getTop());
}
if (mCachingStarted && !child.isDrawingCacheEnabled()) {
child.setDrawingCacheEnabled(true);
}
}
能够看到，setupChild()方法的最后一个參数是recycled。然后在第32行会对这个变量进行推断，由于recycled如今是true。所以会运行attachViewToParent()方法。而第一次Layout过程则是运行的else语句中的addViewInLayout()方法。这两个方法最大的差别在于。假设我们须要向ViewGroup中加入一个新的子View，应该调用addViewInLayout()方法。而假设是想要将一个之前detach的View又一次attach到ViewGroup上，就应该调用attachViewToParent()方法。那么由于前面在layoutChildren()方法其中调用了detachAllViewsFromParent()方法，这样ListView中全部的子View都是处于detach状态的，所以这里attachViewToParent()方法是正确的选择。经历了这样一个detach又attach的过程，ListView中全部的子View又都能够正常显示出来了，那么第二次Layout过程结束。
滑动载入许多其它数据
经历了两次Layout过程，虽说我们已经能够在ListView中看到内容了，然而关于ListView最奇妙的部分我们却还没有接触到，由于眼下ListView中仅仅是载入并显示了第一屏的数据而已。比方说我们的Adapter其中有1000条数据，可是第一屏仅仅显示了10条，ListView中也仅仅有10个子View而已，那么剩下的990是如何工作并显示到界面上的呢？这就要看一下ListView滑动部分的源代码了，由于我们是通过手指滑动来显示许多其它数据的。
由于滑动部分的机制是属于通用型的，即ListView和GridView都会使用相同的机制。因此这部分代码就肯定是写在AbsListView其中的了。那么监听触控事件是在onTouchEvent()方法其中进行的。我们就来看一下AbsListView中的这种方法：
@Override
public boolean onTouchEvent(MotionEvent ev) {
if (!isEnabled()) {
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return isClickable() || isLongClickable();
}
final int action = ev.getAction();
View v;
int deltaY;
if (mVelocityTracker == null) {
mVelocityTracker = VelocityTracker.obtain();
}
mVelocityTracker.addMovement(ev);
switch (action & MotionEvent.ACTION_MASK) {
case MotionEvent.ACTION_DOWN: {
mActivePointerId = ev.getPointerId(0);
final int x = (int) ev.getX();
final int y = (int) ev.getY();
int motionPosition = pointToPosition(x, y);
if (!mDataChanged) {
if ((mTouchMode != TOUCH_MODE_FLING) && (motionPosition >= 0)
&& (getAdapter().isEnabled(motionPosition))) {
// User clicked on an actual view (and was not stopping a
// fling). It might be a
// click or a scroll. Assume it is a click until proven
// otherwise
mTouchMode = TOUCH_MODE_DOWN;
// FIXME Debounce
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
if (ev.getEdgeFlags() != 0 && motionPosition < 0) {
// If we couldn't find a view to click on, but the down
// event was touching
// the edge, we will bail out and try again. This allows
// the edge correcting
// code in ViewRoot to try to find a nearby view to
// select
return false;
}
if (mTouchMode == TOUCH_MODE_FLING) {
// Stopped a fling. It is a scroll.
createScrollingCache();
mTouchMode = TOUCH_MODE_SCROLL;
mMotionCorrection = 0;
motionPosition = findMotionRow(y);
reportScrollStateChange(OnScrollListener.SCROLL_STATE_TOUCH_SCROLL);
}
}
}
if (motionPosition >= 0) {
// Remember where the motion event started
v = getChildAt(motionPosition - mFirstPosition);
mMotionViewOriginalTop = v.getTop();
}
mMotionX = x;
mMotionY = y;
mMotionPosition = motionPosition;
mLastY = Integer.MIN_VALUE;
break;
}
case MotionEvent.ACTION_MOVE: {
final int pointerIndex = ev.findPointerIndex(mActivePointerId);
final int y = (int) ev.getY(pointerIndex);
deltaY = y - mMotionY;
switch (mTouchMode) {
case TOUCH_MODE_DOWN:
case TOUCH_MODE_TAP:
case TOUCH_MODE_DONE_WAITING:
// Check if we have moved far enough that it looks more like a
// scroll than a tap
startScrollIfNeeded(deltaY);
break;
case TOUCH_MODE_SCROLL:
if (PROFILE_SCROLLING) {
if (!mScrollProfilingStarted) {
Debug.startMethodTracing("AbsListViewScroll");
mScrollProfilingStarted = true;
}
}
if (y != mLastY) {
deltaY -= mMotionCorrection;
int incrementalDeltaY = mLastY != Integer.MIN_VALUE ? y - mLastY : deltaY;
// No need to do all this work if we're not going to move
// anyway
boolean atEdge = false;
if (incrementalDeltaY != 0) {
atEdge = trackMotionScroll(deltaY, incrementalDeltaY);
}
// Check to see if we have bumped into the scroll limit
if (atEdge && getChildCount() > 0) {
// Treat this like we're starting a new scroll from the
// current
// position. This will let the user start scrolling back
// into
// content immediately rather than needing to scroll
// back to the
// point where they hit the limit first.
int motionPosition = findMotionRow(y);
if (motionPosition >= 0) {
final View motionView = getChildAt(motionPosition - mFirstPosition);
mMotionViewOriginalTop = motionView.getTop();
}
mMotionY = y;
mMotionPosition = motionPosition;
invalidate();
}
mLastY = y;
}
break;
}
break;
}
case MotionEvent.ACTION_UP: {
switch (mTouchMode) {
case TOUCH_MODE_DOWN:
case TOUCH_MODE_TAP:
case TOUCH_MODE_DONE_WAITING:
final int motionPosition = mMotionPosition;
final View child = getChildAt(motionPosition - mFirstPosition);
if (child != null && !child.hasFocusable()) {
if (mTouchMode != TOUCH_MODE_DOWN) {
child.setPressed(false);
}
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
final AbsListView.PerformClick performClick = mPerformClick;
performClick.mChild = child;
performClick.mClickMotionPosition = motionPosition;
performClick.rememberWindowAttachCount();
mResurrectToPosition = motionPosition;
if (mTouchMode == TOUCH_MODE_DOWN || mTouchMode == TOUCH_MODE_TAP) {
final Handler handler = getHandler();
if (handler != null) {
handler.removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ? mPendingCheckForTap
: mPendingCheckForLongPress);
}
mLayoutMode = LAYOUT_NORMAL;
if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {
mTouchMode = TOUCH_MODE_TAP;
setSelectedPositionInt(mMotionPosition);
layoutChildren();
child.setPressed(true);
positionSelector(child);
setPressed(true);
if (mSelector != null) {
Drawable d = mSelector.getCurrent();
if (d != null && d instanceof TransitionDrawable) {
((TransitionDrawable) d).resetTransition();
}
}
postDelayed(new Runnable() {
public void run() {
child.setPressed(false);
setPressed(false);
if (!mDataChanged) {
post(performClick);
}
mTouchMode = TOUCH_MODE_REST;
}
}, ViewConfiguration.getPressedStateDuration());
} else {
mTouchMode = TOUCH_MODE_REST;
}
return true;
} else if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {
post(performClick);
}
}
mTouchMode = TOUCH_MODE_REST;
break;
case TOUCH_MODE_SCROLL:
final int childCount = getChildCount();
if (childCount > 0) {
if (mFirstPosition == 0
&& getChildAt(0).getTop() >= mListPadding.top
&& mFirstPosition + childCount < mItemCount
&& getChildAt(childCount - 1).getBottom() <= getHeight()
- mListPadding.bottom) {
mTouchMode = TOUCH_MODE_REST;
reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
} else {
final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
final int initialVelocity = (int) velocityTracker
.getYVelocity(mActivePointerId);
if (Math.abs(initialVelocity) > mMinimumVelocity) {
if (mFlingRunnable == null) {
mFlingRunnable = new FlingRunnable();
}
reportScrollStateChange(OnScrollListener.SCROLL_STATE_FLING);
mFlingRunnable.start(-initialVelocity);
} else {
mTouchMode = TOUCH_MODE_REST;
reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
}
}
} else {
mTouchMode = TOUCH_MODE_REST;
reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
}
break;
}
setPressed(false);
// Need to redraw since we probably aren't drawing the selector
// anymore
invalidate();
final Handler handler = getHandler();
if (handler != null) {
handler.removeCallbacks(mPendingCheckForLongPress);
}
if (mVelocityTracker != null) {
mVelocityTracker.recycle();
mVelocityTracker = null;
}
mActivePointerId = INVALID_POINTER;
if (PROFILE_SCROLLING) {
if (mScrollProfilingStarted) {
Debug.stopMethodTracing();
mScrollProfilingStarted = false;
}
}
break;
}
case MotionEvent.ACTION_CANCEL: {
mTouchMode = TOUCH_MODE_REST;
setPressed(false);
View motionView = this.getChildAt(mMotionPosition - mFirstPosition);
if (motionView != null) {
motionView.setPressed(false);
}
clearScrollingCache();
final Handler handler = getHandler();
if (handler != null) {
handler.removeCallbacks(mPendingCheckForLongPress);
}
if (mVelocityTracker != null) {
mVelocityTracker.recycle();
mVelocityTracker = null;
}
mActivePointerId = INVALID_POINTER;
break;
}
case MotionEvent.ACTION_POINTER_UP: {
onSecondaryPointerUp(ev);
final int x = mMotionX;
final int y = mMotionY;
final int motionPosition = pointToPosition(x, y);
if (motionPosition >= 0) {
// Remember where the motion event started
v = getChildAt(motionPosition - mFirstPosition);
mMotionViewOriginalTop = v.getTop();
mMotionPosition = motionPosition;
}
mLastY = y;
break;
}
}
return true;
}
这种方法中的代码就许多了，由于它所处理的逻辑或许多，要监听各种各样的触屏事件。
可是我们眼下所关心的就仅仅有手指在屏幕上滑动这一个事件而已。相应的是ACTION_MOVE这个动作，那么我们就仅仅看这部分代码就能够了。
能够看到。ACTION_MOVE这个case里面又嵌套了一个switch语句，是依据当前的TouchMode来选择的。那这里我能够直接告诉大家，当手指在屏幕上滑动时，TouchMode是等于TOUCH_MODE_SCROLL这个值的，至于为什么那又要牵扯到另外的好几个方法。这里限于篇幅原因就不再展开解说了，喜欢寻根究底的朋友们能够自己去源代码里找一找原因。
这样的话。代码就应该会走到第78行的这个case里面去了，在这个case其中并没有什么太多须要注意的东西。唯一一点很重要的就是第92行调用的trackMotionScroll()方法。相当于我们手指仅仅要在屏幕上略微有一点点移动。这种方法就会被调用，而假设是正常在屏幕上滑动的话，那么这种方法就会被调用许多次。
那么我们进入到这种方法中瞧一瞧，代码例如以下所看到的：
boolean trackMotionScroll(int deltaY, int incrementalDeltaY) {
final int childCount = getChildCount();
if (childCount == 0) {
return true;
}
final int firstTop = getChildAt(0).getTop();
final int lastBottom = getChildAt(childCount - 1).getBottom();
final Rect listPadding = mListPadding;
final int spaceAbove = listPadding.top - firstTop;
final int end = getHeight() - listPadding.bottom;
final int spaceBelow = lastBottom - end;
final int height = getHeight() - getPaddingBottom() - getPaddingTop();
if (deltaY < 0) {
deltaY = Math.max(-(height - 1), deltaY);
} else {
deltaY = Math.min(height - 1, deltaY);
}
if (incrementalDeltaY < 0) {
incrementalDeltaY = Math.max(-(height - 1), incrementalDeltaY);
} else {
incrementalDeltaY = Math.min(height - 1, incrementalDeltaY);
}
final int firstPosition = mFirstPosition;
if (firstPosition == 0 && firstTop >= listPadding.top && deltaY >= 0) {
// Don't need to move views down if the top of the first position
// is already visible
return true;
}
if (firstPosition + childCount == mItemCount && lastBottom <= end && deltaY <= 0) {
// Don't need to move views up if the bottom of the last position
// is already visible
return true;
}
final boolean down = incrementalDeltaY < 0;
final boolean inTouchMode = isInTouchMode();
if (inTouchMode) {
hideSelector();
}
final int headerViewsCount = getHeaderViewsCount();
final int footerViewsStart = mItemCount - getFooterViewsCount();
int start = 0;
int count = 0;
if (down) {
final int top = listPadding.top - incrementalDeltaY;
for (int i = 0; i < childCount; i++) {
final View child = getChildAt(i);
if (child.getBottom() >= top) {
break;
} else {
count++;
int position = firstPosition + i;
if (position >= headerViewsCount && position < footerViewsStart) {
mRecycler.addScrapView(child);
}
}
}
} else {
final int bottom = getHeight() - listPadding.bottom - incrementalDeltaY;
for (int i = childCount - 1; i >= 0; i--) {
final View child = getChildAt(i);
if (child.getTop() <= bottom) {
break;
} else {
start = i;
count++;
int position = firstPosition + i;
if (position >= headerViewsCount && position < footerViewsStart) {
mRecycler.addScrapView(child);
}
}
}
}
mMotionViewNewTop = mMotionViewOriginalTop + deltaY;
mBlockLayoutRequests = true;
if (count > 0) {
detachViewsFromParent(start, count);
}
offsetChildrenTopAndBottom(incrementalDeltaY);
if (down) {
mFirstPosition += count;
}
invalidate();
final int absIncrementalDeltaY = Math.abs(incrementalDeltaY);
if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {
fillGap(down);
}
if (!inTouchMode && mSelectedPosition != INVALID_POSITION) {
final int childIndex = mSelectedPosition - mFirstPosition;
if (childIndex >= 0 && childIndex < getChildCount()) {
positionSelector(getChildAt(childIndex));
}
}
mBlockLayoutRequests = false;
invokeOnItemScrollListener();
awakenScrollBars();
return false;
}这种方法接收两个參数，deltaY表示从手指按下时的位置到当前手指位置的距离，incrementalDeltaY则表示据上次触发event事件手指在Y方向上位置的改变量。那么事实上我们就能够通过incrementalDeltaY的正负值情况来推断用户是向上还是向下滑动的了。如第34行代码所看到的，假设incrementalDeltaY小于0，说明是向下滑动。否则就是向上滑动。
以下将会进行一个边界值检測的过程。能够看到。从第43行開始，当ListView向下滑动的时候。就会进入一个for循环其中。从上往下依次获取子View，第47行其中，假设该子View的bottom值已经小于top值了。就说明这个子View已经移出屏幕了。所以会调用RecycleBin的addScrapView()方法将这个View加入到废弃缓存其中。并将count计数器加1，计数器用于记录有多少个子View被移出了屏幕。
那么假设是ListView向上滑动的话，事实上过程是基本相同的，仅仅只是变成了从下往上依次获取子View，然后推断该子View的top值是不是大于bottom值了，假设大于的话说明子View已经移出了屏幕，相同把它加入到废弃缓存中，并将计数器加1。
接下来在第76行。会依据当前计数器的值来进行一个detach操作，它的作用就是把全部移出屏幕的子View全部detach掉，在ListView的概念其中。全部看不到的View就没有必要为它进行保存，由于屏幕外还有成百上千条数据等着显示呢，一个好的回收策略才干保证ListView的高性能和高效率。
紧接着在第78行调用了offsetChildrenTopAndBottom()方法，并将incrementalDeltaY作为參数传入，这种方法的作用是让ListView中全部的子View都依照传入的參数值进行相应的偏移，这样就实现了随着手指的拖动，ListView的内容也会随着滚动的效果。
然后在第84行会进行推断，假设ListView中最后一个View的底部已经移入了屏幕，或者ListView中第一个View的顶部移入了屏幕，就会调用fillGap()方法，那么因此我们就能够猜出fillGap()方法是用来载入屏幕外数据的，进入到这种方法中瞧一瞧。例如以下所看到的：
/** * Fills the gap left open by a touch-scroll. During a touch scroll, * children that remain on screen are shifted and the other ones are * discarded. The role of this method is to fill the gap thus created by * performing a partial layout in the empty space. * * @param down * true if the scroll is going down, false if it is going up */ abstract void fillGap(boolean down);OK，AbsListView中的fillGap()是一个抽象方法，那么我们立马就能够想到。它的详细实现肯定是在ListView中完毕的了。回到ListView其中。fillGap()方法的代码例如以下所看到的：
void fillGap(boolean down) {
final int count = getChildCount();
if (down) {
final int startOffset = count > 0 ? getChildAt(count - 1).getBottom() + mDividerHeight :
getListPaddingTop();
fillDown(mFirstPosition + count, startOffset);
correctTooHigh(getChildCount());
} else {
final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight :
getHeight() - getListPaddingBottom();
fillUp(mFirstPosition - 1, startOffset);
correctTooLow(getChildCount());
}
}
down參数用于表示ListView是向下滑动还是向上滑动的，能够看到。假设是向下滑动的话就会调用fillDown()方法，而假设是向上滑动的话就会调用fillUp()方法。那么这两个方法我们都已经很熟悉了，内部都是通过一个循环来去对ListView进行填充，所以这两个方法我们就不看了，可是填充ListView会通过调用makeAndAddView()方法来完毕，又是makeAndAddView()方法，但这次的逻辑再次不同了，所以我们还是回到这种方法瞧一瞧：
/**
* Obtain the view and add it to our list of children. The view can be made
* fresh, converted from an unused view, or used as is if it was in the
* recycle bin.
*
* @param position Logical position in the list
* @param y Top or bottom edge of the view to add
* @param flow If flow is true, align top edge to y. If false, align bottom
* edge to y.
* @param childrenLeft Left edge where children should be positioned
* @param selected Is this position selected?
* @return View that was added
*/
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
boolean selected) {
View child;
if (!mDataChanged) {
// Try to use an exsiting view for this position
child = mRecycler.getActiveView(position);
if (child != null) {
// Found it -- we're using an existing child
// This just needs to be positioned
setupChild(child, position, y, flow, childrenLeft, selected, true);
return child;
}
}
// Make a new view for this position, or convert an unused view if possible
child = obtainView(position, mIsScrap);
// This needs to be positioned and measured
setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
return child;
}无论怎么说。这里首先仍然是会尝试调用RecycleBin的getActiveView()方法来获取子布局，仅仅只是肯定是获取不到的了。由于在第二次Layout过程中我们已经从mActiveViews中获取过了数据，而依据RecycleBin的机制。mActiveViews是不能够反复利用的，因此这里返回的值肯定是null。既然getActiveView()方法返回的值是null，那么就还是会走到第28行的obtainView()方法其中。代码例如以下所看到的：
/**
* Get a view and have it show the data associated with the specified
* position. This is called when we have already discovered that the view is
* not available for reuse in the recycle bin. The only choices left are
* converting an old view or making a new one.
*
* @param position
* The position to display
* @param isScrap
* Array of at least 1 boolean, the first entry will become true
* if the returned view was taken from the scrap heap, false if
* otherwise.
*
* @return A view displaying the data associated with the specified position
*/
View obtainView(int position, boolean[] isScrap) {
isScrap[0] = false;
View scrapView;
scrapView = mRecycler.getScrapView(position);
View child;
if (scrapView != null) {
child = mAdapter.getView(position, scrapView, this);
if (child != scrapView) {
mRecycler.addScrapView(scrapView);
if (mCacheColorHint != 0) {
child.setDrawingCacheBackgroundColor(mCacheColorHint);
}
} else {
isScrap[0] = true;
dispatchFinishTemporaryDetach(child);
}
} else {
child = mAdapter.getView(position, null, this);
if (mCacheColorHint != 0) {
child.setDrawingCacheBackgroundColor(mCacheColorHint);
}
}
return child;
}这里在第19行会调用RecyleBin的getScrapView()方法来尝试从废弃缓存中获取一个View，那么废弃缓存有没有View呢？当然有。由于刚才在trackMotionScroll()方法中我们就已经看到了，一旦有不论什么子View被移出了屏幕，就会将它加入到废弃缓存中，而从obtainView()方法中的逻辑来看。一旦有新的数据须要显示到屏幕上。就会尝试从废弃缓存中获取View。所以它们之间就形成了一个生产者和消费者的模式，那么ListView奇妙的地方也就在这里体现出来了。无论你有随意多条数据须要显示。ListView中的子View事实上来来回回就那么几个，移出屏幕的子View会很快被移入屏幕的数据又一次利用起来，因而无论我们载入多少数据都不会出现OOM的情况，甚至内存都不会有所添加。
那么另外另一点是须要大家留意的。这里获取到了一个scrapView，然后我们在第22行将它作为第二个參数传入到了Adapter的getView()方法其中。那么第二个參数是什么意思呢？我们再次看一下一个简单的getView()方法演示样例：
@Override
public View getView(int position, View convertView, ViewGroup parent) {
Fruit fruit = getItem(position);
View view;
if (convertView == null) {
view = LayoutInflater.from(getContext()).inflate(resourceId, null);
} else {
view = convertView;
}
ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);
fruitImage.setImageResource(fruit.getImageId());
fruitName.setText(fruit.getName());
return view;
}第二个參数就是我们最熟悉的convertView呀。难怪平时我们在写getView()方法是要推断一下convertView是不是等于null，假设等于null才调用inflate()方法来载入布局，不等于null就能够直接利用convertView，由于convertView就是我们之间利用过的View。仅仅只是被移出屏幕后进入到了废弃缓存中，如今又又一次拿出来使用而已。然后我们仅仅须要把convertView中的数据更新成当前位置上应该显示的数据。那么看起来就好像是全新载入出来的一个布局一样，这背后的道理你是不是已经全然搞明确了？之后的代码又都是我们熟悉的流程了。从缓存中拿到子View之后再调用setupChild()方法将它又一次attach到ListView其中，由于缓存中的View也是之前从ListView中detach掉的，这部分代码就不再反复进行分析了。
为了方便大家理解，这里我再附上一张图解说明：
那么到眼下为止。我们就把ListView的整个工作流程代码基本分析结束了。文章比較长，希望大家能够理解清晰，下篇文章中会解说我们平时使用ListView时遇到的问题，感兴趣的朋友请继续阅读 Android ListView异步载入图片乱序问题。原因分析及解决方式 。
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