前言
之前已经和大家聊了onLayout的流程，本文将会继续聊一聊onDraw中做了什么？本文将集中关注软件渲染，关于Canvas的api源码解析暂时不会在本文聊，会专门开一个Skia源码解析进行分析。
如果遇到问题可以来https://www.jianshu.com/p/a4fb6a02ad53中讨论
正文
performTravel的方法走完onMeasure和onLayout流程后会走到下面这段代码段。
文件：/frameworks/base/core/java/android/view/ViewRootImpl.java
if (mFirst) {
if (sAlwaysAssignFocus || !isInTouchMode()) {
if (mView != null) {
if (!mView.hasFocus()) {
mView.restoreDefaultFocus();
} else {
...
}
}
} else {
View focused = mView.findFocus();
if (focused instanceof ViewGroup
&& ((ViewGroup) focused).getDescendantFocusability()
== ViewGroup.FOCUS_AFTER_DESCENDANTS) {
focused.restoreDefaultFocus();
}
}
}
final boolean changedVisibility = (viewVisibilityChanged || mFirst) && isViewVisible;
final boolean hasWindowFocus = mAttachInfo.mHasWindowFocus && isViewVisible;
final boolean regainedFocus = hasWindowFocus && mLostWindowFocus;
if (regainedFocus) {
mLostWindowFocus = false;
} else if (!hasWindowFocus && mHadWindowFocus) {
mLostWindowFocus = true;
}
if (changedVisibility || regainedFocus) {
boolean isToast = (mWindowAttributes == null) ? false
: (mWindowAttributes.type == WindowManager.LayoutParams.TYPE_TOAST);
...
}
mFirst = false;
mWillDrawSoon = false;
mNewSurfaceNeeded = false;
mActivityRelaunched = false;
mViewVisibility = viewVisibility;
mHadWindowFocus = hasWindowFocus;
if (hasWindowFocus && !isInLocalFocusMode()) {
final boolean imTarget = WindowManager.LayoutParams
.mayUseInputMethod(mWindowAttributes.flags);
if (imTarget != mLastWasImTarget) {
mLastWasImTarget = imTarget;
InputMethodManager imm = InputMethodManager.peekInstance();
if (imm != null && imTarget) {
imm.onPreWindowFocus(mView, hasWindowFocus);
imm.onPostWindowFocus(mView, mView.findFocus(),
mWindowAttributes.softInputMode,
!mHasHadWindowFocus, mWindowAttributes.flags);
}
}
}
在进入onDraw的流程之前，会先处理焦点。这个过程中可以分为2大步骤：
1.如果是第一次渲染，则说明之前的宽高都是都为0.在requestFocus方法中会有这个判断把整个焦点集中拦截下来：
private boolean canTakeFocus() {
return ((mViewFlags & VISIBILITY_MASK) == VISIBLE)
&& ((mViewFlags & FOCUSABLE) == FOCUSABLE)
&& ((mViewFlags & ENABLED_MASK) == ENABLED)
&& (sCanFocusZeroSized || !isLayoutValid() || hasSize());
}
而在每一次onMeasure之前，都会尝试集中一次焦点的遍历。其中requestFocusNoSearch方法中，如果没有测量过就会直接返回false。因为每一次更换焦点或者集中焦点都可能伴随着如背景drawable，statelistDrawable等切换。没有测量过就没有必要做这无用功(详情请看View的绘制流程(三) onMeasure
)。
因此此时为了弥补之前拒绝焦点的行为，会重新进行一次restoreDefaultFocus的行为进行requestFocus处理。
2.如果存在窗体焦点，同时不是打开了FLAG_LOCAL_FOCUS_MODE标志（这是一种特殊情况，一般打上这个标志位只有在startingWindow的快照中才会有，startingWindow具体是什么可以看看WMS在Activity启动中的职责 添加窗体(三)）。
则会调用InputMethodManager的onPostWindowFocus方法启动带了android.view.InputMethod这个action的软键盘服务。详细的这里暂时不展开讨论。
onDraw流程
if ((relayoutResult & WindowManagerGlobal.RELAYOUT_RES_FIRST_TIME) != 0) {
reportNextDraw();
}
boolean cancelDraw = mAttachInfo.mTreeObserver.dispatchOnPreDraw() || !isViewVisible;
if (!cancelDraw && !newSurface) {
if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
for (int i = 0; i < mPendingTransitions.size(); ++i) {
mPendingTransitions.get(i).startChangingAnimations();
}
mPendingTransitions.clear();
}
performDraw();
} else {
if (isViewVisible) {
scheduleTraversals();
} else if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
for (int i = 0; i < mPendingTransitions.size(); ++i) {
mPendingTransitions.get(i).endChangingAnimations();
}
mPendingTransitions.clear();
}
}
mIsInTraversal = false;
1.判断到如果是第一次调用draw方法，则会调用reportNextDraw。
private void reportNextDraw() {
if (mReportNextDraw == false) {
drawPending();
}
mReportNextDraw = true;
}
void drawPending() {
mDrawsNeededToReport++;
}
能看到实际上就是设置mReportNextDraw为true。我们回顾一下前两个流程mReportNextDraw参与了标志位的判断。在执行onMeasure和onLayout有两个大前提，一个是mStop为false，一个是mReportNextDraw为true。只要满足其一就会执行。
这么做的目的只有一个，保证调用一次onDraw方法。为什么会这样呢？performDraw是整个Draw流程的入口。然而在这个入口，必须要保证cancelDraw为false以及newSurface为false。
注意，如果是第一次渲染因为会添加进新的Surface，此时newSurface为true(可以看View的绘制流程(二) 绘制的准备)。所以会走到下面的分之，如果串口可见则调用scheduleTraversals执行下一次Loop的绘制流程。否则判断是否有需要执行的LayoutTransitions layout动画就执行了。
因此第一次是不会走到onDraw，是从第二次Looper之后View的绘制流程才会执行onDraw。
我们继续关注performDraw的逻辑。
ViewRootImpl performDraw
private void performDraw() {
if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
return;
} else if (mView == null) {
return;
}
final boolean fullRedrawNeeded = mFullRedrawNeeded || mReportNextDraw;
mFullRedrawNeeded = false;
mIsDrawing = true;
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
boolean usingAsyncReport = false;
if (mReportNextDraw && mAttachInfo.mThreadedRenderer != null
&& mAttachInfo.mThreadedRenderer.isEnabled()) {
usingAsyncReport = true;
mAttachInfo.mThreadedRenderer.setFrameCompleteCallback((long frameNr) -> {
pendingDrawFinished();
});
}
try {
boolean canUseAsync = draw(fullRedrawNeeded);
if (usingAsyncReport && !canUseAsync) {
mAttachInfo.mThreadedRenderer.setFrameCompleteCallback(null);
usingAsyncReport = false;
}
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
if (mAttachInfo.mPendingAnimatingRenderNodes != null) {
final int count = mAttachInfo.mPendingAnimatingRenderNodes.size();
for (int i = 0; i < count; i++) {
mAttachInfo.mPendingAnimatingRenderNodes.get(i).endAllAnimators();
}
mAttachInfo.mPendingAnimatingRenderNodes.clear();
}
if (mReportNextDraw) {
mReportNextDraw = false;
if (mWindowDrawCountDown != null) {
try {
mWindowDrawCountDown.await();
} catch (InterruptedException e) {
Log.e(mTag, "Window redraw count down interrupted!");
}
mWindowDrawCountDown = null;
}
if (mAttachInfo.mThreadedRenderer != null) {
mAttachInfo.mThreadedRenderer.setStopped(mStopped);
}
if (mSurfaceHolder != null && mSurface.isValid()) {
SurfaceCallbackHelper sch = new SurfaceCallbackHelper(this::postDrawFinished);
SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();
sch.dispatchSurfaceRedrawNeededAsync(mSurfaceHolder, callbacks);
} else if (!usingAsyncReport) {
if (mAttachInfo.mThreadedRenderer != null) {
mAttachInfo.mThreadedRenderer.fence();
}
pendingDrawFinished();
}
}
}
我们把整个流程抽象出来实际上就是可以分为如下几个步骤：
对于软件渲染：
1.调用draw方法，遍历View的层级。
2.如果Surface是生效的，则在SurfaceHolder.Callback的surfaceRedrawNeededAsync回调中调用pendingDrawFinished。
3.如果是强制同步渲染，则会直接调用pendingDrawFinished。
对于硬件渲染：
1.调用draw方法，遍历View的层级。
2.通过监听mThreadedRenderer的setFrameCompleteCallback回调执行pendingDrawFinished方法。
我们先关注软件渲染的流程。也就是draw和pendingDrawFinished。
ViewRootImpl draw
private boolean draw(boolean fullRedrawNeeded) {
Surface surface = mSurface;
if (!surface.isValid()) {
return false;
}
if (!sFirstDrawComplete) {
synchronized (sFirstDrawHandlers) {
sFirstDrawComplete = true;
final int count = sFirstDrawHandlers.size();
for (int i = 0; i< count; i++) {
mHandler.post(sFirstDrawHandlers.get(i));
}
}
}
scrollToRectOrFocus(null, false);
if (mAttachInfo.mViewScrollChanged) {
mAttachInfo.mViewScrollChanged = false;
mAttachInfo.mTreeObserver.dispatchOnScrollChanged();
}
boolean animating = mScroller != null && mScroller.computeScrollOffset();
final int curScrollY;
if (animating) {
curScrollY = mScroller.getCurrY();
} else {
curScrollY = mScrollY;
}
if (mCurScrollY != curScrollY) {
mCurScrollY = curScrollY;
fullRedrawNeeded = true;
if (mView instanceof RootViewSurfaceTaker) {
((RootViewSurfaceTaker) mView).onRootViewScrollYChanged(mCurScrollY);
}
}
final float appScale = mAttachInfo.mApplicationScale;
final boolean scalingRequired = mAttachInfo.mScalingRequired;
final Rect dirty = mDirty;
if (mSurfaceHolder != null) {
dirty.setEmpty();
if (animating && mScroller != null) {
mScroller.abortAnimation();
}
return false;
}
if (fullRedrawNeeded) {
mAttachInfo.mIgnoreDirtyState = true;
dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
}
mAttachInfo.mTreeObserver.dispatchOnDraw();
int xOffset = -mCanvasOffsetX;
int yOffset = -mCanvasOffsetY + curScrollY;
final WindowManager.LayoutParams params = mWindowAttributes;
final Rect surfaceInsets = params != null ? params.surfaceInsets : null;
if (surfaceInsets != null) {
xOffset -= surfaceInsets.left;
yOffset -= surfaceInsets.top;
dirty.offset(surfaceInsets.left, surfaceInsets.right);
}
...
mAttachInfo.mDrawingTime =
mChoreographer.getFrameTimeNanos() / TimeUtils.NANOS_PER_MS;
boolean useAsyncReport = false;
if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
if (mAttachInfo.mThreadedRenderer != null && mAttachInfo.mThreadedRenderer.isEnabled()) {
boolean invalidateRoot = accessibilityFocusDirty || mInvalidateRootRequested;
mInvalidateRootRequested = false;
mIsAnimating = false;
if (mHardwareYOffset != yOffset || mHardwareXOffset != xOffset) {
mHardwareYOffset = yOffset;
mHardwareXOffset = xOffset;
invalidateRoot = true;
}
if (invalidateRoot) {
mAttachInfo.mThreadedRenderer.invalidateRoot();
}
dirty.setEmpty();
final boolean updated = updateContentDrawBounds();
if (mReportNextDraw) {
mAttachInfo.mThreadedRenderer.setStopped(false);
}
if (updated) {
requestDrawWindow();
}
useAsyncReport = true;
final FrameDrawingCallback callback = mNextRtFrameCallback;
mNextRtFrameCallback = null;
mAttachInfo.mThreadedRenderer.draw(mView, mAttachInfo, this, callback);
} else {
if (mAttachInfo.mThreadedRenderer != null &&
!mAttachInfo.mThreadedRenderer.isEnabled() &&
mAttachInfo.mThreadedRenderer.isRequested() &&
mSurface.isValid()) {
try {
mAttachInfo.mThreadedRenderer.initializeIfNeeded(
mWidth, mHeight, mAttachInfo, mSurface, surfaceInsets);
} catch (OutOfResourcesException e) {
handleOutOfResourcesException(e);
return false;
}
mFullRedrawNeeded = true;
scheduleTraversals();
return false;
}
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
scalingRequired, dirty, surfaceInsets)) {
return false;
}
}
}
if (animating) {
mFullRedrawNeeded = true;
scheduleTraversals();
}
return useAsyncReport;
}
大致上完成了如下流程：
1.如果surface无效则直接返回
sFirstDrawHandlers这个存储着runnable静态对象。实际上是在ActivityThread启动后调用attach方法通过addFirstDrawHandler添加进来的目的只是为了启动jit模式。
3.scrollToRectOrFocus 处理滑动区域或者焦点区域。如果发生了滑动则回调TreeObserver.dispatchOnScrollChanged。接下来则通过全局的mScroller通过computeScrollOffset判断是否需要滑动动画。如果需要执行动画，则调用DeocView的onRootViewScrollYChanged，进行Y轴上的动画执行。
4.通过ViewTreeObserver的dispatchOnDraw开始分发draw开始绘制的监听者。
5.判断是否存在surface面上偏移量，有就矫正一次脏区，把偏移量添加上去。
接下来则会进入到硬件渲染和软件渲染的分支。但是进一步进行调用draw的流程有几个前提条件：脏区不为空，需要执行动画，辅助服务发生了焦点变化
6.如果ThreadedRenderer不为空且可用。ThreadedRenderer通过onPreDraw回调到ViewRootImpl，更新mHardwareYOffset，mHardwareXOffset。如果这两个参数发生了变化，则说明整个发生了硬件绘制的区域变化，需要从头遍历一次所有的区域设置为无效区域，mThreadedRenderer.invalidateRoot。
最后调用ThreadedRenderer.draw方法执行硬件渲染绘制。并且设置通过registerRtFrameCallback设置进来的callback设置到ThreadedRenderer中。
7.如果此时ThreadedRenderer不可用但是不为空，说明此时需要对ThreadedRenderer进行初始化，调用scheduleTraversals在下一轮的绘制流程中才进行硬件渲染。
8.如果以上情况都不满足，说明是软件渲染，则调用drawSoftware进行软件渲染。
9.如果不许要draw方法遍历全局的View树，则判断是否需要执行滑动动画，需要则调用scheduleTraversals进入下一轮的绘制。
本文先抛开硬件渲染，来看看软件渲染drawSoftware中做了什么。还有scrollToRectOrFocus滑动中做了什么？
ViewRootImpl scrollToRectOrFocus
boolean scrollToRectOrFocus(Rect rectangle, boolean immediate) {
final Rect ci = mAttachInfo.mContentInsets;
final Rect vi = mAttachInfo.mVisibleInsets;
int scrollY = 0;
boolean handled = false;
if (vi.left > ci.left || vi.top > ci.top
|| vi.right > ci.right || vi.bottom > ci.bottom) {
final View focus = mView.findFocus();
if (focus == null) {
return false;
}
View lastScrolledFocus = (mLastScrolledFocus != null) ? mLastScrolledFocus.get() : null;
if (focus != lastScrolledFocus) {
rectangle = null;
}
if (focus == lastScrolledFocus && !mScrollMayChange && rectangle == null) {
} else {
mLastScrolledFocus = new WeakReference
mScrollMayChange = false;
if (focus.getGlobalVisibleRect(mVisRect, null)) {
if (rectangle == null) {
focus.getFocusedRect(mTempRect);
if (mView instanceof ViewGroup) {
((ViewGroup) mView).offsetDescendantRectToMyCoords(
focus, mTempRect);
}
} else {
mTempRect.set(rectangle);
}
if (mTempRect.intersect(mVisRect)) {
if (mTempRect.height() >
(mView.getHeight()-vi.top-vi.bottom)) {
}
else if (mTempRect.top < vi.top) {
scrollY = mTempRect.top - vi.top;
} else if (mTempRect.bottom > (mView.getHeight()-vi.bottom)) {
scrollY = mTempRect.bottom - (mView.getHeight()-vi.bottom);
} else {
scrollY = 0;
}
handled = true;
}
}
}
}
if (scrollY != mScrollY) {
if (!immediate) {
if (mScroller == null) {
mScroller = new Scroller(mView.getContext());
}
mScroller.startScroll(0, mScrollY, 0, scrollY-mScrollY);
} else if (mScroller != null) {
mScroller.abortAnimation();
}
mScrollY = scrollY;
}
return handled;
}
能看到在这个过程中实际上就是处理两个区域mVisibleInsets可见区域以及mContentInsets内容区域。
实际上这个过程就是从根部节点开始寻找焦点，然后整个画面定格在焦点处。因为mVisibleInsets一般是屏幕中出去过扫描区的大小，但是内容区域就不一定了，可能内容会超出屏幕大小，因此会通过mScroller滑动定位。
计算原理如下，分为2个情况：
1.可视区域的顶部比起获得了焦点的view的顶部要低，说明这个view在屏幕外了，需要向下滑动：
scrollY = mTempRect.top - vi.top;
2.如果焦点view的底部比起可视区域要比可视区域的低，说明需要向上滑动,注意滑动之后需要展示view，因此滑动的距离要减去view的高度:
scrollY = mTempRect.bottom - (mView.getHeight()-vi.bottom);
稍微变一下如下：
scrollY = mTempRect.bottom +vi.bottom - mView.getHeight();
ViewRootImpl drawSoftware
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty, Rect surfaceInsets) {
final Canvas canvas;
int dirtyXOffset = xoff;
int dirtyYOffset = yoff;
if (surfaceInsets != null) {
dirtyXOffset += surfaceInsets.left;
dirtyYOffset += surfaceInsets.top;
}
try {
dirty.offset(-dirtyXOffset, -dirtyYOffset);
final int left = dirty.left;
final int top = dirty.top;
final int right = dirty.right;
final int bottom = dirty.bottom;
canvas = mSurface.lockCanvas(dirty);
if (left != dirty.left || top != dirty.top || right != dirty.right
|| bottom != dirty.bottom) {
attachInfo.mIgnoreDirtyState = true;
}
canvas.setDensity(mDensity);
} catch (Surface.OutOfResourcesException e) {
handleOutOfResourcesException(e);
return false;
} catch (IllegalArgumentException e) {
mLayoutRequested = true; // ask wm for a new surface next time.
return false;
} finally {
dirty.offset(dirtyXOffset, dirtyYOffset); // Reset to the original value.
}
try {
if (!canvas.isOpaque() || yoff != 0 || xoff != 0) {
canvas.drawColor(0, PorterDuff.Mode.CLEAR);
}
dirty.setEmpty();
mIsAnimating = false;
mView.mPrivateFlags |= View.PFLAG_DRAWN;
try {
canvas.translate(-xoff, -yoff);
if (mTranslator != null) {
mTranslator.translateCanvas(canvas);
}
canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
attachInfo.mSetIgnoreDirtyState = false;
mView.draw(canvas);
drawAccessibilityFocusedDrawableIfNeeded(canvas);
} finally {
if (!attachInfo.mSetIgnoreDirtyState) {
attachInfo.mIgnoreDirtyState = false;
}
}
} finally {
try {
surface.unlockCanvasAndPost(canvas);
} catch (IllegalArgumentException e) {
Log.e(mTag, "Could not unlock surface", e);
mLayoutRequested = true; // ask wm for a new surface next time.
//noinspection ReturnInsideFinallyBlock
return false;
}
}
return true;
}
这里面的逻辑和上面硬件渲染逻辑有点相似：
1.同样还是根据全局的surface的偏移量对整个dirty区域进行偏移
2.通过Surface.lockCanvas方法映射一个Canvas对象，之后所有的绘制行为都在这个Canvas对象上。关于这个方法，详细的原理可以看看我写的SurfaceView和TextureView 源码浅析(上)。
3.获得Canvas后，由于上面是对整个surface的偏移，因此Canvas作为surface映射出来的绘制对象也需要进行一次偏移。
4.调用DecorView的draw方法，开始对整个View树遍历。
5.遍历完整个View树后，说明所有的信息已经会知道Canvas了，就可以通过surface.unlockCanvasAndPost，把记录在SkCanvas中的像素数据发送到SF中渲染到屏幕中。关于第五点详细的可以阅读SurfaceView和TextureView 源码浅析(上)。后续的步骤可以阅读我写的SF系列文章。
我们继续来要来看看draw方法做了什么。
DecorView draw
@Override
public void draw(Canvas canvas) {
super.draw(canvas);
if (mMenuBackground != null) {
mMenuBackground.draw(canvas);
}
}
能看到整个DecorView实际上就是调用了父类的draw方法后，专门给menu栏的drawable绘制到Canvas中。
View draw
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
int saveCount;
if (!dirtyOpaque) {
drawBackground(canvas);
}
// skip step 2 & 5 if possible (common case)
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
if (!dirtyOpaque) onDraw(canvas);
dispatchDraw(canvas);
drawAutofilledHighlight(canvas);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
onDrawForeground(canvas);
drawDefaultFocusHighlight(canvas);
return;
}
boolean drawTop = false;
boolean drawBottom = false;
boolean drawLeft = false;
boolean drawRight = false;
float topFadeStrength = 0.0f;
float bottomFadeStrength = 0.0f;
float leftFadeStrength = 0.0f;
float rightFadeStrength = 0.0f;
int paddingLeft = mPaddingLeft;
final boolean offsetRequired = isPaddingOffsetRequired();
if (offsetRequired) {
paddingLeft += getLeftPaddingOffset();
}
int left = mScrollX + paddingLeft;
int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
int top = mScrollY + getFadeTop(offsetRequired);
int bottom = top + getFadeHeight(offsetRequired);
if (offsetRequired) {
right += getRightPaddingOffset();
bottom += getBottomPaddingOffset();
}
final ScrollabilityCache scrollabilityCache = mScrollCache;
final float fadeHeight = scrollabilityCache.fadingEdgeLength;
int length = (int) fadeHeight;
if (verticalEdges && (top + length > bottom - length)) {
length = (bottom - top) / 2;
}
if (horizontalEdges && (left + length > right - length)) {
length = (right - left) / 2;
}
if (verticalEdges) {
topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
drawTop = topFadeStrength * fadeHeight > 1.0f;
bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
}
if (horizontalEdges) {
leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
drawLeft = leftFadeStrength * fadeHeight > 1.0f;
rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
drawRight = rightFadeStrength * fadeHeight > 1.0f;
}
saveCount = canvas.getSaveCount();
int solidColor = getSolidColor();
if (solidColor == 0) {
if (drawTop) {
canvas.saveUnclippedLayer(left, top, right, top + length);
}
if (drawBottom) {
canvas.saveUnclippedLayer(left, bottom - length, right, bottom);
}
if (drawLeft) {
canvas.saveUnclippedLayer(left, top, left + length, bottom);
}
if (drawRight) {
canvas.saveUnclippedLayer(right - length, top, right, bottom);
}
} else {
scrollabilityCache.setFadeColor(solidColor);
}
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Step 5, draw the fade effect and restore layers
final Paint p = scrollabilityCache.paint;
final Matrix matrix = scrollabilityCache.matrix;
final Shader fade = scrollabilityCache.shader;
if (drawTop) {
matrix.setScale(1, fadeHeight * topFadeStrength);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, right, top + length, p);
}
if (drawBottom) {
matrix.setScale(1, fadeHeight * bottomFadeStrength);
matrix.postRotate(180);
matrix.postTranslate(left, bottom);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, bottom - length, right, bottom, p);
}
if (drawLeft) {
matrix.setScale(1, fadeHeight * leftFadeStrength);
matrix.postRotate(-90);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, left + length, bottom, p);
}
if (drawRight) {
matrix.setScale(1, fadeHeight * rightFadeStrength);
matrix.postRotate(90);
matrix.postTranslate(right, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(right - length, top, right, bottom, p);
}
canvas.restoreToCount(saveCount);
drawAutofilledHighlight(canvas);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
}
大致上draw方法分为如下几个步骤：
1.首先在draw方法中先校验mPrivateFlags中打开的标志位。还记得上一篇文章聊过的PFLAG_DIRTY_MASK的掩码实际上控制的是dirty以及透明两个标志位。
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
首先校验PFLAG_DIRTY_OPAQUE也就是透明标志位是否开启了，开启了dirtyOpaque则为true。同时打开PFLAG_DRAWN标志位，说明该View已经调用过了draw方法了。
2.如果dirtyOpaque为false说明不是透明则调用drawBackground，进行背景的绘制。
3.校验是否有横竖方向的边缘阴影需要绘制，如果不需要则以此执行如下三个流程：
1.执行该View重写的onDraw流程，进行绘制
2.dispatchDraw 把绘制行为分发到子View中
3.判断是否有overlay，有则绘制每一个View的浮层的dispatchDraw
4.onDrawForeground 绘制View的前景drawable
5.drawDefaultFocusHighlight 绘制默认的焦点高亮。
4.如果需要绘制上下左右四个方向的滑轮，则执行如下几个步骤：
1.计算滑轮的上下左右四个方向，根据是横向还是竖向计算其长度
2.把这一块内容作为Canvas的非裁剪区域绘制到外面区域
3.执行3.1以及3.2的步骤
4.根据绘制的上下左右四个方向，对滑轮进行旋转
5.执行3.3-3.5的步骤
我们关注核心行为3.1onDraw以及3.2dispatchDraw 以及drawBackground中完成了什么事情.
View drawBackground
private void drawBackground(Canvas canvas) {
final Drawable background = mBackground;
if (background == null) {
return;
}
setBackgroundBounds();
// Attempt to use a display list if requested.
if (canvas.isHardwareAccelerated() && mAttachInfo != null
&& mAttachInfo.mThreadedRenderer != null) {
mBackgroundRenderNode = getDrawableRenderNode(background, mBackgroundRenderNode);
final RenderNode renderNode = mBackgroundRenderNode;
if (renderNode != null && renderNode.isValid()) {
setBackgroundRenderNodeProperties(renderNode);
((DisplayListCanvas) canvas).drawRenderNode(renderNode);
return;
}
}
final int scrollX = mScrollX;
final int scrollY = mScrollY;
if ((scrollX | scrollY) == 0) {
background.draw(canvas);
} else {
canvas.translate(scrollX, scrollY);
background.draw(canvas);
canvas.translate(-scrollX, -scrollY);
}
}
能看到这里面绘制的逻辑分为硬件渲染和软件渲染：
硬件渲染首先会通过getDrawableRenderNode方法获取一个drawable渲染的renderNode，接着调用canvas的drawRenderNode。从之前我分析的TextureView一文中可以了解到硬件渲染，Canvas实质上就是DisplayListCanvas。
软件渲染则是调用draable的draw方法，把像素绘制到canvas智商。
getDrawableRenderNode
private RenderNode getDrawableRenderNode(Drawable drawable, RenderNode renderNode) {
if (renderNode == null) {
renderNode = RenderNode.create(drawable.getClass().getName(), this);
}
final Rect bounds = drawable.getBounds();
final int width = bounds.width();
final int height = bounds.height();
final DisplayListCanvas canvas = renderNode.start(width, height);
canvas.translate(-bounds.left, -bounds.top);
try {
drawable.draw(canvas);
} finally {
renderNode.end(canvas);
}
renderNode.setLeftTopRightBottom(bounds.left, bounds.top, bounds.right, bounds.bottom);
renderNode.setProjectBackwards(drawable.isProjected());
renderNode.setProjectionReceiver(true);
renderNode.setClipToBounds(false);
return renderNode;
}
能看到在绘制一个硬件渲染的drawable对象时候，会先生成一个RenderNode，调用start之后获取Drawable对象对应DisplayListCanvas，在调用drawable的draw方法，把信息绘制到DisplayListCanvas，最后返回renderNode。
再把这个drawable对应的renderNode添加到当前View的Canvas中。
ViewGroup dispatchDraw
View默认是留下一个onDraw的空方法。我们看看dispatchDraw中做了什么。
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;
final ArrayList
? 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) {
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
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 (clipToPadding) {
canvas.restoreToCount(clipSaveCount);
}
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) {
mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER;
final Runnable end = new Runnable() {
@Override
public void run() {
notifyAnimationListener();
}
};
post(end);
}
}
这个过程中做了如下几件事情：
1.判断是否打开了FLAG_RUN_ANIMATION标志位，且允许Layout动画。首先遍历该viewGroup中所有子View中所有的可见的子View，并且bindLayoutAnimation设置好每一个子View对应的Layout动画。
private void bindLayoutAnimation(View child) {
Animation a = mLayoutAnimationController.getAnimationForView(child);
child.setAnimation(a);
}
LayoutAnimationController 控制Layout动画的控制者调用start启动动画，并且回调监听。
3.判断是否被padding裁剪内容区域，默认是开启的。这个情况下，则会滑动的区域，padding区域，以及viewgroup的区域，进行裁剪，而不是统统都画到Canvas中。
canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,
mScrollX + mRight - mLeft - mPaddingRight,
mScrollY + mBottom - mTop - mPaddingBottom);
4.判断是否打开了硬件加速。如果没有，buildOrderedChildList对当前该ViewGroup下所有子View进行z轴上的插入排序，从而得知谁将绘制在更加更加上方。这个过程中z轴上的数值越小，越先调用drawChild方法绘制到canvas中，也就是层级越低，被其他子View覆盖在其上。在处理每一个孩子对应的drawChild方法之前，会先处理通过addTransientView添加进来的临时View。这种方式十分少见，你可以看成临时动画一样的效果，不参加view的onmeasure，onLayout，但是会绘制出来。需要手动的remove掉。
5.绘制通过addDisappearingView添加的消失临时View。
6.如果FLAG_NOTIFY_ANIMATION_LISTENER，FLAG_ANIMATION_DONE标志位都关闭了，同时Layout动画也完成了，所有的drawChild都返回了false，则在下一个Looper中开始时机调用notifyAnimationListener，通知监听者本次动画已经完成。
整个核心都是drawChild方法，我们来看看drawChild做了什么？
drawChild
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
}
核心调用了view的draw方法。但是注意了，这个draw和上面那个draw方法不太一样。
boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
final boolean hardwareAcceleratedCanvas = canvas.isHardwareAccelerated();
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) {
renderNode = updateDisplayListIfDirty();
if (!renderNode.isValid()) {
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 {
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);
}
}
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 {
mPrivateFlags |= PFLAG_ALPHA_SET;
}
}
}
} else if ((mPrivateFlags & PFLAG_ALPHA_SET) == PFLAG_ALPHA_SET) {
onSetAlpha(255);
mPrivateFlags &= ~PFLAG_ALPHA_SET;
}
if (!drawingWithRenderNode) {
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 {
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) {
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 {
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) {
invalidate(true);
}
}
mRecreateDisplayList = false;
return more;
}
上面这个方法做了一个很重要的事情，那就是绘制每一个View的缓存。在这个过程中有两个比较重要的标志位：
boolean drawingWithRenderNode = mAttachInfo != null
&& mAttachInfo.mHardwareAccelerated
&& hardwareAcceleratedCanvas;
drawingWithRenderNode判断是否需要通过硬件绘制RenderNode。
final boolean drawingWithDrawingCache = cache != null && !drawingWithRenderNode;
drawingWithDrawingCache是否绘制view中的缓存由2点决定，一个是关闭硬件渲染，另一个是缓存的bitmap不为空。
做到的事情如下：
1.首先判断View中是否通过setAnimation设置了一个变换矩阵Transformation动画到View中。如果有，则调用applyLegacyAnimation方法确定整个变化动画刷新的范围在这个View范围内；transformToApply设置为父容器的Transformation。如果没有设置Animation，则判断是否关闭了硬件渲染且打开了FLAG_SUPPORT_STATIC_TRANSFORMATIONS。也就是说当前的View的父容器是否存在一个静态的变换矩阵，存在则更新到transformToApply中。
2.如果mLayerType为LAYER_TYPE_SOFTWARE或者关闭了硬件渲染，说明是一个软件渲染，则调用buildDrawingCache构建一个绘制的缓存，通过getDrawingCache获取这个缓存bitmap到cache中。
3.如果drawingWithRenderNode为true，说明在使用硬件渲染。则调用updateDisplayListIfDirty更新DisplayList中的脏区。
4.drawingWithRenderNode为false，则调用computeScroll计算滑动区域，赋值给sx和sy。
5.如果存在一个变换矩阵或者关闭硬件渲染，则调用canvas的save方法保存当前的状态。如果offsetForScroll为true(说明此时是软件渲染同时没有缓存)，则调用canvas的translate方法参数第四步骤中计算出来平移距离。
横向平移：mLeft - sx
纵向平移： mTop - sy
这么做可以把绘制的原点移动到平移 平移后的位置，之后所有的绘制都是基于这个点进行的
6.offsetForScroll为false，说明此时可能需要绘制缓存或者是硬件渲染，只做了两件事情：平移当前的画布的绘制原点，如果需要则对整个画布进行伸缩。
7.如果transformToApply不为空，前提下。发现打开了硬件渲染，则调用RenderNode.setAnimationMatrix方法设置动画的矩阵。如果关闭，则先回退经过平移的Canvas原点，先对动画矩阵进行合并后在进行滑动的移动。
8.如果alpha小于1，且判断到drawingWithDrawingCache关闭的。则说明可以在当前的绘制结果中进行透明度处理。判断onSetAlpha为false，如果是硬件渲染则调用renderNode.setAlpha，如果是软件渲染则调用canvas.saveLayerAlpha。如果绘制缓存是打开。onSetAlpha如果为true说明整个透明是由子View决定的，因此先打开PFLAG_ALPHA_SET标志位，等待后续的处理。
9.如果父容器打开了FLAG_CLIP_CHILDREN标志位且当前的View没有缓存。说明当前的View绘制的结果需要被父容器裁剪了：
如果进行了滑动，则裁剪区域如下：
canvas.clipRect(sx, sy, sx + getWidth(), sy + getHeight());
如果没有缓存，则直接裁剪当前的View
canvas.clipRect(0, 0, getWidth(), getHeight());
有缓存：
canvas.clipRect(0, 0, cache.getWidth(), cache.getHeight());
如果需要根据边缘进行裁剪：
canvas.clipRect(mClipBounds);
10.drawingWithDrawingCache如果是关闭，且drawingWithRenderNode是打开的，则调用DisplayListCanvas.drawRenderNode(renderNode) 方法。参数中的renderNode是updateDisplayListIfDirty方法生成一个新的DisplayListCanvas。通过drawRenderNode把结果绘制到父容器的DisplayListCanvas。
11.drawingWithDrawingCache关闭，drawingWithRenderNode也是关闭的。说明此时是直接进行绘制。如果打开了PFLAG_SKIP_DRAW标志位说明需要直接掉过当前的View，直接调用dispatchDraw分发View的绘制命令。如果没有打开，则调用draw方法。就会继续调用onDraw后并且dispatchDraw分发View的绘制方法。
12.drawingWithDrawingCache是打开的，同时cache缓存不为空，则把cache中的结果绘制到Canvas中。
13.当所有子View都绘制结束之后，则调用canvas.restoreToCount方法一层层的恢复绘制状态。主要还是恢复绘制原点。调用父容器的finishAnimatingView清空所有的Animation以及disappearingAnimation，回调onAnimationEnd。
在这几点中，除去Canvas操作(关于Canvas操作，我会专门开一个Skia源码解析专题进行分析)有几个比较重要的方法：
1.buildDrawingCache 构建一个绘制缓存对象
2.updateDisplayListIfDirty 硬件渲染更新脏区
buildDrawingCache
public void buildDrawingCache(boolean autoScale) {
if ((mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == 0 || (autoScale ?
mDrawingCache == null : mUnscaledDrawingCache == null)) {
try {
buildDrawingCacheImpl(autoScale);
} finally {
}
}
}
能看到实际上就是调用buildDrawingCacheImpl.
buildDrawingCacheImpl
private void buildDrawingCacheImpl(boolean autoScale) {
mCachingFailed = false;
int width = mRight - mLeft;
int height = mBottom - mTop;
final AttachInfo attachInfo = mAttachInfo;
final boolean scalingRequired = attachInfo != null && attachInfo.mScalingRequired;
if (autoScale && scalingRequired) {
width = (int) ((width * attachInfo.mApplicationScale) + 0.5f);
height = (int) ((height * attachInfo.mApplicationScale) + 0.5f);
}
final int drawingCacheBackgroundColor = mDrawingCacheBackgroundColor;
final boolean opaque = drawingCacheBackgroundColor != 0 || isOpaque();
final boolean use32BitCache = attachInfo != null && attachInfo.mUse32BitDrawingCache;
final long projectedBitmapSize = width * height * (opaque && !use32BitCache ? 2 : 4);
final long drawingCacheSize =
ViewConfiguration.get(mContext).getScaledMaximumDrawingCacheSize();
if (width <= 0 || height <= 0 || projectedBitmapSize > drawingCacheSize) {
if (width > 0 && height > 0) {
Log.w(VIEW_LOG_TAG, getClass().getSimpleName() + " not displayed because it is"
+ " too large to fit into a software layer (or drawing cache), needs "
+ projectedBitmapSize + " bytes, only "
+ drawingCacheSize + " available");
}
destroyDrawingCache();
mCachingFailed = true;
return;
}
boolean clear = true;
Bitmap bitmap = autoScale ? mDrawingCache : mUnscaledDrawingCache;
if (bitmap == null || bitmap.getWidth() != width || bitmap.getHeight() != height) {
Bitmap.Config quality;
if (!opaque) {
// Never pick ARGB_4444 because it looks awful
// Keep the DRAWING_CACHE_QUALITY_LOW flag just in case
switch (mViewFlags & DRAWING_CACHE_QUALITY_MASK) {
case DRAWING_CACHE_QUALITY_AUTO:
case DRAWING_CACHE_QUALITY_LOW:
case DRAWING_CACHE_QUALITY_HIGH:
default:
quality = Bitmap.Config.ARGB_8888;
break;
}
} else {
quality = use32BitCache ? Bitmap.Config.ARGB_8888 : Bitmap.Config.RGB_565;
}
if (bitmap != null) bitmap.recycle();
try {
bitmap = Bitmap.createBitmap(mResources.getDisplayMetrics(),
width, height, quality);
bitmap.setDensity(getResources().getDisplayMetrics().densityDpi);
if (autoScale) {
mDrawingCache = bitmap;
} else {
mUnscaledDrawingCache = bitmap;
}
if (opaque && use32BitCache) bitmap.setHasAlpha(false);
} catch (OutOfMemoryError e) {
if (autoScale) {
mDrawingCache = null;
} else {
mUnscaledDrawingCache = null;
}
mCachingFailed = true;
return;
}
clear = drawingCacheBackgroundColor != 0;
}
Canvas canvas;
if (attachInfo != null) {
canvas = attachInfo.mCanvas;
if (canvas == null) {
canvas = new Canvas();
}
canvas.setBitmap(bitmap);
attachInfo.mCanvas = null;
} else {
canvas = new Canvas(bitmap);
}
if (clear) {
bitmap.eraseColor(drawingCacheBackgroundColor);
}
computeScroll();
final int restoreCount = canvas.save();
if (autoScale && scalingRequired) {
final float scale = attachInfo.mApplicationScale;
canvas.scale(scale, scale);
}
canvas.translate(-mScrollX, -mScrollY);
mPrivateFlags |= PFLAG_DRAWN;
if (mAttachInfo == null || !mAttachInfo.mHardwareAccelerated ||
mLayerType != LAYER_TYPE_NONE) {
mPrivateFlags |= PFLAG_DRAWING_CACHE_VALID;
}
if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
dispatchDraw(canvas);
drawAutofilledHighlight(canvas);
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().draw(canvas);
}
} else {
draw(canvas);
}
canvas.restoreToCount(restoreCount);
canvas.setBitmap(null);
if (attachInfo != null) {
attachInfo.mCanvas = canvas;
}
}
1.在创建绘制的缓存bitmap之前，如果当前View的宽高其中之一小于等于0，或者当前View需要内存大于最大允许的缓存View大小。
这个过程中，View绘制后的缓存计算方法如下：
projectedBitmapSize = width * height * (opaque && !use32BitCache ? 2 : 4);
能看到这个过程中判断是否需要透明且关闭32的缓存，一个像素就会2位，否则则是4位。
如果计算出来的结果比MAXIMUM_DRAWING_CACHE_SIZE大则销毁绘制缓存。
private static final int MAXIMUM_DRAWING_CACHE_SIZE = 480 * 800 * 4; // ARGB8888
能看到每一个View最大只能是由宽480，高800且是ARGB8888模式内存大小。
2.如果View的大小发生了变化，则调用Bitmap.createBitmap的方法创建一个对应View大小的Bitmap。
3.如果attachInfo不为空，则判断是否存在一个全局的Canvas，如果不存在就创建一个新的Canvas，并把bitmap设置到Canvas中。
4.如果需要进行伸缩，则伸缩缓存bitmap。如果需要滑动，则移动整个Canvas的绘制原点。
5.如果PFLAG_SKIP_DRAW 打开了，则直接调用dispatchDraw，继续分发绘制流程。关闭了则调用draw方法，先调用onDraw后调用dispatchDraw。
能看到在上面draw方法中因为检测存在绘制缓存而跳过的流程，在这个方法中都进行处理了。
updateDisplayListIfDirty
public RenderNode updateDisplayListIfDirty() {
final RenderNode renderNode = mRenderNode;
if (!canHaveDisplayList()) {
return renderNode;
}
if ((mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == 0
|| !renderNode.isValid()
|| (mRecreateDisplayList)) {
if (renderNode.isValid()
&& !mRecreateDisplayList) {
mPrivateFlags |= PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID;
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
dispatchGetDisplayList();
return renderNode; // no work needed
}
mRecreateDisplayList = true;
int width = mRight - mLeft;
int height = mBottom - mTop;
int layerType = getLayerType();
final DisplayListCanvas canvas = renderNode.start(width, height);
try {
if (layerType == LAYER_TYPE_SOFTWARE) {
buildDrawingCache(true);
Bitmap cache = getDrawingCache(true);
if (cache != null) {
canvas.drawBitmap(cache, 0, 0, mLayerPaint);
}
} else {
computeScroll();
canvas.translate(-mScrollX, -mScrollY);
mPrivateFlags |= PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID;
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
// Fast path for layouts with no backgrounds
if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
dispatchDraw(canvas);
drawAutofilledHighlight(canvas);
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().draw(canvas);
}
if (debugDraw()) {
debugDrawFocus(canvas);
}
} else {
draw(canvas);
}
}
} finally {
renderNode.end(canvas);
setDisplayListProperties(renderNode);
}
} else {
mPrivateFlags |= PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID;
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
}
return renderNode;
}
1.如果canHaveDisplayList为 false也就是mThreadedRenderer为null，则直接返回。
如果PFLAG_DRAWING_CACHE_VALID关闭，或者renderNode是无效的，或者mRecreateDisplayList是false。则进入到RenderNode的Canvas绘制中。
2.如果renderNode是有效的，且不需要进行重新构建整个硬件渲染的DisplayList(mRecreateDisplayList为false)，说明不是第一次绘制了已经有绘制结果了，则调用dispatchGetDisplayList。
3.接下来的情景说明是第一次绘制，renderNode还是属于无效状态。其初始化流程如下：
1.renderNode.start(width, height) 调用start方法创建一个全新的DisplayListCanvas
2.如果LayerType是LAYER_TYPE_SOFTWARE，就算是Layer的模式是软件渲染模式，如果打开了硬件渲染模式，还是会把当前View对应绘制缓存bitmap通过setBitmap的方式设置到DisplayListCanvas中。
3.如果不是LAYER_TYPE_SOFTWARE，调用computeScroll进行滑动计算后。把整个RenderNode的绘制原点退回到滑动之前的状态，并且打上PFLAG_DRAWN和PFLAG_DRAWING_CACHE_VALID两个标志位。
4.如果打开PFLAG_SKIP_DRAW，则直接调用dispatchDraw，分发绘制流程。
5.没有打开PFLAG_SKIP_DRAW，则直接调用draw方法，先回调onDraw再调用dispatchDraw进行分发。
4.最后调用renderNode.end(canvas) 结束整个RenderNode的绘制。
我们来看看非第一次绘制时候dispatchGetDisplayList做了什么？
ViewGroup dispatchGetDisplayList
protected void dispatchGetDisplayList() {
final int count = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < count; i++) {
final View child = children[i];
if (((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null)) {
recreateChildDisplayList(child);
}
}
final int transientCount = mTransientViews == null ? 0 : mTransientIndices.size();
for (int i = 0; i < transientCount; ++i) {
View child = mTransientViews.get(i);
if (((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null)) {
recreateChildDisplayList(child);
}
}
if (mOverlay != null) {
View overlayView = mOverlay.getOverlayView();
recreateChildDisplayList(overlayView);
}
if (mDisappearingChildren != null) {
final ArrayList
final int disappearingCount = disappearingChildren.size();
for (int i = 0; i < disappearingCount; ++i) {
final View child = disappearingChildren.get(i);
recreateChildDisplayList(child);
}
}
}
能看到实际上很简单，对4种View进行recreateChildDisplayList处理。
1.所有的可见子View或者带着动画的子View
2.mTransientViews 通过addTransientView添加进来的临时View
3.overlayView 每一个View的浮层
4.mDisappearingChildren 通过addDisappearingView 添加进来的当View移除时候需要的动画View。
ViewGroup recreateChildDisplayList
private void recreateChildDisplayList(View child) {
child.mRecreateDisplayList = (child.mPrivateFlags & PFLAG_INVALIDATED) != 0;
child.mPrivateFlags &= ~PFLAG_INVALIDATED;
child.updateDisplayListIfDirty();
child.mRecreateDisplayList = false;
}
能看到这个过程实际上就是调用了子View的updateDisplayListIfDirty方法。
pendingDrawFinished
当一切都处理完毕之后，就会调用pendingDrawFinished。如果mDrawsNeededToReport计数为0，则说明所有需要绘制的命令全部完成了。最后调用reportDrawFinished。
void pendingDrawFinished() {
if (mDrawsNeededToReport == 0) {
throw new RuntimeException("Unbalanced drawPending/pendingDrawFinished calls");
}
mDrawsNeededToReport--;
if (mDrawsNeededToReport == 0) {
reportDrawFinished();
}
}
reportDrawFinished
private void reportDrawFinished() {
try {
mDrawsNeededToReport = 0;
mWindowSession.finishDrawing(mWindow);
} catch (RemoteException e) {
// Have fun!
}
}
能看到最后会调用reportDrawFinished，通知WindowSession已经finishDrawing。
public void finishDrawing(IWindow window) {
if (WindowManagerService.localLOGV) Slog.v(
TAG_WM, "IWindow finishDrawing called for " + window);
mService.finishDrawingWindow(this, window);
}
WMS finishDrawingWindow
void finishDrawingWindow(Session session, IWindow client) {
final long origId = Binder.clearCallingIdentity();
try {
synchronized (mWindowMap) {
WindowState win = windowForClientLocked(session, client, false);
if (win != null && win.mWinAnimator.finishDrawingLocked()) {
if ((win.mAttrs.flags & FLAG_SHOW_WALLPAPER) != 0) {
win.getDisplayContent().pendingLayoutChanges |=
WindowManagerPolicy.FINISH_LAYOUT_REDO_WALLPAPER;
}
win.setDisplayLayoutNeeded();
mWindowPlacerLocked.requestTraversal();
}
}
} finally {
Binder.restoreCallingIdentity(origId);
}
}
会调用WindowState的setDisplayLayoutNeeded。设置DisplayContent的mLayoutNeeded为true。
调用WindowSurfacePlacer的requestTraversal。而这个方法会在AnimationHandler 窗体动画的handler中调用performSurfacePlacement。而这里的逻辑可以阅读WMS在Activity启动中的职责 计算窗体的大小
总结
到这列就完成了onDraw的解析。从onMeasure，onLayout，onDraw四个流程已经过了一遍。但是还没有仔细聊聊硬件渲染，但是没关系，从软件渲染也能一窥整个核心流程了。
老规矩，先来一副时序图：
View的绘制流程.jpg
整个onDraw的入口依次执行了如下的方法：
1.执行该View的background 背景的绘制
2.执行该View重写的onDraw流程，进行绘制
3.dispatchDraw 把绘制行为分发到子View中
4.判断是否有overlay，有则绘制每一个View的浮层的dispatchDraw
5.onDrawForeground 绘制View的前景drawable
6.drawDefaultFocusHighlight 绘制默认的焦点高亮。
每一次进行一次onDraw之前对dirtyOpaque标志位进行判断，实际上就是判断是否是透明的，是透明的就不会调用该View的onDraw方法。
其中dispatchDraw进行绘制行为分发后，就会调用drawChild的方法会每一个子View的draw方法。同时会通过buildOrderedChildList插入排序计算z轴上的绘制的顺序，保证z轴上会根据大小顺序调用drawChild进行覆盖绘制。
这个过程中，软件渲染过程中会伴随着绘制一个Bitmap的缓存。每一个View都能够申请到的缓存最大数值就是：
(宽/高)480 * (宽/高)800 * 4(ARGB8888)
如果超出这个数值就不会出现缓存，或者直到OOM了也会销毁缓存。
当执行完毕之后，必定会调用WMS的finishDrawingWindow，告诉WMS ViewRootImpl已经完成了绘制工作。这个方法会设置DisplayContent的mLayoutNeeded为true。这样就能告诉WMS，当下一轮WMS的relayoutWindow对窗体进行重新测量的时候，允许遍历DisplayContent所有的内容窗体(详细的可以看看我写的WMS在Activity启动中的职责 计算窗体的大小)。
本文已经涉及到不少关于硬件渲染的逻辑，下一篇就来聊聊硬件绘制的原理。
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