Android-自定义View前传-View的三大流程-Layout
参考
- 《Android开发艺术探索》
- https://github.com/hongyangAndroid/FlowLayout
写在前头
在之前的文章中 ， 我们学习了Android View的 Measure的流程， 本篇文章来学习一下View的 Layout 的过程。 学完了这一篇文章后，我们可以尝试自己去自定义一个自己的Layout。
Overview
我对于Layout过程的理解：Layout的过程就是给Child安家的过程
Layout的过程主要是放在 ViewGroup 中的，ViewGroup不仅需要定位自己，还需要定位Child。
View和ViewGroup
Layout流程的起点也是在 ViewRootImpl 中的 performTraversals 方法中。
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
mLayoutRequested = false;
mScrollMayChange = true;
mInLayout = true;
final View host = mView;
if (host == null) {
return;
}
try {
//首先调用了host的layout方法 host = mView = DecorView
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
//....
}
我们接着来看View的layout方法
public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
//调用 setOpticalFrame或者 setFrame 来确定自己的位置
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {、
//调用onLayout
onLayout(changed, l, t, r, b);
//.....
}
protected boolean setFrame(int left, int top, int right, int bottom) {
boolean changed = false;
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
changed = true;
// Remember our drawn bit
int drawn = mPrivateFlags & PFLAG_DRAWN;
int oldWidth = mRight - mLeft;
int oldHeight = mBottom - mTop;
int newWidth = right - left;
int newHeight = bottom - top;
boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);
// Invalidate our old position
invalidate(sizeChanged);
//确定4个点
//这四个点一旦确定了那么View在ViewGroup中的位置也就确定了
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);
//......
View的 layout 方法主要是做了:
- 通过
setFrame确定了自己的位置，一篇Left,Top,Right,Bottom这几个值确定了,那么View的位置也就确定了。 - 紧接着调用了
onLayout方法。
LinearLayout的onLayout
View是不需要实现onLayout方法的，只用ViewGroup才需要实现。由于各种ViewGroup的布局方式的不同，无法统一，所以ViewGroup也并没有实现onLayout. 而是将onLayout的过程放到了子类中。我们还是通过 LinearLayout 来学习。
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
if (mOrientation == VERTICAL) {
layoutVertical(l, t, r, b);
} else {
layoutHorizontal(l, t, r, b);
}
}
onLayout根据 Orientation 属性来调用 layoutVertical 或者 layoutHorizontal
void layoutVertical(int left, int top, int right, int bottom) {
final int paddingLeft = mPaddingLeft;
int childTop;
int childLeft;
// Where right end of child should go
final int width = right - left;
int childRight = width - mPaddingRight;
// Space available for child
//获取Child可用的空间
int childSpace = width - paddingLeft - mPaddingRight;
//Child的Group
final int count = getVirtualChildCount();
final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
//根据Gravity确定初始的childTop
switch (majorGravity) {
case Gravity.BOTTOM:
// mTotalLength contains the padding already
childTop = mPaddingTop + bottom - top - mTotalLength;
break;
// mTotalLength contains the padding already
case Gravity.CENTER_VERTICAL:
childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
break;
case Gravity.TOP:
default:
childTop = mPaddingTop;
break;
}
//LayoutView
//遍历
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {//过滤Child的Visibility是GONE的情况
//获取Child的大小
final int childWidth = child.getMeasuredWidth();
final int childHeight = child.getMeasuredHeight();
//获取LP
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams();
int gravity = lp.gravity;
if (gravity < 0) {
gravity = minorGravity;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
//处理Child的Gravity
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
//水平居中
case Gravity.CENTER_HORIZONTAL:
childLeft = paddingLeft + ((childSpace - childWidth) / 2)
+ lp.leftMargin - lp.rightMargin;
break;
//居右
case Gravity.RIGHT:
childLeft = childRight - childWidth - lp.rightMargin;
break;
case Gravity.LEFT:
default:
childLeft = paddingLeft + lp.leftMargin;
break;
}
//childTop 加上 Divider
if (hasDividerBeforeChildAt(i)) {
childTop += mDividerHeight;
}
//加上Margin
childTop += lp.topMargin;
//调用Child的layout方法
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
}
private void setChildFrame(View child, int left, int top, int width, int height) {
child.layout(left, top, left + width, top + height);
}
上面的代码还是比较清晰的，首先是有一个childTop变量，来确定child与ViewGroup顶部的距离，通过不断的遍历Child然后不断增加childTop的值，这样就实现了LinearLayout的垂直布局的效果。当然其中也有处理LinearLayout和Child的Gravity的过程。
FlowLayout
通过学习LinearLayout的Layout的过程，发现其实Layout的过程就是确定View的Left,Top,Right,Bottom 4个值的过程，学习了 Measure 和 Layout 的过程以后，我们就已经可以着手做一个自己的Layout了，这里我选的是模仿 hongyang大神的FlowLayout。效果图如下:
package layout
import android.annotation.SuppressLint
import android.content.Context
import android.util.AttributeSet
import android.view.View
import android.view.ViewGroup
/**
* Created by ShyCoder on 2019/1/16.
*/
class MyFlowLayout(context: Context?, attrs: AttributeSet?, defStyleAttr: Int)
: ViewGroup(context, attrs, defStyleAttr) {
constructor(context: Context?, attributeSet: AttributeSet?) : this(context, attributeSet, 0)
constructor(context: Context?) : this(context, null)
init {
}
/**
* 存贮所有的View根据行来存贮
* */
private val mAllViews = mutableListOf<List<View>>()
/**
* 存贮每一行View的高度
* */
private val mHeightList = mutableListOf<Int>()
override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
//从MeasureSpec获取Mode和Size
val widthMode = MeasureSpec.getMode(widthMeasureSpec)
val widthSize = MeasureSpec.getMode(widthMeasureSpec)
val heightMode = MeasureSpec.getMode(heightMeasureSpec)
val heightSize = MeasureSpec.getMode(heightMeasureSpec)
//计算Wrap_content的情况
var totalHeight = 0//总高度
var totalWidth = 0//总宽度
var lineWidth = 0//当前行的宽度
var lineHeight = 0//当前行的高度
val viewCount = childCount
for (i in 0.until(viewCount)) {
val child = getChildAt(i)
//如果是GONE状态不用测量
if (child.visibility == View.GONE) {
continue
}
//测量Child
measureChild(child, widthMeasureSpec, heightMeasureSpec)
val lp = child.layoutParams as MarginLayoutParams
//计算child的width = 测量后宽度+左右的两个Margin
val childWidth = child.measuredWidth + lp.leftMargin + lp.rightMargin
//计算child的Height = 测量后高度 + 上下两个Margin
val childHeight = child.measuredHeight + lp.bottomMargin + lp.topMargin
//需要换行时候的处理方式
//如果已有的行宽+当前child的宽度> FlowLayout的宽度(减去左右的Padding)
if (childWidth + lineWidth > widthSize - this.paddingLeft + this.paddingRight) {
totalWidth = Math.max(totalWidth, lineWidth)
lineWidth = childWidth
totalHeight += lineHeight
lineHeight = childHeight
} else {//如果不需要换行增加行宽
lineWidth += childWidth
//获取最大高度
lineHeight = Math.max(lineHeight, childHeight)
}
//最后一个View
if (i == viewCount - 1) {
totalWidth = Math.max(totalWidth, lineWidth)
totalHeight += lineHeight
}
}
this.setMeasuredDimension(
//如果MeasureSpec的Mode是EXACTLY 的话，测量后大小等会传进来的测量大小，
//否则则是我们自己计算的大小
if (widthMode == MeasureSpec.EXACTLY) widthSize else totalWidth,
if (heightMode == MeasureSpec.EXACTLY) heightSize else totalHeight
)
}
override fun onLayout(changed: Boolean, l: Int, t: Int, r: Int, b: Int) {
this.mAllViews.clear()
this.mHeightList.clear()
val viewCount = this.childCount
var lineHeight = 0
var lineWidth = 0
var lineViews = mutableListOf<View>()
for (i in 0.until(viewCount)) {
val child = this.getChildAt(i)
if (child.visibility == View.GONE) {
continue
}
val lp = child.layoutParams as MarginLayoutParams
val childWidth = child.measuredWidth + lp.leftMargin + lp.rightMargin
val childHeight = child.measuredHeight + lp.topMargin + lp.bottomMargin
//行上无法继续放置View
if (childWidth + lineWidth > this.width - this.paddingLeft - this.paddingRight) {
//添加line height
mHeightList.add(lineHeight)
lineWidth = 0
//添加正行的View到集合中
this.mAllViews.add(lineViews)
lineViews = mutableListOf()
}
//区当前行的View的最大高度
lineHeight = StrictMath.max(lineHeight, childHeight)
lineWidth += childWidth
//向行上添加View
lineViews.add(child)
}
//进行Layout
var left = this.paddingLeft
var top = this.paddingTop
for (i in 0.until(this.mAllViews.size)) {
val lineViews = mAllViews[i]
val lineHeight = mHeightList[i]
left = this.paddingLeft
for (j in 0.until(lineViews.size)) {
val child = lineViews[j]
val lp = child.layoutParams as MarginLayoutParams
//view的四个边
val l = left + lp.leftMargin
val t = top + lp.topMargin
val r = l + child.measuredWidth
val b = t + child.measuredHeight
//调用Child的Layout方法
child.layout(l, t, r, b)
left += child.measuredWidth + lp.leftMargin + lp.rightMargin
}
top += lineHeight
}
}
}
在onMeasure对wrap_content的情况进行了处理，计算出所需要的大小。
在onLayout方法中，首先计算出每一行的高度并存储和获取每一行的View的List进行存储，当这些都计算完之后，进行layout操作。
在layout操作的时候，首先是在同一行上的View的top是统一的，每当这一行的View处理完成之后，就执行换行的操作，即增加view的大小。
写在最后
本篇文章就到此结束了，Layout的过程还是相对简单的，在下一篇文章呢，我们将会学习View的最后一个流程 Draw 流程。