Android 带你从源码的角度解析Scroller的滚动实现原理

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- <span class="annotation">@Override</span>

- <span class="keyword">protected</span> <span class="keyword">void</span> dispatchDraw(Canvas canvas) {

- <span class="keyword">final</span> <span class="keyword">int</span> count = mChildrenCount;

- <span class="keyword">final</span> View[] children = mChildren;

- <span class="keyword">int</span> flags = mGroupFlags;

- 
- <span class="keyword">if</span> ((flags & FLAG_RUN_ANIMATION) != <span class="number"></span> && canAnimate()) {

- <span class="keyword">final</span> <span class="keyword">boolean</span> cache = (mGroupFlags & FLAG_ANIMATION_CACHE) == FLAG_ANIMATION_CACHE;

- 
- <span class="keyword">final</span> <span class="keyword">boolean</span> buildCache = !isHardwareAccelerated();

- <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number"></span>; i < count; i++) {

- <span class="keyword">final</span> View child = children[i];

- <span class="keyword">if</span> ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {

- <span class="keyword">final</span> LayoutParams params = child.getLayoutParams();

- attachLayoutAnimationParameters(child, params, i, count);

- bindLayoutAnimation(child);

- <span class="keyword">if</span> (cache) {

- child.setDrawingCacheEnabled(<span class="keyword">true</span>);

- <span class="keyword">if</span> (buildCache) {

- child.buildDrawingCache(<span class="keyword">true</span>);

- }

- }

- }

- }

- 
- <span class="keyword">final</span> LayoutAnimationController controller = mLayoutAnimationController;

- <span class="keyword">if</span> (controller.willOverlap()) {

- mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;

- }

- 
- controller.start();

- 
- mGroupFlags &= ~FLAG_RUN_ANIMATION;

- mGroupFlags &= ~FLAG_ANIMATION_DONE;

- 
- <span class="keyword">if</span> (cache) {

- mGroupFlags |= FLAG_CHILDREN_DRAWN_WITH_CACHE;

- }

- 
- <span class="keyword">if</span> (mAnimationListener != <span class="keyword">null</span>) {

- mAnimationListener.onAnimationStart(controller.getAnimation());

- }

- }

- 
- <span class="keyword">int</span> saveCount = <span class="number"></span>;

- <span class="keyword">final</span> <span class="keyword">boolean</span> clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;

- <span class="keyword">if</span> (clipToPadding) {

- saveCount = canvas.save();

- canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,

- mScrollX + mRight &#8211; mLeft &#8211; mPaddingRight,

- mScrollY + mBottom &#8211; mTop &#8211; mPaddingBottom);

- 
- }

- 
- <span class="comment">// We will draw our child&#8217;s animation, let&#8217;s reset the flag</span>

- mPrivateFlags &= ~PFLAG_DRAW_ANIMATION;

- mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED;

- 
- <span class="keyword">boolean</span> more = <span class="keyword">false</span>;

- <span class="keyword">final</span> <span class="keyword">long</span> drawingTime = getDrawingTime();

- 
- <span class="keyword">if</span> ((flags & FLAG_USE_CHILD_DRAWING_ORDER) == <span class="number"></span>) {

- <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number"></span>; i < count; i++) {

- <span class="keyword">final</span> View child = children[i];

- <span class="keyword">if</span> ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != <span class="keyword">null</span>) {

- more |= drawChild(canvas, child, drawingTime);

- }

- }

- } <span class="keyword">else</span> {

- <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number"></span>; i < count; i++) {

- <span class="keyword">final</span> View child = children[getChildDrawingOrder(count, i)];

- <span class="keyword">if</span> ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != <span class="keyword">null</span>) {

- more |= drawChild(canvas, child, drawingTime);

- }

- }

- }

- 
- <span class="comment">// Draw any disappearing views that have animations</span>

- <span class="keyword">if</span> (mDisappearingChildren != <span class="keyword">null</span>) {

- <span class="keyword">final</span> ArrayList<View> disappearingChildren = mDisappearingChildren;

- <span class="keyword">final</span> <span class="keyword">int</span> disappearingCount = disappearingChildren.size() &#8211; <span class="number">1</span>;

- <span class="comment">// Go backwards &#8212; we may delete as animations finish</span>

- <span class="keyword">for</span> (<span class="keyword">int</span> i = disappearingCount; i >= <span class="number"></span>; i&#8211;) {

- <span class="keyword">final</span> View child = disappearingChildren.get(i);

- more |= drawChild(canvas, child, drawingTime);

- }

- }

- 
- <span class="keyword">if</span> (debugDraw()) {

- onDebugDraw(canvas);

- }

- 
- <span class="keyword">if</span> (clipToPadding) {

- canvas.restoreToCount(saveCount);

- }

- 
- <span class="comment">// mGroupFlags might have been updated by drawChild()</span>

- flags = mGroupFlags;

- 
- <span class="keyword">if</span> ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) {

- invalidate(<span class="keyword">true</span>);

- }

- 
- <span class="keyword">if</span> ((flags & FLAG_ANIMATION_DONE) == <span class="number"></span> && (flags & FLAG_NOTIFY_ANIMATION_LISTENER) == <span class="number"></span> &&

- mLayoutAnimationController.isDone() && !more) {

- <span class="comment">// We want to erase the drawing cache and notify the listener after the</span>

- <span class="comment">// next frame is drawn because one extra invalidate() is caused by</span>

- <span class="comment">// drawChild() after the animation is over</span>

- mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER;

- <span class="keyword">final</span> Runnable end = <span class="keyword">new</span> Runnable() {

- <span class="keyword">public</span> <span class="keyword">void</span> run() {

- notifyAnimationListener();

- }

- };

- post(end);

- }

- }