Scroller、ScrollTo、ScrollBy源码解析

序言

  本着写了忘,忘了写。还是写篇博文分析了工作流程加深记忆,滑动方向每次都出错。

ScrollTo与ScrollBy两兄弟

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
/*Set scrollX,scrollY.inValidate 
mSrollX:The offset, in pixels, by which the content of this view is scrolled horizontally.
mSrollY:The offset, in pixels, by which the content of this view is scrolled
vertically.
*/
public void scrollTo(int x, int y) {
if (mScrollX != x || mScrollY != y) {
int oldX = mScrollX;
int oldY = mScrollY;
mScrollX = x;
mScrollY = y;
invalidateParentCaches();
onScrollChanged(mScrollX, mScrollY, oldX, oldY);
if (!awakenScrollBars()) {
postInvalidateOnAnimation();
}
}
}
public void scrollBy(int x, int y) {
scrollTo(mScrollX + x, mScrollY + y);
}

可以看出来ScrollBy是调用scrollTo.这样只需要分析scrollTo源码即可。先设置mScrollX,mScrollY.而ScrollX,Y是View内容的滑动距离。所以这就解释为什么只能ViewGroup调用该方法有效。Scroll传入偏移距离加上起始滑动距离 = 绝对距离。进入onScrollChanged方法

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
protected void onScrollChanged(int l, int t, int oldl, int oldt) {
notifySubtreeAccessibilityStateChangedIfNeeded();

if (AccessibilityManager.getInstance(mContext).isEnabled()) {
postSendViewScrolledAccessibilityEventCallback();
}

mBackgroundSizeChanged = true;
mDefaultFocusHighlightSizeChanged = true;
if (mForegroundInfo != null) {
mForegroundInfo.mBoundsChanged = true;
}

final AttachInfo ai = mAttachInfo;
if (ai != null) {
ai.mViewScrollChanged = true;
}

if (mListenerInfo != null && mListenerInfo.mOnScrollChangeListener != null) {
mListenerInfo.mOnScrollChangeListener.onScrollChange(this, l, t, oldl, oldt);
}
}

当View注册了OnScrollChangeListener,onScrollChange才会被调用。在onScrollChanged()后调用了postInvalidateOnAnimation()。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
  public void postInvalidateOnAnimation() {
// We try only with the AttachInfo because there's no point in invalidating
// if we are not attached to our window
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
attachInfo.mViewRootImpl.dispatchInvalidateOnAnimation(this);
}
}
public void dispatchInvalidateOnAnimation(View view) {
mInvalidateOnAnimationRunnable.addView(view);
}
final class InvalidateOnAnimationRunnable implements Runnable {
private boolean mPosted;
private final ArrayList<View> mViews = new ArrayList<View>();
private final ArrayList<AttachInfo.InvalidateInfo> mViewRects =
new ArrayList<AttachInfo.InvalidateInfo>();
private View[] mTempViews;
private AttachInfo.InvalidateInfo[] mTempViewRects;

public void addView(View view) {
synchronized (this) {
mViews.add(view);
postIfNeededLocked();
}
}

public void addViewRect(AttachInfo.InvalidateInfo info) {
synchronized (this) {
mViewRects.add(info);
postIfNeededLocked();
}
}

public void removeView(View view) {
synchronized (this) {
mViews.remove(view);

for (int i = mViewRects.size(); i-- > 0; ) {
AttachInfo.InvalidateInfo info = mViewRects.get(i);
if (info.target == view) {
mViewRects.remove(i);
info.recycle();
}
}

if (mPosted && mViews.isEmpty() && mViewRects.isEmpty()) {
mChoreographer.removeCallbacks(Choreographer.CALLBACK_ANIMATION, this, null);
mPosted = false;
}
}
}

@Override
public void run() {
final int viewCount;
final int viewRectCount;
synchronized (this) {
mPosted = false;

viewCount = mViews.size();
if (viewCount != 0) {
mTempViews = mViews.toArray(mTempViews != null
? mTempViews : new View[viewCount]);
mViews.clear();
}

viewRectCount = mViewRects.size();
if (viewRectCount != 0) {
mTempViewRects = mViewRects.toArray(mTempViewRects != null
? mTempViewRects : new AttachInfo.InvalidateInfo[viewRectCount]);
mViewRects.clear();
}
}

for (int i = 0; i < viewCount; i++) {
mTempViews[i].invalidate();
mTempViews[i] = null;
}

for (int i = 0; i < viewRectCount; i++) {
final View.AttachInfo.InvalidateInfo info = mTempViewRects[i];
info.target.invalidate(info.left, info.top, info.right, info.bottom);
info.recycle();
}
}

private void postIfNeededLocked() {
if (!mPosted) {
mChoreographer.postCallback(Choreographer.CALLBACK_ANIMATION, this, null);
mPosted = true;
}
}
}

在addView中postIfNeededLocked()会调用到run(),在run()里遍历所有添加进去的View,然后对每个View调用invalidate(),

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
   public void invalidate() {
invalidate(true);
}
public void invalidate(boolean invalidateCache) {
invalidateInternal(0, 0, mRight - mLeft, mBottom - mTop, invalidateCache, true);
}
void invalidateInternal(int l, int t, int r, int b, boolean invalidateCache,
boolean fullInvalidate) {
...
// Propagate the damage rectangle to the parent view.
final AttachInfo ai = mAttachInfo;
final ViewParent p = mParent;
if (p != null && ai != null && l < r && t < b) {
final Rect damage = ai.mTmpInvalRect;
damage.set(l, t, r, b);
p.invalidateChild(this, damage);// Invalidate child
}

// Damage the entire projection receiver, if necessary.
if (mBackground != null && mBackground.isProjected()) {
final View receiver = getProjectionReceiver();
if (receiver != null) {
receiver.damageInParent();
}
}
}
}

p.invalidateChild()这就是scrollTo移动的是View的内容的真相。再来揭示为何方向相反呢?进入ViewGroup的invalidateChild().

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
@Deprecated
@Override
public final void invalidateChild(View child, final Rect dirty) {
···
parent = parent.invalidateChildInParent(location, dirty);
if (view != null) {
// Account for transform on current parent
Matrix m = view.getMatrix();
if (!m.isIdentity()) {
RectF boundingRect = attachInfo.mTmpTransformRect;
boundingRect.set(dirty);
m.mapRect(boundingRect);
dirty.set((int) Math.floor(boundingRect.left),
(int) Math.floor(boundingRect.top),
(int) Math.ceil(boundingRect.right),
(int) Math.ceil(boundingRect.bottom));
}
}
} while (parent != null);
}
}

里面有一行代码parent = parent.invalidateChildInParent(location, dirty);进去invalidateChildInParent()

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
@Override
public ViewParent invalidateChildInParent(int[] location, Rect dirty) {
if (mHostView != null) {
dirty.offset(location[0], location[1]);
if (mHostView instanceof ViewGroup) {
location[0] = 0;
location[1] = 0;
super.invalidateChildInParent(location, dirty);
return ((ViewGroup) mHostView).invalidateChildInParent(location, dirty);
} else {
invalidate(dirty);
}
}
return null;
}
}

进入 invalidate(dirty);

1
2
3
4
5
6
public void invalidate(Rect dirty) {
final int scrollX = mScrollX;
final int scrollY = mScrollY;
invalidateInternal(dirty.left - scrollX, dirty.top - scrollY,
dirty.right - scrollX, dirty.bottom - scrollY, true, false);
}

这就是为什么方向会相反的原因啦。整个scrollTo透露着一股观察者模式的气味,当调用scrollTo()时,InvalidateOnAnimationRunnable中的run()会通知所有子View。让其重绘。

Scroller源码

Scroller典型用法:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
@Override
public void computeScroll() {
super.computeScroll();
// 判断Scroller是否执行完毕
if (mScroller.computeScrollOffset()) {
((View) getParent()).scrollTo(
mScroller.getCurrX(),//获取当前的滑动坐标
mScroller.getCurrY());
// 通过重绘来不断调用computeScroll
postInvalidate();
}
}

@Override
public boolean onTouchEvent(MotionEvent event) {
int x = (int) event.getX();
int y = (int) event.getY();
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
lastX = (int) event.getX();
lastY = (int) event.getY();
break;
case MotionEvent.ACTION_MOVE:
int offsetX = x - lastX;
int offsetY = y - lastY;
((View) getParent()).scrollBy(-offsetX, -offsetY);
break;
case MotionEvent.ACTION_UP:
// 手指离开时,执行滑动过程
View viewGroup = ((View) getParent());
mScroller.startScroll(
viewGroup.getScrollX(),
viewGroup.getScrollY(),
-viewGroup.getScrollX(),
-viewGroup.getScrollY());
invalidate();
break;
}
return true;
}

与scrollTo()、scrollBy()同样,滑动的是View的内容。首先进入startScroll()

1
2
3
4
5
6
7
8
9
10
11
12
13
public void startScroll(int startX, int startY, int dx, int dy, int duration) {
mMode = SCROLL_MODE;
mFinished = false;
mDuration = duration;
mStartTime = AnimationUtils.currentAnimationTimeMillis();
mStartX = startX;
mStartY = startY;
mFinalX = startX + dx;
mFinalY = startY + dy;
mDeltaX = dx;
mDeltaY = dy;
mDurationReciprocal = 1.0f / (float) mDuration;
}

里面除了设值什么也没做,那Scroller是怎么让View滑起来的呢?秘密就在startScroll()的下一行invalidate()重绘中调用View中的draw()

1
2
3
4
5
6
7
8
  boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
...
if (!drawingWithRenderNode) {
computeScroll();
sx = mScrollX;
sy = mScrollY;
}

里面会调用我们重写的computeScroll(),而在我们重写的computeScroll()里我们利用scrollTo完成了滑动操作。并同时启动了postInvalidate();开启了第二次重绘。反复直至滑动结束。最后再看看computeScrollOffset()

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
   public boolean computeScrollOffset() {
if (mFinished) {
return false;
}

int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);

if (timePassed < mDuration) {
switch (mMode) {
case SCROLL_MODE:
final float x = mInterpolator.getInterpolation(timePassed * mDurationReciprocal);
mCurrX = mStartX + Math.round(x * mDeltaX);
mCurrY = mStartY + Math.round(x * mDeltaY);
break;
...
}
return true;


根据时间流逝的百分比算出scrollX和scrollY。返回为true意味着滑动还未结束,false:滑动已结束

文章目录
  1. 1. 序言
  2. 2. ScrollTo与ScrollBy两兄弟
  3. 3. Scroller源码
|