开源Android-PullToRefresh下拉刷新源代码分析

  PullToRefresh 这个库用的是很至多。github  今天主要分析一下源代码实现.

我们通过ListView的下拉刷新进行分析。其他的类似。

整个下拉刷新  父View是LinearLayout， 在LinearLayout加入了Header View ,Footer View,和ListView

PullToRefreshBase 是父类 扩展了 LinearLayout水平布局   假设我们使用ListView 须要观看子类  PullToRefreshAdapterViewBase -> PullToRefreshListView  

 初始化代码在PullToRefreshBase init方法中

重点代码:

		// Refreshable View
		// By passing the attrs, we can add ListView/GridView params via XML
		mRefreshableView = createRefreshableView(context, attrs);//通过子类传入的View,ListView或者ScrollView等
		addRefreshableView(context, mRefreshableView);//加入view到布局中

		// We need to create now layouts now  创建Header和Footer视图,默认是INVISIBLE。要加入到父窗体
		mHeaderLayout = createLoadingLayout(context, Mode.PULL_FROM_START, a);
		mFooterLayout = createLoadingLayout(context, Mode.PULL_FROM_END, a);


		handleStyledAttributes(a);//加入loadingView效果。这里是把View加入到ListView HeaderView里面去
		updateUIForMode(); //把布局加入到父View中

	protected void updateUIForMode() {
		final LinearLayout.LayoutParams lp = getLoadingLayoutLayoutParams();
		// Remove Header, and then add Header Loading View again if needed
		if (this == mHeaderLayout.getParent()) {
			removeView(mHeaderLayout);
		}
		if (mMode.showHeaderLoadingLayout()) {
			addViewInternal(mHeaderLayout, 0, lp);//增加View到LinearLayout
		}

		// Remove Footer, and then add Footer Loading View again if needed
		if (this == mFooterLayout.getParent()) {
			removeView(mFooterLayout);
		}
		if (mMode.showFooterLoadingLayout()) {
			addViewInternal(mFooterLayout, lp);//增加View到LinearLayout
		}

		// Hide Loading Views
		refreshLoadingViewsSize();//把headerView隐藏起来，其有用的是padding的方式 设置为负值 就到屏幕顶部的外面了 

		// If we're not using Mode.BOTH, set mCurrentMode to mMode, otherwise
		// set it to pull down
		mCurrentMode = (mMode != Mode.BOTH) ?
 mMode : Mode.PULL_FROM_START;
	}

//这里有2个LoadingView，一个是增加到LinearLayout中去了，另一个是增加到ListView本身的Header里面

看看handleStyledAttributes方法 定位到子类复写的地方

FrameLayout frame = new FrameLayout(getContext());
mHeaderLoadingView = createLoadingLayout(getContext(), Mode.PULL_FROM_START, a);
mHeaderLoadingView.setVisibility(View.GONE);
frame.addView(mHeaderLoadingView, lp);
mRefreshableView.addHeaderView(frame, null, false);//加入LoadingView到ListView Header上

        //headerView一共同拥有2个LoadingView,一个是被增加到LinearLayout一个是被增加到ListView的HeaderView
addViewInternal方法就是增加到LinearLayout父类中 


看看LoadingLayout 有2种  FlipLoadingLayout 和  RotateLoadingLayout 一般我们用旋转的载入动画
左边一个旋转图片，右边是文字和时间提示
    
第一个LoadingLayout主要显示 ：下拉刷新，放开以刷新
第二个LoadingLayout显示松手后的文字：正在加载...

结构是这样

当UI初始化好，以下看看onTouch 下拉捕获事件

	public final boolean onTouchEvent(MotionEvent event) {
		if (!isPullToRefreshEnabled()) {
			return false;
		}
		// If we're refreshing, and the flag is set. Eat the event
		if (!mScrollingWhileRefreshingEnabled && isRefreshing()) {
			return true;
		}
		if (event.getAction() == MotionEvent.ACTION_DOWN && event.getEdgeFlags() != 0) {
			return false;
		}
		switch (event.getAction()) {
			case MotionEvent.ACTION_MOVE: {
				if (mIsBeingDragged) {
					mLastMotionY = event.getY();
					mLastMotionX = event.getX();
					pullEvent();//開始下拉，移动 
					return true;
				}
				break;
			}

			case MotionEvent.ACTION_DOWN: {
				if (isReadyForPull()) {//按下 開始下拉
					mLastMotionY = mInitialMotionY = event.getY();
					mLastMotionX = mInitialMotionX = event.getX();
					return true;
				}
				break;
			}

			case MotionEvent.ACTION_CANCEL:
			case MotionEvent.ACTION_UP: { //停止下拉的时候
				if (mIsBeingDragged) {
					mIsBeingDragged = false;
					if (mState == State.RELEASE_TO_REFRESH
							&& (null != mOnRefreshListener || null != mOnRefreshListener2)) {
						setState(State.REFRESHING, true);//放下手指開始回调，运行我们的回调任务
						return true;
					}

					// If we're already refreshing, just scroll back to the top
					if (isRefreshing()) {
						smoothScrollTo(0);
						return true;
					}

					// If we haven't returned by here, then we're not in a state
					// to pull, so just reset
					setState(State.RESET); //恢复到原来的UI状态

					return true;
				}
				break;
			}
		}

		return false;
	}

看看pullEvent方法
	private void pullEvent() {
		final int newScrollValue;
		final int itemDimension;
		final float initialMotionValue, lastMotionValue;

		switch (getPullToRefreshScrollDirection()) {
			case HORIZONTAL:
				initialMotionValue = mInitialMotionX;
				lastMotionValue = mLastMotionX;
				break;
			case VERTICAL:
			default:
				initialMotionValue = mInitialMotionY;
				lastMotionValue = mLastMotionY;
				break;
		}
		//计算下拉移动了多少
		switch (mCurrentMode) {
			case PULL_FROM_END://上拉
				newScrollValue = Math.round(Math.max(initialMotionValue - lastMotionValue, 0) / FRICTION);
				itemDimension = getFooterSize();
				break;
			case PULL_FROM_START://下拉
			default:
				newScrollValue = Math.round(Math.min(initialMotionValue - lastMotionValue, 0) / FRICTION);
				itemDimension = getHeaderSize();
				break;
		}

		//显示HeaderView 得到移动的值。能够让LoadingView显示出来
		setHeaderScroll(newScrollValue);

		if (newScrollValue != 0 && !isRefreshing()) {
			float scale = Math.abs(newScrollValue) / (float) itemDimension;
			switch (mCurrentMode) {
				case PULL_FROM_END:
					mFooterLayout.onPull(scale);
					break;
				case PULL_FROM_START:
				default:
				 mHeaderLayout.onPull(scale);//旋转左边的载入图片，显示文字和图片 这个地方终于会运行LoadingLayout中的 onPullImpl方法
					break;
			}
			//更新状态 包含2中 释放按下触摸，还有就是 没释放手的触摸
			if (mState != State.PULL_TO_REFRESH && itemDimension >= Math.abs(newScrollValue)) {
				setState(State.PULL_TO_REFRESH);
			} else if (mState == State.PULL_TO_REFRESH && itemDimension < Math.abs(newScrollValue)) {
				setState(State.RELEASE_TO_REFRESH);//下拉松手 能够松手了
			}
		}
	}

再看看setHeaderScroll方法代码
	protected final void setHeaderScroll(int value) {
		if (DEBUG) {
			Log.d(LOG_TAG, "setHeaderScroll: " + value);
		}
		
		if (DEBUG) {
			Log.d(LOG_TAG, "setHeaderScroll:" + value );
		}

		// Clamp value to with pull scroll range
		final int maximumPullScroll = getMaximumPullScroll();
		value = Math.min(maximumPullScroll, Math.max(-maximumPullScroll, value));

		if (mLayoutVisibilityChangesEnabled) {
			if (value < 0) { //有位移才显示
				mHeaderLayout.setVisibility(View.VISIBLE);
			} else if (value > 0) {  <span style="font-family: Arial, Helvetica, sans-serif;">//有位移才显示</span>
				mFooterLayout.setVisibility(View.VISIBLE);
			} else { 
				mHeaderLayout.setVisibility(View.INVISIBLE);
				mFooterLayout.setVisibility(View.INVISIBLE);
			}
		}

		if (USE_HW_LAYERS) {
			/**
			 * Use a Hardware Layer on the Refreshable View if we've scrolled at
			 * all. We don't use them on the Header/Footer Views as they change
			 * often, which would negate any HW layer performance boost.
			 */
			ViewCompat.setLayerType(mRefreshableViewWrapper, value != 0 ? View.LAYER_TYPE_HARDWARE
					: View.LAYER_TYPE_NONE);
		}

		//回到最原始的scrollTo 最经常使用的 移动布局
		switch (getPullToRefreshScrollDirection()) {
			case VERTICAL:
				scrollTo(0, value);
				break;
			case HORIZONTAL:
				scrollTo(value, 0);
				break;
		}
	}

setState(State.REFRESHING, true);//拉倒最顶部 松手，会运行onRefreshing方法。回调我们实现的任务接口 也就是OnRefreshListener

protected void onRefreshing(final boolean doScroll) {
		if (mMode.showHeaderLoadingLayout()) {
			mHeaderLayout.refreshing();
		}
		if (mMode.showFooterLoadingLayout()) {
			mFooterLayout.refreshing();
		}

		if (doScroll) {
			if (mShowViewWhileRefreshing) {

				// Call Refresh Listener when the Scroll has finished
				OnSmoothScrollFinishedListener listener = new OnSmoothScrollFinishedListener() {
					@Override
					public void onSmoothScrollFinished() {
						callRefreshListener();//回调接口运行
					}
				};

				switch (mCurrentMode) {
					case MANUAL_REFRESH_ONLY:
					case PULL_FROM_END:
						smoothScrollTo(getFooterSize(), listener);
						break;
					default:
					case PULL_FROM_START:
						smoothScrollTo(-getHeaderSize(), listener);
						break;
				}
			} else {
				smoothScrollTo(0);//回到原来的位置
			}
		} else {
			// We're not scrolling, so just call Refresh Listener now
			callRefreshListener();//回调接口运行
		}
	}

	private void callRefreshListener() {
		if (null != mOnRefreshListener) {
			mOnRefreshListener.onRefresh(this);//回调
		} else if (null != mOnRefreshListener2) { //这个是上拉，下拉都能够的情况，使用 onRefreshListener2
			if (mCurrentMode == Mode.PULL_FROM_START) {
				mOnRefreshListener2.onPullDownToRefresh(this);
			} else if (mCurrentMode == Mode.PULL_FROM_END) {
				mOnRefreshListener2.onPullUpToRefresh(this);
			}
		}
	}

总结：状态包含下拉刷新。松手刷新，正在刷新，Loading隐藏。移动UI还是用的scrollTo最主要的代码. 动画部分能够看LoadingLayout的2个子类

基本的就这些。还有非常多细节没有分析。若有问题请指出谢谢。