红橙Darren视频笔记 view的绘制流程（下）基于API27

关于View的测量代码是如何执行到的以及performMeasure的具体流程可以参见上一篇笔记
之前忘了说明 view的绘制流程里面用到的设计模式是模板设计模式可以参考这篇文章
https://blog.csdn.net/u011109881/article/details/82453771
模板设计模式就是父类制定了操作流程（空方法）等着子类去实现（类似hook的用法）
上一篇讲到ViewRootImpl的performTraversals 方法种执行了三个方法
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);//关键方法1
performLayout(lp, mWidth, mHeight);//关键方法2
performDraw();//关键方法3
并着重分析了performMeasure的后续流程我们从这里开始接着讲述View的绘制流程

一.View的绘制整体流程

如前所述 View的绘制分成三步
1.performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
整个测量过程由外至内传递MeasureSpec 有了父布局的MeasureSpec再计算子view 然后一级级由内至外执行measure方法
子布局的大小是由父布局和子布局一起决定的
2.performLayout(lp, mWidth, mHeight);//关键方法2
用于摆放布局大致流程是for循环遍历子view 使用setChildFrame来摆放每一个布局
3.performDraw();//关键方法3
用于绘制自己还有子view（包括背景等）

二.继performMeasure之后执行的方法performLayout方法

ViewRootImpl performLayout(lp, mWidth, mHeight);–>View layout()–>onLayout()

        private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,int desiredWindowHeight) {mLayoutRequested = false;mScrollMayChange = true;mInLayout = true;final View host = mView;if (host == null) {return;}if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {Log.v(mTag, "Laying out " + host + " to (" +host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")");}Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");try {host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());//关键点mInLayout = false;int numViewsRequestingLayout = mLayoutRequesters.size();if (numViewsRequestingLayout > 0) {// requestLayout() was called during layout.// If no layout-request flags are set on the requesting views, there is no problem.// If some requests are still pending, then we need to clear those flags and do// a full request/measure/layout pass to handle this situation.ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters,false);if (validLayoutRequesters != null) {// Set this flag to indicate that any further requests are happening during// the second pass, which may result in posting those requests to the next// frame insteadmHandlingLayoutInLayoutRequest = true;// Process fresh layout requests, then measure and layoutint numValidRequests = validLayoutRequesters.size();for (int i = 0; i < numValidRequests; ++i) {final View view = validLayoutRequesters.get(i);Log.w("View", "requestLayout() improperly called by " + view +" during layout: running second layout pass");view.requestLayout();}measureHierarchy(host, lp, mView.getContext().getResources(),desiredWindowWidth, desiredWindowHeight);mInLayout = true;host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());mHandlingLayoutInLayoutRequest = false;// Check the valid requests again, this time without checking/clearing the// layout flags, since requests happening during the second pass get noop'dvalidLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true);if (validLayoutRequesters != null) {final ArrayList<View> finalRequesters = validLayoutRequesters;// Post second-pass requests to the next framegetRunQueue().post(new Runnable() {@Overridepublic void run() {int numValidRequests = finalRequesters.size();for (int i = 0; i < numValidRequests; ++i) {final View view = finalRequesters.get(i);Log.w("View", "requestLayout() improperly called by " + view +" during second layout pass: posting in next frame");view.requestLayout();}}});}}}} finally {Trace.traceEnd(Trace.TRACE_TAG_VIEW);}mInLayout = false;}

View

    public void layout(int l, int t, int r, int b) {if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;}int oldL = mLeft;int oldT = mTop;int oldB = mBottom;int oldR = mRight;boolean changed = isLayoutModeOptical(mParent) ?setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {onLayout(changed, l, t, r, b);//关键点if (shouldDrawRoundScrollbar()) {if(mRoundScrollbarRenderer == null) {mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);}} else {mRoundScrollbarRenderer = null;}mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;ListenerInfo li = mListenerInfo;if (li != null && li.mOnLayoutChangeListeners != null) {ArrayList<OnLayoutChangeListener> listenersCopy =(ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();int numListeners = listenersCopy.size();for (int i = 0; i < numListeners; ++i) {listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);}}}final boolean wasLayoutValid = isLayoutValid();mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;if (!wasLayoutValid && isFocused()) {mPrivateFlags &= ~PFLAG_WANTS_FOCUS;if (canTakeFocus()) {// We have a robust focus, so parents should no longer be wanting focus.clearParentsWantFocus();} else if (getViewRootImpl() == null || !getViewRootImpl().isInLayout()) {// This is a weird case. Most-likely the user, rather than ViewRootImpl, called// layout. In this case, there's no guarantee that parent layouts will be evaluated// and thus the safest action is to clear focus here.clearFocusInternal(null, /* propagate */ true, /* refocus */ false);clearParentsWantFocus();} else if (!hasParentWantsFocus()) {// original requestFocus was likely on this view directly, so just clear focusclearFocusInternal(null, /* propagate */ true, /* refocus */ false);}// otherwise, we let parents handle re-assigning focus during their layout passes.} else if ((mPrivateFlags & PFLAG_WANTS_FOCUS) != 0) {mPrivateFlags &= ~PFLAG_WANTS_FOCUS;View focused = findFocus();if (focused != null) {// Try to restore focus as close as possible to our starting focus.if (!restoreDefaultFocus() && !hasParentWantsFocus()) {// Give up and clear focus once we've reached the top-most parent which wants// focus.focused.clearFocusInternal(null, /* propagate */ true, /* refocus */ false);}}}if ((mPrivateFlags3 & PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT) != 0) {mPrivateFlags3 &= ~PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT;notifyEnterOrExitForAutoFillIfNeeded(true);}}

跟到最后我们发现View onLayout是个空实现那么如果以我们上一篇举的例子

<?xml version="1.0" encoding="utf-8"?><!-- step2 在xml中使用自定义属性 -->
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"xmlns:tools="http://schemas.android.com/tools"android:id="@+id/outer"android:layout_width="match_parent"android:layout_height="match_parent"android:orientation="vertical"tools:context=".MainActivity"><LinearLayoutandroid:id="@+id/inner"android:orientation="horizontal"android:layout_width="match_parent"android:layout_height="wrap_content"><TextViewandroid:layout_width="wrap_content"android:layout_height="wrap_content"android:text="text1" /><TextViewandroid:layout_width="wrap_content"android:layout_height="wrap_content"android:text="text2" /><TextViewandroid:layout_width="wrap_content"android:layout_height="wrap_content"android:text="text3" /></LinearLayout></LinearLayout>

对于第二层的LinearLayout inner而言执行的就是LinearLayout 的onLayout方法

    @Overrideprotected void onLayout(boolean changed, int l, int t, int r, int b) {if (mOrientation == VERTICAL) {layoutVertical(l, t, r, b);} else {layoutHorizontal(l, t, r, b);}}

因为我指定了属性排序方式为horizontal 那么我们看看子布局如何摆放

    void layoutHorizontal(int left, int top, int right, int bottom) {final boolean isLayoutRtl = isLayoutRtl();//默认falsefinal int paddingTop = mPaddingTop;int childTop;//子布局开始摆放的y坐标int childLeft;//子布局开始摆放的x坐标// Where bottom of child should gofinal int height = bottom - top;int childBottom = height - mPaddingBottom;// Space available for childint childSpace = height - paddingTop - mPaddingBottom;//可以摆放的高度（需要减去父容器的上下padding）final int count = getVirtualChildCount();final int majorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;final int minorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;final boolean baselineAligned = mBaselineAligned;final int[] maxAscent = mMaxAscent;final int[] maxDescent = mMaxDescent;final int layoutDirection = getLayoutDirection();switch (Gravity.getAbsoluteGravity(majorGravity, layoutDirection)) {//根据LinearLayout不同的Gravity 计算childLeft（子节点的坐标起点x坐标）case Gravity.RIGHT:// mTotalLength contains the padding alreadychildLeft = mPaddingLeft + right - left - mTotalLength;break;case Gravity.CENTER_HORIZONTAL:// mTotalLength contains the padding alreadychildLeft = mPaddingLeft + (right - left - mTotalLength) / 2;break;case Gravity.LEFT:default:childLeft = mPaddingLeft;break;}int start = 0;int dir = 1;//In case of RTL, start drawing from the last child.if (isLayoutRtl) {start = count - 1;dir = -1;}for (int i = 0; i < count; i++) {final int childIndex = start + dir * i;final View child = getVirtualChildAt(childIndex);if (child == null) {//跳过childLeft += measureNullChild(childIndex);} else if (child.getVisibility() != GONE) {final int childWidth = child.getMeasuredWidth();final int childHeight = child.getMeasuredHeight();int childBaseline = -1;final LinearLayout.LayoutParams lp =(LinearLayout.LayoutParams) child.getLayoutParams();if (baselineAligned && lp.height != LayoutParams.MATCH_PARENT) {childBaseline = child.getBaseline();}int gravity = lp.gravity;if (gravity < 0) {gravity = minorGravity;}switch (gravity & Gravity.VERTICAL_GRAVITY_MASK) {//根据子view的gravity计算y方向的起始坐标（每一个子view都是从0开始计算）case Gravity.TOP:childTop = paddingTop + lp.topMargin;if (childBaseline != -1) {childTop += maxAscent[INDEX_TOP] - childBaseline;}break;case Gravity.CENTER_VERTICAL:// Removed support for baseline alignment when layout_gravity or// gravity == center_vertical. See bug #1038483.// Keep the code around if we need to re-enable this feature// if (childBaseline != -1) {//     // Align baselines vertically only if the child is smaller than us//     if (childSpace - childHeight > 0) {//         childTop = paddingTop + (childSpace / 2) - childBaseline;//     } else {//         childTop = paddingTop + (childSpace - childHeight) / 2;//     }// } else {childTop = paddingTop + ((childSpace - childHeight) / 2)+ lp.topMargin - lp.bottomMargin;break;case Gravity.BOTTOM:childTop = childBottom - childHeight - lp.bottomMargin;if (childBaseline != -1) {int descent = child.getMeasuredHeight() - childBaseline;childTop -= (maxDescent[INDEX_BOTTOM] - descent);}break;default:childTop = paddingTop;break;}if (hasDividerBeforeChildAt(childIndex)) {childLeft += mDividerWidth;}childLeft += lp.leftMargin;setChildFrame(child, childLeft + getLocationOffset(child), childTop,childWidth, childHeight);//摆放当前循环的child的位置childLeft += childWidth + lp.rightMargin +getNextLocationOffset(child);//累加 记录下一个摆放child的x起点位置 （注意对比y方向起始点的计算）i += getChildrenSkipCount(child, childIndex);}}}

最终是调用setChildFrame去摆放每一个子view

三.继performLayout之后执行的方法performDraw方法

ViewRootImpl performDraw()–>draw(fullRedrawNeeded);–>drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)
View–>mView.draw(canvas);
draw的几个绘制步骤中有三个比较重要:
1 drawBackground(canvas);画背景
2 onDraw(canvas);绘制自己（ViewGroup默认不会调用原因以及如何修改这一特点参见https://blog.csdn.net/u011109881/article/details/110729417）
3 dispatchDraw(canvas);绘制子view

    private void performDraw() {...try {draw(fullRedrawNeeded);//关键点} finally {mIsDrawing = false;Trace.traceEnd(Trace.TRACE_TAG_VIEW);}...}private void draw(boolean fullRedrawNeeded) {...if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {return;}...}private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,boolean scalingRequired, Rect dirty) {...try {canvas.translate(-xoff, -yoff);if (mTranslator != null) {mTranslator.translateCanvas(canvas);}canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);attachInfo.mSetIgnoreDirtyState = false;mView.draw(canvas);//关键点drawAccessibilityFocusedDrawableIfNeeded(canvas);} finally {if (!attachInfo.mSetIgnoreDirtyState) {// Only clear the flag if it was not set during the mView.draw() callattachInfo.mIgnoreDirtyState = false;}}...return true;}

View

    public void draw(Canvas canvas) {final int privateFlags = mPrivateFlags;mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;/** Draw traversal performs several drawing steps which must be executed* in the appropriate order:**      1. Draw the background*      2. If necessary, save the canvas' layers to prepare for fading*      3. Draw view's content*      4. Draw children*      5. If necessary, draw the fading edges and restore layers*      6. Draw decorations (scrollbars for instance)*/// Step 1, draw the background, if neededint saveCount;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) {// Step 3, draw the contentonDraw(canvas);// Step 4, draw the childrendispatchDraw(canvas);drawAutofilledHighlight(canvas);// Overlay is part of the content and draws beneath Foregroundif (mOverlay != null && !mOverlay.isEmpty()) {mOverlay.getOverlayView().dispatchDraw(canvas);}// Step 6, draw decorations (foreground, scrollbars)onDrawForeground(canvas);// Step 7, draw the default focus highlightdrawDefaultFocusHighlight(canvas);if (debugDraw()) {debugDrawFocus(canvas);}// we're done...return;}/** Here we do the full fledged routine...* (this is an uncommon case where speed matters less,* this is why we repeat some of the tests that have been* done above)*/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;// Step 2, save the canvas' layersint 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;// clip the fade length if top and bottom fades overlap// overlapping fades produce odd-looking artifactsif (verticalEdges && (top + length > bottom - length)) {length = (bottom - top) / 2;}// also clip horizontal fades if necessaryif (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 topSaveCount = -1;int bottomSaveCount = -1;int leftSaveCount = -1;int rightSaveCount = -1;int solidColor = getSolidColor();if (solidColor == 0) {if (drawTop) {topSaveCount = canvas.saveUnclippedLayer(left, top, right, top + length);}if (drawBottom) {bottomSaveCount = canvas.saveUnclippedLayer(left, bottom - length, right, bottom);}if (drawLeft) {leftSaveCount = canvas.saveUnclippedLayer(left, top, left + length, bottom);}if (drawRight) {rightSaveCount = canvas.saveUnclippedLayer(right - length, top, right, bottom);}} else {scrollabilityCache.setFadeColor(solidColor);}// Step 3, draw the contentonDraw(canvas);// Step 4, draw the childrendispatchDraw(canvas);// Step 5, draw the fade effect and restore layersfinal Paint p = scrollabilityCache.paint;final Matrix matrix = scrollabilityCache.matrix;final Shader fade = scrollabilityCache.shader;// must be restored in the reverse order that they were savedif (drawRight) {matrix.setScale(1, fadeHeight * rightFadeStrength);matrix.postRotate(90);matrix.postTranslate(right, top);fade.setLocalMatrix(matrix);p.setShader(fade);if (solidColor == 0) {canvas.restoreUnclippedLayer(rightSaveCount, p);} else {canvas.drawRect(right - length, top, right, bottom, p);}}if (drawLeft) {matrix.setScale(1, fadeHeight * leftFadeStrength);matrix.postRotate(-90);matrix.postTranslate(left, top);fade.setLocalMatrix(matrix);p.setShader(fade);if (solidColor == 0) {canvas.restoreUnclippedLayer(leftSaveCount, p);} else {canvas.drawRect(left, top, left + length, bottom, p);}}if (drawBottom) {matrix.setScale(1, fadeHeight * bottomFadeStrength);matrix.postRotate(180);matrix.postTranslate(left, bottom);fade.setLocalMatrix(matrix);p.setShader(fade);if (solidColor == 0) {canvas.restoreUnclippedLayer(bottomSaveCount, p);} else {canvas.drawRect(left, bottom - length, right, bottom, p);}}if (drawTop) {matrix.setScale(1, fadeHeight * topFadeStrength);matrix.postTranslate(left, top);fade.setLocalMatrix(matrix);p.setShader(fade);if (solidColor == 0) {canvas.restoreUnclippedLayer(topSaveCount, p);} else {canvas.drawRect(left, top, right, top + length, p);}}canvas.restoreToCount(saveCount);drawAutofilledHighlight(canvas);// Overlay is part of the content and draws beneath Foregroundif (mOverlay != null && !mOverlay.isEmpty()) {mOverlay.getOverlayView().dispatchDraw(canvas);}// Step 6, draw decorations (foreground, scrollbars)onDrawForeground(canvas);if (debugDraw()) {debugDrawFocus(canvas);}}

再次注意draw里面使用了设计模式种的模板设计模式即规定了方法的顺序（模板）但是可能没给出具体的实现，具体的实现由不同的子类根据各自的情况实现。
注意draw步骤
* 1. Draw the background
* 2. If necessary, save the canvas’ layers to prepare for fading
* 3. Draw view’s content
* 4. Draw children
* 5. If necessary, draw the fading edges and restore layers
* 6. Draw decorations (scrollbars for instance)

四思考

比如 addView setVisibility invalidate setBackground等都会调用到performTraversals然后触发view的绘制流程但是可能跳过某些步骤
代码优化：onDraw的调用由外到内又由内至外非常复杂尽量不要嵌套并且不要过度依赖性能优化工具，再厉害的工具往往也有检测不出的问题了解源码的基本运行原理才是王道