Android开发中View的滑动处理源码，该怎么看？
Android 带你从源码的角度解析Scroller的滚动实现原理，
今天给大家讲解的是Scroller类的滚动实现原理，可能很多朋友不太了解该类是用来干嘛的，但是研究Launcher的朋友应该对他很熟悉，Scroller类是滚动的一个封装类，可以实现View的平滑滚动效果，什么是实现View的平滑滚动效果呢，举个简单的例子，一个View从在我们指定的时间内从一个位置滚动到另外一个位置，我们利用Scroller类可以实现匀速滚动，可以先加速后减速，可以先减速后加速等等效果，而不是瞬间的移动的效果，所以Scroller可以帮我们实现很多滑动的效果。
在介绍Scroller类之前，我们先去了解View的scrollBy() 和scrollTo()方法的区别，在区分这两个方法的之前，我们要先理解View 里面的两个成员变量mScrollX， mScrollY，X轴方向的偏移量和Y轴方向的偏移量，这个是一个相对距离，相对的不是屏幕的原点，而是View的左边缘，举个通俗易懂的例子，一列火车从吉安到深圳，途中经过赣州，那么原点就是赣州，偏移量就是 负的吉安到赣州的距离，大家从getScrollX()方法中的注释中就能看出答案来
/**
* Return the scrolled left position of this view. This is the left edge of
* the displayed part of your view. You do not need to draw any pixels
* farther left, since those are outside of the frame of your view on
* screen.
*
* @return The left edge of the displayed part of your view, in pixels.
*/
public final int getScrollX() {
return mScrollX;
}
现在我们知道了向右滑动 mScrollX就为负数，向左滑动mScrollX为正数，接下来我们先来看看scrollTo()方法的源码
注 : 此处应该参考: 艺术探索 里所说,移动的是 盖板
/**
* Set the scrolled position of your view. This will cause a call to
* {@link #onScrollChanged(int, int, int, int)} and the view will be
* invalidated.
* @param x the x position to scroll to
* @param y the y position to scroll to
*/
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();
}
}
}
从该方法中我们可以看出，先判断传进来的(x, y)值是否和View的X, Y偏移量相等，如果不相等，就调用onScrollChanged()方法来通知界面发生改变，然后重绘界面，所以这样子就实现了移动效果啦， 现在我们知道了scrollTo()方法是滚动到(x, y)这个偏移量的点，他是相对于View的开始位置来滚动的。在看看scrollBy()这个方法的代码
/**
* Move the scrolled position of your view. This will cause a call to
* {@link #onScrollChanged(int, int, int, int)} and the view will be
* invalidated.
* @param x the amount of pixels to scroll by horizontally
* @param y the amount of pixels to scroll by vertically
*/
public void scrollBy(int x, int y) {
scrollTo(mScrollX + x, mScrollY + y);
}
原来他里面调用了scrollTo()方法，那就好办了，他就是相对于View上一个位置根据(x, y)来进行滚动，可能大家脑海中对这两个方法还有点模糊，没关系，还是举个通俗的例子帮大家理解下，假如一个View,调用两次scrollTo(-10, 0)，第一次向右滚动10，第二次就不滚动了，因为mScrollX和x相等了，当我们调用两次scrollBy(-10, 0),第一次向右滚动10，第二次再向右滚动10，他是相对View的上一个位置来滚动的。
对于scrollTo()和scrollBy()方法还有一点需要注意，这点也很重要，假如你给一个LinearLayout调用scrollTo()方法，并不是LinearLayout滚动，而是LinearLayout里面的内容进行滚动，比如你想对一个按钮进行滚动，直接用Button调用scrollTo()一定达不到你的需求，大家可以试一试，如果真要对某个按钮进行scrollTo()滚动的话，我们可以在Button外面包裹一层Layout，然后对Layout调用scrollTo()方法。
了解了scrollTo()和scrollBy()方法之后我们就了解下Scroller类了，先看其构造方法
/**
* Create a Scroller with the default duration and interpolator.
*/
public Scroller(Context context) {
this(context, null);
}
/**
* Create a Scroller with the specified interpolator. If the interpolator is
* null, the default (viscous) interpolator will be used. "Flywheel" behavior will
* be in effect for apps targeting Honeycomb or newer.
*/
public Scroller(Context context, Interpolator interpolator) {
this(context, interpolator,
context.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.HONEYCOMB);
}
/**
* Create a Scroller with the specified interpolator. If the interpolator is
* null, the default (viscous) interpolator will be used. Specify whether or
* not to support progressive "flywheel" behavior in flinging.
*/
public Scroller(Context context, Interpolator interpolator, boolean flywheel) {
mFinished = true;
if (interpolator == null) {
mInterpolator = new ViscousFluidInterpolator();
} else {
mInterpolator = interpolator;
}
mPpi = context.getResources().getDisplayMetrics().density * 160.0f;
mDeceleration = computeDeceleration(ViewConfiguration.getScrollFriction());
mFlywheel = flywheel;
mPhysicalCoeff = computeDeceleration(0.84f); // look and feel tuning
}
共计三个构造方法，第一个只有一个Context参数，第二个构造方法中指定了Interpolator，什么Interpolator呢?中文意思插补器，了解Android动画的朋友都应该熟悉
Interpolator，他指定了动画的变化率，比如说匀速变化，先加速后减速，正弦变化等等，不同的Interpolator可以做出不同的效果出来，第一个使用默认的Interpolator(viscous)
第三个构造方法,是对滑动的 "惯性轮"特性的支持.
接下来我们就要在Scroller类里面找滚动的方法，我们从名字上面可以看出startScroll()应该是个滚动的方法，我们来看看其源码吧
/**
* Start scrolling by providing a starting point, the distance to travel,
* and the duration of the scroll.
*
* @param startX Starting horizontal scroll offset in pixels. Positive
* numbers will scroll the content to the left.
* @param startY Starting vertical scroll offset in pixels. Positive numbers
* will scroll the content up.
* @param dx Horizontal distance to travel. Positive numbers will scroll the
* content to the left.
* @param dy Vertical distance to travel. Positive numbers will scroll the
* content up.
* @param duration Duration of the scroll in milliseconds.
*/
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;
}
在这个方法中我们只看到了对一些滚动的基本设置动作，比如设置滚动模式，开始时间，持续时间等等，并没有任何对View的滚动操作，也许你正纳闷，不是滚动的方法干嘛还叫做startScroll(),稍安勿躁，既然叫开始滚动，那就是对滚动的滚动之前的基本设置咯。
接下来是computeScrollOffset()方法:
/**
* Call this when you want to know the new location. If it returns true,
* the animation is not yet finished.
*/
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;
case FLING_MODE:
final float t = (float) timePassed / mDuration;
final int index = (int) (NB_SAMPLES * t);
float distanceCoef = 1.f;
float velocityCoef = 0.f;
if (index < NB_SAMPLES) {
final float t_inf = (float) index / NB_SAMPLES;
final float t_sup = (float) (index + 1) / NB_SAMPLES;
final float d_inf = SPLINE_POSITION[index];
final float d_sup = SPLINE_POSITION[index + 1];
velocityCoef = (d_sup - d_inf) / (t_sup - t_inf);
distanceCoef = d_inf + (t - t_inf) * velocityCoef;
}
mCurrVelocity = velocityCoef * mDistance / mDuration * 1000.0f;
mCurrX = mStartX + Math.round(distanceCoef * (mFinalX - mStartX));
// Pin to mMinX <= mCurrX <= mMaxX
mCurrX = Math.min(mCurrX, mMaxX);
mCurrX = Math.max(mCurrX, mMinX);
mCurrY = mStartY + Math.round(distanceCoef * (mFinalY - mStartY));
// Pin to mMinY <= mCurrY <= mMaxY
mCurrY = Math.min(mCurrY, mMaxY);
mCurrY = Math.max(mCurrY, mMinY);
if (mCurrX == mFinalX && mCurrY == mFinalY) {
mFinished = true;
}
break;
}
}
else {
mCurrX = mFinalX;
mCurrY = mFinalY;
mFinished = true;
}
return true;
}
《注：》这里想补充一句个人理解：isFinish()方法返回的就是该方法里面的mFinished。当时间截止或者路程走完都会使mFinished =true，但是computeScrollOffset()的返回值是依赖于mFinished的
我们在startScroll()方法的时候获取了当前的动画毫秒赋值给了mStartTime，在computeScrollOffset()中再一次调用AnimationUtils.currentAnimationTimeMillis()来获取动画毫秒减去mStartTime就是动画已过时间了，然后进去if判断，如果动画已过时间时间小于我们设置的滚动持续时间mDuration，进去switch的SCROLL_MODE，然后根据Interpolator来计算出在该时间段里面移动的距离，赋值给mCurrX， mCurrY， 所以该方法的作用是，计算在0到mDuration时间段内滚动的偏移量，并且判断滚动是否结束，true代表还没结束，false则表示滚动介绍了，Scroller类的其他的方法我就不提了，大都是一些get(), set()方法。
看了这么多，到底要怎么才能触发滚动，你心里肯定有很多疑惑，在说滚动之前我要先提另外一个方法View 的 computeScroll()，该方法是滑动的控制方法，在绘制View时，会在draw()过程调用该方法。我们先看看computeScroll()的源码:
/**
* Called by a parent to request that a child update its values for mScrollX
* and mScrollY if necessary. This will typically be done if the child is
* animating a scroll using a {@link android.widget.Scroller Scroller}
* object.
*/
public void computeScroll() {
}
没错，他是一个空的方法，需要子类去重写该方法来实现逻辑，到底该方法在哪里被触发呢。我们继续看看View的绘制方法draw()
/**
* This method is called by ViewGroup.drawChild() to have each child view draw itself.
*
* This is where the View specializes rendering behavior based on layer type,
* and hardware acceleration.
*/
boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
final boolean hardwareAcceleratedCanvas = canvas.isHardwareAccelerated();
/* If an attached view draws to a HW canvas, it may use its RenderNode + DisplayList.
*
* If a view is dettached, its DisplayList shouldn't exist. If the canvas isn't
* HW accelerated, it can't handle drawing RenderNodes.
*/
boolean drawingWithRenderNode = mAttachInfo != null
&& mAttachInfo.mHardwareAccelerated
&& hardwareAcceleratedCanvas;
boolean more = false;
final boolean childHasIdentityMatrix = hasIdentityMatrix();
final int parentFlags = parent.mGroupFlags;
if ((parentFlags & ViewGroup.FLAG_CLEAR_TRANSFORMATION) != 0) {
parent.getChildTransformation().clear();
parent.mGroupFlags &= ~ViewGroup.FLAG_CLEAR_TRANSFORMATION;
}
Transformation transformToApply = null;
boolean concatMatrix = false;
final boolean scalingRequired = mAttachInfo != null && mAttachInfo.mScalingRequired;
final Animation a = getAnimation();
if (a != null) {
more = applyLegacyAnimation(parent, drawingTime, a, scalingRequired);
concatMatrix = a.willChangeTransformationMatrix();
if (concatMatrix) {
mPrivateFlags3 |= PFLAG3_VIEW_IS_ANIMATING_TRANSFORM;
}
transformToApply = parent.getChildTransformation();
} else {
if ((mPrivateFlags3 & PFLAG3_VIEW_IS_ANIMATING_TRANSFORM) != 0) {
// No longer animating: clear out old animation matrix
mRenderNode.setAnimationMatrix(null);
mPrivateFlags3 &= ~PFLAG3_VIEW_IS_ANIMATING_TRANSFORM;
}
if (!drawingWithRenderNode
&& (parentFlags & ViewGroup.FLAG_SUPPORT_STATIC_TRANSFORMATIONS) != 0) {
final Transformation t = parent.getChildTransformation();
final boolean hasTransform = parent.getChildStaticTransformation(this, t);
if (hasTransform) {
final int transformType = t.getTransformationType();
transformToApply = transformType != Transformation.TYPE_IDENTITY ? t : null;
concatMatrix = (transformType & Transformation.TYPE_MATRIX) != 0;
}
}
}
concatMatrix |= !childHasIdentityMatrix;
// Sets the flag as early as possible to allow draw() implementations
// to call invalidate() successfully when doing animations
mPrivateFlags |= PFLAG_DRAWN;
if (!concatMatrix &&
(parentFlags & (ViewGroup.FLAG_SUPPORT_STATIC_TRANSFORMATIONS |
ViewGroup.FLAG_CLIP_CHILDREN)) == ViewGroup.FLAG_CLIP_CHILDREN &&
canvas.quickReject(mLeft, mTop, mRight, mBottom, Canvas.EdgeType.BW) &&
(mPrivateFlags & PFLAG_DRAW_ANIMATION) == 0) {
mPrivateFlags2 |= PFLAG2_VIEW_QUICK_REJECTED;
return more;
}
mPrivateFlags2 &= ~PFLAG2_VIEW_QUICK_REJECTED;
if (hardwareAcceleratedCanvas) {
// Clear INVALIDATED flag to allow invalidation to occur during rendering, but
// retain the flag's value temporarily in the mRecreateDisplayList flag
mRecreateDisplayList = (mPrivateFlags & PFLAG_INVALIDATED) != 0;
mPrivateFlags &= ~PFLAG_INVALIDATED;
}
RenderNode renderNode = null;
Bitmap cache = null;
int layerType = getLayerType(); // TODO: signify cache state with just 'cache' local
if (layerType == LAYER_TYPE_SOFTWARE || !drawingWithRenderNode) {
if (layerType != LAYER_TYPE_NONE) {
// If not drawing with RenderNode, treat HW layers as SW
layerType = LAYER_TYPE_SOFTWARE;
buildDrawingCache(true);
}
cache = getDrawingCache(true);
}
if (drawingWithRenderNode) {
// Delay getting the display list until animation-driven alpha values are
// set up and possibly passed on to the view
renderNode = updateDisplayListIfDirty();
if (!renderNode.isValid()) {
// Uncommon, but possible. If a view is removed from the hierarchy during the call
// to getDisplayList(), the display list will be marked invalid and we should not
// try to use it again.
renderNode = null;
drawingWithRenderNode = false;
}
}
int sx = 0;
int sy = 0;
if (!drawingWithRenderNode) {
//******************************************
computeScroll();
//******************************************
sx = mScrollX;
sy = mScrollY;
}
final boolean drawingWithDrawingCache = cache != null && !drawingWithRenderNode;
final boolean offsetForScroll = cache == null && !drawingWithRenderNode;
int restoreTo = -1;
if (!drawingWithRenderNode || transformToApply != null) {
restoreTo = canvas.save();
}
if (offsetForScroll) {
canvas.translate(mLeft - sx, mTop - sy);
} else {
if (!drawingWithRenderNode) {
canvas.translate(mLeft, mTop);
}
if (scalingRequired) {
if (drawingWithRenderNode) {
// TODO: Might not need this if we put everything inside the DL
restoreTo = canvas.save();
}
// mAttachInfo cannot be null, otherwise scalingRequired == false
final float scale = 1.0f / mAttachInfo.mApplicationScale;
canvas.scale(scale, scale);
}
}
float alpha = drawingWithRenderNode ? 1 : (getAlpha() * getTransitionAlpha());
if (transformToApply != null
|| alpha < 1
|| !hasIdentityMatrix()
|| (mPrivateFlags3 & PFLAG3_VIEW_IS_ANIMATING_ALPHA) != 0) {
if (transformToApply != null || !childHasIdentityMatrix) {
int transX = 0;
int transY = 0;
if (offsetForScroll) {
transX = -sx;
transY = -sy;
}
if (transformToApply != null) {
if (concatMatrix) {
if (drawingWithRenderNode) {
renderNode.setAnimationMatrix(transformToApply.getMatrix());
} else {
// Undo the scroll translation, apply the transformation matrix,
// then redo the scroll translate to get the correct result.
canvas.translate(-transX, -transY);
canvas.concat(transformToApply.getMatrix());
canvas.translate(transX, transY);
}
parent.mGroupFlags |= ViewGroup.FLAG_CLEAR_TRANSFORMATION;
}
float transformAlpha = transformToApply.getAlpha();
if (transformAlpha < 1) {
alpha *= transformAlpha;
parent.mGroupFlags |= ViewGroup.FLAG_CLEAR_TRANSFORMATION;
}
}
if (!childHasIdentityMatrix && !drawingWithRenderNode) {
canvas.translate(-transX, -transY);
canvas.concat(getMatrix());
canvas.translate(transX, transY);
}
}
// Deal with alpha if it is or used to be <1
if (alpha < 1 || (mPrivateFlags3 & PFLAG3_VIEW_IS_ANIMATING_ALPHA) != 0) {
if (alpha < 1) {
mPrivateFlags3 |= PFLAG3_VIEW_IS_ANIMATING_ALPHA;
} else {
mPrivateFlags3 &= ~PFLAG3_VIEW_IS_ANIMATING_ALPHA;
}
parent.mGroupFlags |= ViewGroup.FLAG_CLEAR_TRANSFORMATION;
if (!drawingWithDrawingCache) {
final int multipliedAlpha = (int) (255 * alpha);
if (!onSetAlpha(multipliedAlpha)) {
if (drawingWithRenderNode) {
renderNode.setAlpha(alpha * getAlpha() * getTransitionAlpha());
} else if (layerType == LAYER_TYPE_NONE) {
canvas.saveLayerAlpha(sx, sy, sx + getWidth(), sy + getHeight(),
multipliedAlpha);
}
} else {
// Alpha is handled by the child directly, clobber the layer's alpha
mPrivateFlags |= PFLAG_ALPHA_SET;
}
}
}
} else if ((mPrivateFlags & PFLAG_ALPHA_SET) == PFLAG_ALPHA_SET) {
onSetAlpha(255);
mPrivateFlags &= ~PFLAG_ALPHA_SET;
}
if (!drawingWithRenderNode) {
// apply clips directly, since RenderNode won't do it for this draw
if ((parentFlags & ViewGroup.FLAG_CLIP_CHILDREN) != 0 && cache == null) {
if (offsetForScroll) {
canvas.clipRect(sx, sy, sx + getWidth(), sy + getHeight());
} else {
if (!scalingRequired || cache == null) {
canvas.clipRect(0, 0, getWidth(), getHeight());
} else {
canvas.clipRect(0, 0, cache.getWidth(), cache.getHeight());
}
}
}
if (mClipBounds != null) {
// clip bounds ignore scroll
canvas.clipRect(mClipBounds);
}
}
if (!drawingWithDrawingCache) {
if (drawingWithRenderNode) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
((DisplayListCanvas) canvas).drawRenderNode(renderNode);
} else {
// Fast path for layouts with no backgrounds
if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
//******************************************
dispatchDraw(canvas);
//******************************************
} else {
draw(canvas);
}
}
} else if (cache != null) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
if (layerType == LAYER_TYPE_NONE || mLayerPaint == null) {
// no layer paint, use temporary paint to draw bitmap
Paint cachePaint = parent.mCachePaint;
if (cachePaint == null) {
cachePaint = new Paint();
cachePaint.setDither(false);
parent.mCachePaint = cachePaint;
}
cachePaint.setAlpha((int) (alpha * 255));
canvas.drawBitmap(cache, 0.0f, 0.0f, cachePaint);
} else {
// use layer paint to draw the bitmap, merging the two alphas, but also restore
int layerPaintAlpha = mLayerPaint.getAlpha();
if (alpha < 1) {
mLayerPaint.setAlpha((int) (alpha * layerPaintAlpha));
}
canvas.drawBitmap(cache, 0.0f, 0.0f, mLayerPaint);
if (alpha < 1) {
mLayerPaint.setAlpha(layerPaintAlpha);
}
}
}
if (restoreTo >= 0) {
canvas.restoreToCount(restoreTo);
}
if (a != null && !more) {
if (!hardwareAcceleratedCanvas && !a.getFillAfter()) {
onSetAlpha(255);
}
parent.finishAnimatingView(this, a);
}
if (more && hardwareAcceleratedCanvas) {
if (a.hasAlpha() && (mPrivateFlags & PFLAG_ALPHA_SET) == PFLAG_ALPHA_SET) {
// alpha animations should cause the child to recreate its display list
invalidate(true);
}
}
mRecreateDisplayList = false;
return more;
}
这上面星号标注的部分,写出了绘制一个View的几个步骤，我们看看第四步绘制孩子的时候会触发dispatchDraw()这个方法，来看看源码是什么内容
@Override
protected void dispatchDraw(Canvas canvas) {
boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
final int childrenCount = mChildrenCount;
final View[] children = mChildren;
int flags = mGroupFlags;
if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {
final boolean buildCache = !isHardwareAccelerated();
for (int i = 0; i < childrenCount; i++) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {
final LayoutParams params = child.getLayoutParams();
attachLayoutAnimationParameters(child, params, i, childrenCount);
bindLayoutAnimation(child);
}
}
final LayoutAnimationController controller = mLayoutAnimationController;
if (controller.willOverlap()) {
mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;
}
controller.start();
mGroupFlags &= ~FLAG_RUN_ANIMATION;
mGroupFlags &= ~FLAG_ANIMATION_DONE;
if (mAnimationListener != null) {
mAnimationListener.onAnimationStart(controller.getAnimation());
}
}
int clipSaveCount = 0;
final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
if (clipToPadding) {
clipSaveCount = canvas.save(Canvas.CLIP_SAVE_FLAG);
canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,
mScrollX + mRight - mLeft - mPaddingRight,
mScrollY + mBottom - mTop - mPaddingBottom);
}
// We will draw our child's animation, let's reset the flag
mPrivateFlags &= ~PFLAG_DRAW_ANIMATION;
mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED;
boolean more = false;
final long drawingTime = getDrawingTime();
if (usingRenderNodeProperties) canvas.insertReorderBarrier();
final int transientCount = mTransientIndices == null ? 0 : mTransientIndices.size();
int transientIndex = transientCount != 0 ? 0 : -1;
// Only use the preordered list if not HW accelerated, since the HW pipeline will do the
// draw reordering internally
final ArrayList preorderedList = usingRenderNodeProperties
? null : buildOrderedChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
for (int i = 0; i < childrenCount; i++) {
while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
final View transientChild = mTransientViews.get(transientIndex);
if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
transientChild.getAnimation() != null) {
more |= drawChild(canvas, transientChild, drawingTime);
}
transientIndex++;
if (transientIndex >= transientCount) {
transientIndex = -1;
}
}
final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
while (transientIndex >= 0) {
// there may be additional transient views after the normal views
final View transientChild = mTransientViews.get(transientIndex);
if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
transientChild.getAnimation() != null) {
more |= drawChild(canvas, transientChild, drawingTime);
}
transientIndex++;
if (transientIndex >= transientCount) {
break;
}
}
if (preorderedList != null) preorderedList.clear();
// Draw any disappearing views that have animations
if (mDisappearingChildren != null) {
final ArrayList disappearingChildren = mDisappearingChildren;
final int disappearingCount = disappearingChildren.size() - 1;
// Go backwards -- we may delete as animations finish
for (int i = disappearingCount; i >= 0; i--) {
final View child = disappearingChildren.get(i);
//*************************
more |= drawChild(canvas, child, drawingTime);
//*************************
}
}
if (usingRenderNodeProperties) canvas.insertInorderBarrier();
if (debugDraw()) {
onDebugDraw(canvas);
}
if (clipToPadding) {
canvas.restoreToCount(clipSaveCount);
}
// mGroupFlags might have been updated by drawChild()
flags = mGroupFlags;
if ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) {
invalidate(true);
}
if ((flags & FLAG_ANIMATION_DONE) == 0 && (flags & FLAG_NOTIFY_ANIMATION_LISTENER) == 0 &&
mLayoutAnimationController.isDone() && !more) {
// We want to erase the drawing cache and notify the listener after the
// next frame is drawn because one extra invalidate() is caused by
// drawChild() after the animation is over
mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER;
final Runnable end = new Runnable() {
@Override
public void run() {
notifyAnimationListener();
}
};
post(end);
}
}
这个方法代码有点多，但是我们还是挑重点看吧，从星号标注的行可以看出 在dispatchDraw()里面会对ViewGroup里面的子View调用drawChild()来进行绘制，接下来我们来看看drawChild()方法的代码
/**
* Draw one child of this View Group. This method is responsible for getting
* the canvas in the right state. This includes clipping, translating so
* that the child's scrolled origin is at 0, 0, and applying any animation
* transformations.
*
* @param canvas The canvas on which to draw the child
* @param child Who to draw
* @param drawingTime The time at which draw is occurring
* @return True if an invalidate() was issued
*/
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
}
最终又回到了View的draw() 方法,我们可以看到如下代码:
int sx = 0;
int sy = 0;
if (!drawingWithRenderNode) {
//******************************************
computeScroll();
//******************************************
sx = mScrollX;
sy = mScrollY;
}
看到computeScroll()你大概明白什么了吧，转了老半天终于找到了computeScroll()方法被触发，就是ViewGroup在分发绘制自己的孩子的时候，会对其子View调用computeScroll()方法
因此我们在使用Scroller可以这么写:
Scroller scroller = new Scroller(getContext());
//缓慢滑动到指定位置
private void smoothScrollXTo(int targetX) {
int scrollX = getScrollX();
int deltaX
= targetX - scrollX;
//1000ms内 缓慢 滑动到targetX处
scroller.startScroll(scrollX, 0, deltaX, 0, 1000);
invalidate();
}
@Override
public void computeScroll() {
//动画未完成
if (scroller.computeScrollOffset()) {
scrollTo(scroller.getCurrX(), scroller.getCurrY());
//触发draw()
postInvalidate();
}
}
整理下思路，来看看View滚动的实现原理，我们先调用Scroller的startScroll()方法来进行一些滚动的初始化设置，然后迫使View进行绘制，我们调用View的invalidate()或postInvalidate()就可以重新绘制View，绘制View的时候会触发computeScroll()方法，我们重写computeScroll()，在computeScroll()里面先调用Scroller的computeScrollOffset()方法来判断滚动有没有结束，如果滚动没有结束我们就调用scrollTo()方法来进行滚动，该scrollTo()方法虽然会重新绘制View,但是我们还是要手动调用下invalidate()或者postInvalidate()来触发界面重绘，重新绘制View又触发computeScroll()，所以就进入一个循环阶段，这样子就实现了在某个时间段里面滚动某段距离的一个平滑的滚动效果也许有人会问，干嘛还要调用来调用去最后在调用scrollTo()方法，还不如直接调用scrollTo()方法来实现滚动，其实直接调用是可以，只不过scrollTo()是瞬间滚动的，给人的用户体验不太好，所以Android提供了Scroller类实现平滑滚动的效果。为了方面大家理解，我画了一个简单的调用示意图
scroller_原理.jpg
好了，讲到这里就已经讲完了Scroller类的滚动实现原理啦，我们来总结一下：
（1）Scroller类能够帮助我们实现高级的滑动功能，如手指抬起后的惯性滑动功能。使用流程为，首先通过Scroller类的startScroll()+invalidate()触发View的computeScroll()，在computeScroll()中让Scroller类去计算最新的坐标信息，拿到最新的坐标偏移信息后还是要调用View的scrollTo来实现滑动。可以看到，使用Scroller的整个流程比较简单，关键的是控制滑动的一些逻辑计算，比如上面例子中的计算什么时候该往哪一页滑动...（2）Android后面推出了OverScroller类，OverScroller在整体功能上和Scroller类似，使用也相同。OverScroller类可以完全代替Scroller，相比Scroller，OverScroller主要是增加了对滑动到边界的一些控制，如增加一些回弹效果等，功能更加强大。如果对android技术比较感兴趣，可以关注一下微信公众号：终端研发部，和我一块交流和学习。终端研发部是一个以技术为主的学习交流技术号，谈的是技术，是产品，更是我们的人生。做东半球最会思考，最有味道的互联网开发者
作者：终端研发部
链接：https://www.zhihu.com/question/278916471/answer/403341655
来源：知乎
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