源码版本：28
本篇要点：滑动的时候是怎么回收View和填充新的View的。
AbsListView的onTouchEvent方法。
@Override
public boolean onTouchEvent(MotionEvent ev) {
//...
initVelocityTrackerIfNotExists();
final MotionEvent vtev = MotionEvent.obtain(ev);
final int actionMasked = ev.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
mNestedYOffset = 0;
}
vtev.offsetLocation(0, mNestedYOffset);
switch (actionMasked) {
case MotionEvent.ACTION_DOWN: {
onTouchDown(ev);
break;
}
case MotionEvent.ACTION_MOVE: {
onTouchMove(ev, vtev);
break;
}
case MotionEvent.ACTION_UP: {
onTouchUp(ev);
break;
}
//...
}
if (mVelocityTracker != null) {
mVelocityTracker.addMovement(vtev);
}
vtev.recycle();
return true;
}
AbsListView的onTouchDown方法。
private void onTouchDown(MotionEvent ev) {
//...
if (mTouchMode == TOUCH_MODE_OVERFLING) {
//MotionEvent.ACTION_DOWN的时候，如果正在OVERFLING，则停止
mFlingRunnable.endFling();
if (mPositionScroller != null) {
mPositionScroller.stop();
}
//将触摸模式置为TOUCH_MODE_OVERSCROLL
mTouchMode = TOUCH_MODE_OVERSCROLL;
mMotionX = (int) ev.getX();
mMotionY = (int) ev.getY();
mLastY = mMotionY;
mMotionCorrection = 0;
mDirection = 0;
} else {
final int x = (int) ev.getX();
final int y = (int) ev.getY();
//将坐标转化为转化为ListView中子View的位置
int motionPosition = pointToPosition(x, y);
if (!mDataChanged) {
if (mTouchMode == TOUCH_MODE_FLING) {
// Stopped a fling. It is a scroll.
createScrollingCache();
mTouchMode = TOUCH_MODE_SCROLL;
mMotionCorrection = 0;
motionPosition = findMotionRow(y);
//注释1处，根据当前速度检查是否要停止fling
mFlingRunnable.flywheelTouch();
} else if ((motionPosition >= 0) && getAdapter().isEnabled(motionPosition)) {
//用户正常的按下操作，不是停止一个fling的操作，将mTouchMode置为TOUCH_MODE_DOWN
mTouchMode = TOUCH_MODE_DOWN;
//...
}
}
if (motionPosition >= 0) {
// Remember where the motion event started
final View v = getChildAt(motionPosition - mFirstPosition);
mMotionViewOriginalTop = v.getTop();
}
//按下时候的水平坐标
mMotionX = x;
//按下时候的竖直坐标
mMotionY = y;
mMotionPosition = motionPosition;
//按下的时候，将mLastY重置为Integer.MIN_VALUE。
mLastY = Integer.MIN_VALUE;
}
//...
}
注释1处，用户按下时，如果ListView当前正在fling，则需要检查根据速度检查是否要停止fling。
AbsListView的onTouchMove方法。
private void onTouchMove(MotionEvent ev, MotionEvent vtev) {
int pointerIndex = ev.findPointerIndex(mActivePointerId);
if (pointerIndex == -1) {
pointerIndex = 0;
mActivePointerId = ev.getPointerId(pointerIndex);
}
if (mDataChanged) {
layoutChildren();
}
final int y = (int) ev.getY(pointerIndex);
switch (mTouchMode) {
case TOUCH_MODE_DOWN:
case TOUCH_MODE_TAP:
case TOUCH_MODE_DONE_WAITING:
//注释1处，检查是否需要滑动，true的话，直接break。
if (startScrollIfNeeded((int) ev.getX(pointerIndex), y, vtev)) {
break;
}
//...
break;
case TOUCH_MODE_SCROLL:
case TOUCH_MODE_OVERSCROLL:
//注释2处，如果需要直接滑动
scrollIfNeeded((int) ev.getX(pointerIndex), y, vtev);
break;
}
}
注释1处，AbsListView的startScrollIfNeeded方法。
private boolean startScrollIfNeeded(int x, int y, MotionEvent vtev) {
//检查是否移动了足够远的距离，足够远的话，则认为是滚动。
//移动的距离，指从下向上滑动，rawDeltaY小于0
final int deltaY = y - mMotionY;
final int distance = Math.abs(deltaY);
final boolean overscroll = mScrollY != 0;
if ((overscroll || distance > mTouchSlop) &&
(getNestedScrollAxes() & SCROLL_AXIS_VERTICAL) == 0) {
//（处于overscroll状态或者移动的距离足够远）并且（不是嵌套滑动状态）
createScrollingCache();
if (overscroll) {
mTouchMode = TOUCH_MODE_OVERSCROLL;
mMotionCorrection = 0;
} else {
mTouchMode = TOUCH_MODE_SCROLL;
//指从下向上滑动，rawDeltaY为小于0，mMotionCorrection小于0
mMotionCorrection = deltaY > 0 ? mTouchSlop : -mTouchSlop;
}
removeCallbacks(mPendingCheckForLongPress);
setPressed(false);
final View motionView = getChildAt(mMotionPosition - mFirstPosition);
if (motionView != null) {
motionView.setPressed(false);
}
//报告滑动状态是SCROLL_STATE_TOUCH_SCROLL
reportScrollStateChange(OnScrollListener.SCROLL_STATE_TOUCH_SCROLL);
// Time to start stealing events! Once we've stolen them, don't let anyone
// steal from us
final ViewParent parent = getParent();
if (parent != null) {
//告诉父级不要拦截事件
parent.requestDisallowInterceptTouchEvent(true);
}
//注释1处，重点方法
scrollIfNeeded(x, y, vtev);
return true;
}
return false;
}
注释1处，重点方法，AbsListView的scrollIfNeeded方法。
private void scrollIfNeeded(int x, int y, MotionEvent vtev) {
//手指从下向上滑动，rawDeltaY小于0
int rawDeltaY = y - mMotionY;
int scrollOffsetCorrection = 0;
int scrollConsumedCorrection = 0;
//正常MotionEvent的down事件的时候会将mLastY置为Integer.MIN_VALUE
if (mLastY == Integer.MIN_VALUE) {
//减去一个mTouchSlop的量，不影响
rawDeltaY -= mMotionCorrection;
}
//...
final int deltaY = rawDeltaY;
//滑动增加的距离
int incrementalDeltaY =
mLastY != Integer.MIN_VALUE ? y - mLastY : deltaY;
int lastYCorrection = 0;
//我们只看TOUCH_MODE_SCROLL的情况，忽略TOUCH_MODE_OVERSCROLL的情况。哈哈，偷个懒。
if (mTouchMode == TOUCH_MODE_SCROLL) {
if (y != mLastY) {
//...
final int motionIndex;
if (mMotionPosition >= 0) {
motionIndex = mMotionPosition - mFirstPosition;
} else {
// If we don't have a motion position that we can reliably track,
// pick something in the middle to make a best guess at things below.
motionIndex = getChildCount() / 2;
}
int motionViewPrevTop = 0;
View motionView = this.getChildAt(motionIndex);
if (motionView != null) {
motionViewPrevTop = motionView.getTop();
}
// No need to do all this work if we're not going to move anyway
boolean atEdge = false;
if (incrementalDeltaY != 0) {
//注释1处
atEdge = trackMotionScroll(deltaY, incrementalDeltaY);
}
// Check to see if we have bumped into the scroll limit
motionView = this.getChildAt(motionIndex);
if (motionView != null) {
// Check if the top of the motion view is where it is
// supposed to be
final int motionViewRealTop = motionView.getTop();
if (atEdge) {
// 应用 overscroll，忽略 ...
}
mMotionY = y + lastYCorrection + scrollOffsetCorrection;
}
mLastY = y + lastYCorrection + scrollOffsetCorrection;
}
}
}
注释1处，AbsListView的trackMotionScroll方法。
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;
int effectivePaddingTop = 0;
int effectivePaddingBottom = 0;
//...
final int spaceAbove = effectivePaddingTop - firstTop;
final int end = getHeight() - effectivePaddingBottom;
final int spaceBelow = lastBottom - end;
final int height = getHeight() - mPaddingBottom - mPaddingTop;
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;
// Update our guesses for where the first and last views are
if (firstPosition == 0) {
mFirstPositionDistanceGuess = firstTop - listPadding.top;
} else {
mFirstPositionDistanceGuess += incrementalDeltaY;
}
if (firstPosition + childCount == mItemCount) {
mLastPositionDistanceGuess = lastBottom + listPadding.bottom;
} else {
mLastPositionDistanceGuess += incrementalDeltaY;
}
//判断是否在最顶部且手指从上向下滑动，是的话即不能向下滑动了
final boolean cannotScrollDown = (firstPosition == 0 &&
firstTop >= listPadding.top && incrementalDeltaY >= 0);
//判断是否在最底部且手指从下向上滑动,是的话即不能向上滑动了
final boolean cannotScrollUp = (firstPosition + childCount == mItemCount &&
lastBottom <= getHeight() - listPadding.bottom && incrementalDeltaY <= 0);
//注释0处，不能滑动，直接返回
if (cannotScrollDown || cannotScrollUp) {
return incrementalDeltaY != 0;
}
//注释1处，手指是从下向上滑动，incrementalDeltaY < 0
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) {
//注释2处，手指是从下向上滑动，incrementalDeltaY < 0，所以top是大于0的。
int top = -incrementalDeltaY;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
top += listPadding.top;
}
//注释3处，for循环
for (int i = 0; i < childCount; i++) {
final View child = getChildAt(i);
if (child.getBottom() >= top) {
break;
} else {
//注释3.1处，
count++;
int position = firstPosition + i;
if (position >= headerViewsCount && position < footerViewsStart) {
// The view will be rebound to new data, clear any
// system-managed transient state.
child.clearAccessibilityFocus();
//注释3.2处
mRecycler.addScrapView(child, position);
}
}
}
} else {
//注释4处，手指是从上向下滑动，incrementalDeltaY > 0
int bottom = getHeight() - incrementalDeltaY;
//...
for (int i = childCount - 1; i >= 0; i--) {
final View child = getChildAt(i);
if (child.getTop() <= bottom) {
break;
} else {
//注释4.1
start = i;
count++;
int position = firstPosition + i;
if (position >= headerViewsCount && position < footerViewsStart) {
// The view will be rebound to new data, clear any
// system-managed transient state.
child.clearAccessibilityFocus();
//注释4.2
mRecycler.addScrapView(child, position);
}
}
}
}
mMotionViewNewTop = mMotionViewOriginalTop + deltaY;
mBlockLayoutRequests = true;
//注释5处，滚动出去的子View和ListView解除关联
if (count > 0) {
detachViewsFromParent(start, count);
mRecycler.removeSkippedScrap();
}
//...
//注释6处，核心滚动代码，根据incrementalDeltaY同步偏移所有的子View
offsetChildrenTopAndBottom(incrementalDeltaY);
if (down) {
mFirstPosition += count;
}
final int absIncrementalDeltaY = Math.abs(incrementalDeltaY);
//手指从上向下滑动，down为false，spaceAbove < absIncrementalDeltaY，表示第一个子View已经完全滑进屏幕了。需要填充新的子View。
//手指从下向上滑动，down为true，spaceBelow < absIncrementalDeltaY，表示最后一个子View已经完全滑进屏幕了。需要填充新的子View。
if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {
//注释7处，填充新的子View
fillGap(down);
}
mRecycler.fullyDetachScrapViews();
//...
invokeOnItemScrollListener();
return false;
}
注释0处，不能滑动，直接返回。
注释1处，手指是从下向上滑动，incrementalDeltaY < 0。
注释2处，手指是从下向上滑动，incrementalDeltaY < 0，所以top是大于0的。
注释3处，for循环是什么意思呢？解释一下：
- 比如我们手指从下向上滑动，滑动了500像素，那么
incrementalDeltaY = -500，top = -incrementalDeltaY =500。 - 那么ListView整体是要向上滑动500像素的。遍历ListView的所有的子View，View的bottom坐标小于500像素的都要滑出屏幕。
注释3.1处，记录要滑出的View的数量，注释3.2处将要滑出屏幕的View加入到 RecyclerBin的mScrapViews数组中等待以后复用。
同理，注释4处，手指是从上向下滑动，incrementalDeltaY > 0。
-
比如我们手指从上向下滑动，滑动了500像素，那么
incrementalDeltaY = 500。比如ListView的getHeight()=2000，那么bottom = getHeight() - incrementalDeltaY = 1500。 -
那么ListView整体是要向下滑动500像素的。遍历ListView的所有的子View，View的top坐标大于1500像素的都要从滑出屏幕。
注释4.1处，记录要滑出的View的数量，注释4.2处将要滑出屏幕的View加入到RecyclerBin的mScrapViews数组中等待以后复用。
注释5处，滚动出去的子View和ListView解除关联。
注释6处，核心滚动代码,根据incrementalDeltaY同步偏移所有的子View。
/**
* 将所有子View的竖直方向上的位置偏移指定的像素数。
*
* @param offset 指定的像素偏移量。
*/
public void offsetChildrenTopAndBottom(int offset) {
final int count = mChildrenCount;
final View[] children = mChildren;
boolean invalidate = false;
for (int i = 0; i < count; i++) {
final View v = children[i];
v.mTop += offset;
v.mBottom += offset;
if (v.mRenderNode != null) {
invalidate = true;
v.mRenderNode.offsetTopAndBottom(offset);
}
}
if (invalidate) {
invalidateViewProperty(false, false);
}
notifySubtreeAccessibilityStateChangedIfNeeded();
}
ListView正是通过这种将所以子View偏移指定的像素数来实现滚动效果的，偏移出屏幕的子View会和ListView取消关联，并加入到RecyclerBin的mScrapViews数组中等待以后复用。
注意：这里顺便提一嘴，ScrollView是修改自身的scrollY来实现的滚动，从而显示所有的子View的，和ListView是有区别的，注意一下。
注释7处，当我们滑动的时候，会将一些子View滑出屏幕，也需要填充新的子View到屏幕中。
@Override
void fillGap(boolean down) {
final int count = getChildCount();
if (down) {
int paddingTop = 0;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
paddingTop = getListPaddingTop();
}
final int startOffset = count > 0 ? getChildAt(count - 1).getBottom() + mDividerHeight :
paddingTop;
//手指从下向上滑动，down为true，向下填充新的View。
fillDown(mFirstPosition + count, startOffset);
correctTooHigh(getChildCount());
} else {
int paddingBottom = 0;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
paddingBottom = getListPaddingBottom();
}
final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight :
getHeight() - paddingBottom;
//手指从上向下滑动，down为false，向上填充新的View。
fillUp(mFirstPosition - 1, startOffset);
correctTooLow(getChildCount());
}
}
AbsListView的onTouchUp方法
private void onTouchUp(MotionEvent ev) {
switch (mTouchMode) {
case TOUCH_MODE_SCROLL:
final int childCount = getChildCount();
if (childCount > 0) {
final int firstChildTop = getChildAt(0).getTop();
final int lastChildBottom = getChildAt(childCount - 1).getBottom();
final int contentTop = mListPadding.top;
final int contentBottom = getHeight() - mListPadding.bottom;
if (mFirstPosition == 0 && firstChildTop >= contentTop &&
mFirstPosition + childCount < mItemCount &&
lastChildBottom <= getHeight() - contentBottom) {
mTouchMode = TOUCH_MODE_REST;
//注释1处
reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
} else {
final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
final int initialVelocity = (int)
(velocityTracker.getYVelocity(mActivePointerId) * mVelocityScale);
// Fling if we have enough velocity and we aren't at a boundary.
// Since we can potentially overfling more than we can overscroll, don't
// allow the weird behavior where you can scroll to a boundary then
// fling further.
boolean flingVelocity = Math.abs(initialVelocity) > mMinimumVelocity;
if (flingVelocity &&
!((mFirstPosition == 0 &&
firstChildTop == contentTop - mOverscrollDistance) ||
(mFirstPosition + childCount == mItemCount &&
lastChildBottom == contentBottom + mOverscrollDistance))) {
if (!dispatchNestedPreFling(0, -initialVelocity)) {
if (mFlingRunnable == null) {
mFlingRunnable = new FlingRunnable();
}
reportScrollStateChange(OnScrollListener.SCROLL_STATE_FLING);
//注释2处，可以fling
mFlingRunnable.start(-initialVelocity);
}
}
}
}
break;
//...
}
//...
}
注释1处，手指抬起的时候没有触发fling，就将滑动状态置为SCROLL_STATE_IDLE。
注释2处，手指抬起的时候可以fling，直接起飞。
FlingRunnable类实现了Runnable接口。
FlingRunnable的start方法。
void start(int initialVelocity) {
int initialY = initialVelocity < 0 ? Integer.MAX_VALUE : 0;
mLastFlingY = initialY;
mScroller.setInterpolator(null);
//注释1处，调用OverScroller的fling方法开始根据速度计算下一帧的坐标
mScroller.fling(0, initialY, 0, initialVelocity,
0, Integer.MAX_VALUE, 0, Integer.MAX_VALUE);
//注释2处，将mTouchMode置为TOUCH_MODE_FLING
mTouchMode = TOUCH_MODE_FLING;
mSuppressIdleStateChangeCall = false;
//注释3处，post一个runnable对象，下一帧到来的时候会调用run方法。
postOnAnimation(this);
if (PROFILE_FLINGING) {
if (!mFlingProfilingStarted) {
Debug.startMethodTracing("AbsListViewFling");
mFlingProfilingStarted = true;
}
}
}
注释1处，调用OverScroller的fling方法开始根据速度计算下一帧的坐标。
注释2处，将mTouchMode置为TOUCH_MODE_FLING。
注释3处，post一个runnable对象，下一帧到来的时候会调用FlingRunnable的run方法。
FlingRunnable的run方法。
@Override
public void run() {
switch (mTouchMode) {
default:
//停止fling
endFling();
return;
case TOUCH_MODE_SCROLL:
//fling已经结束了，直接return
if (mScroller.isFinished()) {
return;
}
// Fall through
case TOUCH_MODE_FLING: {
if (mDataChanged) {
layoutChildren();
}
if (mItemCount == 0 || getChildCount() == 0) {
endFling();
return;
}
final OverScroller scroller = mScroller;
//注释1处，more为true表示fling未结束
boolean more = scroller.computeScrollOffset();
//要到达的坐标
final int y = scroller.getCurrY();
// Flip sign to convert finger direction to list items direction
//注释2处，上一帧和本次的滚动距离差值
int delta = mLastFlingY - y;
// Pretend that each frame of a fling scroll is a touch scroll
if (delta > 0) {
//向上滚动。 List is moving towards the top. Use first view as mMotionPosition
mMotionPosition = mFirstPosition;
final View firstView = getChildAt(0);
mMotionViewOriginalTop = firstView.getTop();
// Don't fling more than 1 screen
delta = Math.min(getHeight() - mPaddingBottom - mPaddingTop - 1, delta);
} else {
//向下滚动。 List is moving towards the bottom. Use last view as mMotionPosition
int offsetToLast = getChildCount() - 1;
mMotionPosition = mFirstPosition + offsetToLast;
final View lastView = getChildAt(offsetToLast);
mMotionViewOriginalTop = lastView.getTop();
// Don't fling more than 1 screen
delta = Math.max(-(getHeight() - mPaddingBottom - mPaddingTop - 1), delta);
}
// Check to see if we have bumped into the scroll limit
View motionView = getChildAt(mMotionPosition - mFirstPosition);
int oldTop = 0;
if (motionView != null) {
oldTop = motionView.getTop();
}
// Don't stop just because delta is zero (it could have been rounded)
//注释3处，调用trackMotionScroll回收View，填充新的View。
final boolean atEdge = trackMotionScroll(delta, delta);
final boolean atEnd = atEdge && (delta != 0);
if (atEnd) {
//处理overScroll的情况，忽略。。。
}
if (more && !atEnd) {
if (atEdge) invalidate();
//注释4处
mLastFlingY = y;
postOnAnimation(this);
} else {
//注释5处
endFling();
}
break;
}
}
}
注释1处，more为true表示fling未结束。
注释2处，计算上一帧和本次的滚动距离差值 。delta > 0 表示向上滚动否则是向下滚动。
注释3处，调用trackMotionScroll缓存滑出屏幕View，填充新的View。
注释4处，fling还没有结束，重新为mLastFlingY赋值，并继续postFlingRunnable，再下一帧到来的时候继续处理。
注释5处，如果fling结束那就结束。
FlingRunnable的endFling方法。
void endFling() {
//将mTouchMode置为TOUCH_MODE_REST
mTouchMode = TOUCH_MODE_REST;
//移除掉FlingRunnable，不再响应下一帧
removeCallbacks(this);
removeCallbacks(mCheckFlywheel);
//...
}
总结：ListView的源码学习到此告一段落。源码阅读起来感觉不是太难，哈哈，主要是没有去抠每一个细节。接下来没有变化的话，要开始写RecyclerView相关的源码分析了。
参考链接：
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