PullToRefreshListView原理解析(二)

PullToRefreshListView的父类是PullToRefreshAdapterViewBase<T extends AbsListView>，PullToRefreshAdapterViewBase的父类是

PullToRefreshBase<T extends View>。

PullToRefreshBase是公共父类,它的结构如下。从上往下，分别为headView,泛型T,footView
headView和footView不用说了，上拉或者下拉显示出来的UI。至于显示成什么样子，那事他们自己的事，重点是PullToRefreshBase要告诉他们什么时候是

上拉了什么时候是下拉了。PullToRefreshListView的重点也都在这个类里面。至于不需要显示headView和footView时，怎么操作那是泛型T的事。泛型T就是

需要对什么控件添加上拉和下拉效果，可能是WebView,也可能是ListView,这里提高了扩展性。最终PullToRefreshListView传入的泛型T就是ListView。所以

PullToRefreshBase的重点就是什么时候拦截事件来通知headView和footView执行上拉下拉效果。
1.拦截事件onInterceptTouchEvent
public final boolean onInterceptTouchEvent(MotionEvent event) {

		if (!isPullToRefreshEnabled()) {
			return false;
		}

		final int action = event.getAction();
		// 事件事
		if (mMode == Mode.REFROTATE && action == MotionEvent.ACTION_UP) {

		}
		if (action == MotionEvent.ACTION_CANCEL
				|| action == MotionEvent.ACTION_UP) {
			mIsBeingDragged = false;
			return false;
		}

		if (action != MotionEvent.ACTION_DOWN && mIsBeingDragged) {
			return true;
		}

		switch (action) {
		case MotionEvent.ACTION_MOVE: {
			// If we're refreshing, and the flag is set. Eat all MOVE events
			if (!mScrollingWhileRefreshingEnabled && isRefreshing()) {
				return true;
			}

			if (isReadyForPull()) {
				final float y = event.getY(), x = event.getX();
				final float diff, oppositeDiff, absDiff;

				// We need to use the correct values, based on scroll
				// direction
				switch (getPullToRefreshScrollDirection()) {
				case HORIZONTAL:
					diff = x - mLastMotionX;
					oppositeDiff = y - mLastMotionY;
					break;
				case VERTICAL:
				default:
					diff = y - mLastMotionY;
					oppositeDiff = x - mLastMotionX;
					break;
				}
				absDiff = Math.abs(diff);

				if (absDiff > mTouchSlop
						&& (!mFilterTouchEvents || absDiff > Math
								.abs(oppositeDiff))) {
					if (mMode.showHeaderLoadingLayout() && diff >= 1f
							&& isReadyForPullStart()) {
						mLastMotionY = y;
						mLastMotionX = x;
						mIsBeingDragged = true;
						if (mMode == Mode.BOTH || mMode == Mode.FLIP_ROTATE) {
							mCurrentMode = Mode.PULL_FROM_START;
						}
					} else if (mMode.showFooterLoadingLayout() && diff <= -1f
							&& isReadyForPullEnd()) {
						mLastMotionY = y;
						mLastMotionX = x;
						mIsBeingDragged = true;
						if (mMode == Mode.BOTH || mMode == Mode.FLIP_ROTATE
								|| mMode == Mode.REFROTATE) {
							mCurrentMode = Mode.PULL_FROM_END;
						}
					}
				}
			}
			break;
		}
		case MotionEvent.ACTION_DOWN: {
			if (isReadyForPull()) {
				mLastMotionY = mInitialMotionY = event.getY();
				mLastMotionX = mInitialMotionX = event.getX();
				mIsBeingDragged = false;
			}
			break;
		}
		}

		return mIsBeingDragged;
	}
1.首先ACTION_DOWN肯定要返回false,因为按下时还不知道是否要显示headView或者footView,要是返回true,滑动事件就传递不到ListView了。

2.action != MotionEvent.ACTION_DOWN && mIsBeingDragged
ACTION_MOVE时,是否返回true，主要看mIsBeingDragged是否为true,那它什么时候为true呢。
2.1  isRefreshing()的时候
public final boolean isRefreshing() {
		return mState == State.REFRESHING || mState == State.MANUAL_REFRESHING;
	}
2.2
if (absDiff > mTouchSlop
						&& (!mFilterTouchEvents || absDiff > Math
								.abs(oppositeDiff))) {
					if (mMode.showHeaderLoadingLayout() && diff >= 1f
							&& isReadyForPullStart()) {
						mLastMotionY = y;
						mLastMotionX = x;
						mIsBeingDragged = true;
						if (mMode == Mode.BOTH || mMode == Mode.FLIP_ROTATE) {
							mCurrentMode = Mode.PULL_FROM_START;
						}
					} else if (mMode.showFooterLoadingLayout() && diff <= -1f
							&& isReadyForPullEnd()) {
						mLastMotionY = y;
						mLastMotionX = x;
						mIsBeingDragged = true;
						if (mMode == Mode.BOTH || mMode == Mode.FLIP_ROTATE
								|| mMode == Mode.REFROTATE) {
							mCurrentMode = Mode.PULL_FROM_END;
						}
					}
				}

只要有上下滑动就能进入第一层if,

absDiff > Math.abs(oppositeDiff)
就是判断滑动时垂直移动距离大于水平移动距离,即上下滑动。

里面一层if主要就是判断，

mMode.showHeaderLoadingLayout()
判断mode，真正使用时自己会设置的，只要设置没问题就是true.

diff >= 1f
就不说了

isReadyForPullStart()
是抽象方法，看子类。子类主要在isFirstItemVisible方法

private boolean isFirstItemVisible() {
		final Adapter adapter = mRefreshableView.getAdapter();

		if (null == adapter || adapter.isEmpty()) {
			if (DEBUG) {
				Log.d(LOG_TAG, "isFirstItemVisible. Empty View.");
			}
			return true;

		} else {

			/**
			 * This check should really just be:
			 * mRefreshableView.getFirstVisiblePosition() == 0, but PtRListView
			 * internally use a HeaderView which messes the positions up. For
			 * now we'll just add one to account for it and rely on the inner
			 * condition which checks getTop().
			 */
			if (mRefreshableView.getFirstVisiblePosition() <= 1) {
				final View firstVisibleChild = mRefreshableView.getChildAt(0);
				if (firstVisibleChild != null) {
					return firstVisibleChild.getTop() >= mRefreshableView
							.getTop();
				}
			}
		}

		return false;
	}
可以看到，泛型是ListView时，就是判断是不是在最顶部。另一个if代码块意思也差不多

2.执行事件onTouchEvent

不用想也知道，当onInterceptTouchEvent返回true时(mIsBeingDragged为true)，onTouchEvent肯定要做处理了.比如ACTION_MOVE的时候

case MotionEvent.ACTION_MOVE: {
			if (mIsBeingDragged) {
				mLastMotionY = event.getY();
				mLastMotionX = event.getX();
				pullEvent();
				return true;
			}
			break;
		}
执行pullEvent，主要就是一系列计算，让headView或者footView滑动。这里就不说了

over