android draw的绘制流程与measure测量流程类似，我们先来回顾一下measure的调用流程图，如下所示
今天我们要讲的是draw的绘制流程，所以直接从PerformDraw来开始分析（顺便纠正一下上图中的英文单词perfrom,应该是form）.
这里就不贴ViewRootImpl.PerformDraw函数的源码了，通过看源码,它内部的调用过程如下：
ViewRootImpl.PerformDraw函数的源码了，通过看源码,它内部的调用过程如下：
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* 1. Draw the background
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* 2. If necessary, save the canvas' layers to prepare for fading
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* 3. Draw view's content
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* 4. Draw children
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* 5. If necessary, draw the fading edges and restore layers
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* 6. Draw decorations (scrollbars for instance)
上述6个步骤就是在View的draw(Canvas canvas)函数里调用的，我们在此贴出draw函数的代码：
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public void draw(Canvas canvas) {
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final int privateFlags = mPrivateFlags;
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final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
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(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
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mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
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/*
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* Draw traversal performs several drawing steps which must be executed
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* in the appropriate order:
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*
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* 1. Draw the background
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* 2. If necessary, save the canvas' layers to prepare for fading
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* 3. Draw view's content
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* 4. Draw children
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* 5. If necessary, draw the fading edges and restore layers
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* 6. Draw decorations (scrollbars for instance)
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*/
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// Step 1, draw the background, if needed
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int saveCount;
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if (!dirtyOpaque) {
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drawBackground(canvas);
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}
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// skip step 2 & 5 if possible (common case)
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final int viewFlags = mViewFlags;
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boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
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boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
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if (!verticalEdges && !horizontalEdges) {
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// Step 3, draw the content
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if (!dirtyOpaque) onDraw(canvas);
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// Step 4, draw the children
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dispatchDraw(canvas);
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drawAutofilledHighlight(canvas);
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// Overlay is part of the content and draws beneath Foreground
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if (mOverlay != null && !mOverlay.isEmpty()) {
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mOverlay.getOverlayView().dispatchDraw(canvas);
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}
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// Step 6, draw decorations (foreground, scrollbars)
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onDrawForeground(canvas);
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// Step 7, draw the default focus highlight
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drawDefaultFocusHighlight(canvas);
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if (debugDraw()) {
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debugDrawFocus(canvas);
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}
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// we're done...
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return;
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}
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/*
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* Here we do the full fledged routine...
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* (this is an uncommon case where speed matters less,
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* this is why we repeat some of the tests that have been
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* done above)
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*/
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boolean drawTop = false;
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boolean drawBottom = false;
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boolean drawLeft = false;
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boolean drawRight = false;
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float topFadeStrength = 0.0f;
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float bottomFadeStrength = 0.0f;
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float leftFadeStrength = 0.0f;
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float rightFadeStrength = 0.0f;
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// Step 2, save the canvas' layers
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int paddingLeft = mPaddingLeft;
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final boolean offsetRequired = isPaddingOffsetRequired();
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if (offsetRequired) {
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paddingLeft += getLeftPaddingOffset();
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}
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int left = mScrollX + paddingLeft;
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int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
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int top = mScrollY + getFadeTop(offsetRequired);
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int bottom = top + getFadeHeight(offsetRequired);
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if (offsetRequired) {
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right += getRightPaddingOffset();
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bottom += getBottomPaddingOffset();
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}
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final ScrollabilityCache scrollabilityCache = mScrollCache;
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final float fadeHeight = scrollabilityCache.fadingEdgeLength;
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int length = (int) fadeHeight;
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// clip the fade length if top and bottom fades overlap
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// overlapping fades produce odd-looking artifacts
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if (verticalEdges && (top + length > bottom - length)) {
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length = (bottom - top) / 2;
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}
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// also clip horizontal fades if necessary
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if (horizontalEdges && (left + length > right - length)) {
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length = (right - left) / 2;
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}
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if (verticalEdges) {
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topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
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drawTop = topFadeStrength * fadeHeight > 1.0f;
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bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
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drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
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}
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if (horizontalEdges) {
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leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
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drawLeft = leftFadeStrength * fadeHeight > 1.0f;
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rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
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drawRight = rightFadeStrength * fadeHeight > 1.0f;
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}
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saveCount = canvas.getSaveCount();
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int solidColor = getSolidColor();
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if (solidColor == 0) {
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final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;
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if (drawTop) {
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canvas.saveLayer(left, top, right, top + length, null, flags);
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}
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if (drawBottom) {
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canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
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}
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if (drawLeft) {
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canvas.saveLayer(left, top, left + length, bottom, null, flags);
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}
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if (drawRight) {
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canvas.saveLayer(right - length, top, right, bottom, null, flags);
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}
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} else {
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scrollabilityCache.setFadeColor(solidColor);
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}
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// Step 3, draw the content
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if (!dirtyOpaque) onDraw(canvas);
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// Step 4, draw the children
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dispatchDraw(canvas);
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// Step 5, draw the fade effect and restore layers
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final Paint p = scrollabilityCache.paint;
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final Matrix matrix = scrollabilityCache.matrix;
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final Shader fade = scrollabilityCache.shader;
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if (drawTop) {
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matrix.setScale(1, fadeHeight * topFadeStrength);
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matrix.postTranslate(left, top);
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fade.setLocalMatrix(matrix);
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p.setShader(fade);
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canvas.drawRect(left, top, right, top + length, p);
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}
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if (drawBottom) {
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matrix.setScale(1, fadeHeight * bottomFadeStrength);
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matrix.postRotate(180);
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matrix.postTranslate(left, bottom);
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fade.setLocalMatrix(matrix);
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p.setShader(fade);
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canvas.drawRect(left, bottom - length, right, bottom, p);
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}
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if (drawLeft) {
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matrix.setScale(1, fadeHeight * leftFadeStrength);
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matrix.postRotate(-90);
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matrix.postTranslate(left, top);
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fade.setLocalMatrix(matrix);
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p.setShader(fade);
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canvas.drawRect(left, top, left + length, bottom, p);
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}
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if (drawRight) {
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matrix.setScale(1, fadeHeight * rightFadeStrength);
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matrix.postRotate(90);
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matrix.postTranslate(right, top);
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fade.setLocalMatrix(matrix);
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p.setShader(fade);
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canvas.drawRect(right - length, top, right, bottom, p);
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}
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canvas.restoreToCount(saveCount);
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drawAutofilledHighlight(canvas);
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// Overlay is part of the content and draws beneath Foreground
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if (mOverlay != null && !mOverlay.isEmpty()) {
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mOverlay.getOverlayView().dispatchDraw(canvas);
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}
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// Step 6, draw decorations (foreground, scrollbars)
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onDrawForeground(canvas);
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if (debugDraw()) {
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debugDrawFocus(canvas);
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}
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}
1. step 1 第一步
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if (!dirtyOpaque) {
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drawBackground(canvas);
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}
不透明时，绘制背景。
// skip step 2 & 5 if possible (common case)
这条注释表示通常情况下step2和step5是不需要的，那我们直接看step3:draw the content.
2. step 3 第三步
if (!dirtyOpaque) onDraw(canvas);不透明时，绘制内容，通常见我们自定义View需要重写onDraw函数。
3. step 4 第四步
dispatchDraw(canvas);
绘制分发，主要是绘制当前父容器的子View，然后各个子View也会调用它自己的draw方法，即递归调用当前的draw方法，当然如果子View正好也是一个ViewGroup的话，它又会去调用dispatchDraw(canvas)方法，
以此类推，直到整个递归完成；如果子View不是一个ViewGroup容器，那就直接调用View.dispatchDraw,我们定位到View.dispatchDraw发现它是一个空方法，如下：
/**
* Called by draw to draw the child views. This may be overridden
* by derived classes to gain control just before its children are drawn
* (but after its own view has been drawn).
* @param canvas the canvas on which to draw the view
*/
protected void dispatchDraw(Canvas canvas) {
}
通过注释可以看到，这个函数通常是被View的子类重写，用于绘制容器内部的子View的。这里我们可以把View的子类当作ViewGroup，在ViewGroup里会重写dispatchDraw这个方法，用来绘制child。
以下是ViewGroup重写后的dispatchDraw函数：
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@Override
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protected void dispatchDraw(Canvas canvas) {
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boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
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final int childrenCount = mChildrenCount;
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final View[] children = mChildren;
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int flags = mGroupFlags;
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if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {
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final boolean buildCache = !isHardwareAccelerated();
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for (int i = 0; i < childrenCount; i++) {
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final View child = children[i];
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if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {
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final LayoutParams params = child.getLayoutParams();
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attachLayoutAnimationParameters(child, params, i, childrenCount);
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bindLayoutAnimation(child);
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}
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}
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final LayoutAnimationController controller = mLayoutAnimationController;
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if (controller.willOverlap()) {
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mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;
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}
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controller.start();
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mGroupFlags &= ~FLAG_RUN_ANIMATION;
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mGroupFlags &= ~FLAG_ANIMATION_DONE;
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if (mAnimationListener != null) {
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mAnimationListener.onAnimationStart(controller.getAnimation());
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}
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}
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int clipSaveCount = 0;
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final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
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if (clipToPadding) {
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clipSaveCount = canvas.save(Canvas.CLIP_SAVE_FLAG);
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canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,
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mScrollX + mRight - mLeft - mPaddingRight,
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mScrollY + mBottom - mTop - mPaddingBottom);
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}
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// We will draw our child's animation, let's reset the flag
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mPrivateFlags &= ~PFLAG_DRAW_ANIMATION;
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mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED;
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boolean more = false;
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final long drawingTime = getDrawingTime();
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if (usingRenderNodeProperties) canvas.insertReorderBarrier();
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final int transientCount = mTransientIndices == null ? 0 : mTransientIndices.size();
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int transientIndex = transientCount != 0 ? 0 : -1;
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// Only use the preordered list if not HW accelerated, since the HW pipeline will do the
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// draw reordering internally
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final ArrayList<View> preorderedList = usingRenderNodeProperties
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? null : buildOrderedChildList();
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final boolean customOrder = preorderedList == null
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&& isChildrenDrawingOrderEnabled();
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for (int i = 0; i < childrenCount; i++) {
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while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
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final View transientChild = mTransientViews.get(transientIndex);
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if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
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transientChild.getAnimation() != null) {
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more |= drawChild(canvas, transientChild, drawingTime);
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}
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transientIndex++;
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if (transientIndex >= transientCount) {
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transientIndex = -1;
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}
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}
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final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
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final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
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if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
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more |= drawChild(canvas, child, drawingTime);
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}
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}
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while (transientIndex >= 0) {
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// there may be additional transient views after the normal views
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final View transientChild = mTransientViews.get(transientIndex);
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if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
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transientChild.getAnimation() != null) {
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more |= drawChild(canvas, transientChild, drawingTime);
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}
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transientIndex++;
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if (transientIndex >= transientCount) {
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break;
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}
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}
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if (preorderedList != null) preorderedList.clear();
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// Draw any disappearing views that have animations
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if (mDisappearingChildren != null) {
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final ArrayList<View> disappearingChildren = mDisappearingChildren;
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final int disappearingCount = disappearingChildren.size() - 1;
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// Go backwards -- we may delete as animations finish
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for (int i = disappearingCount; i >= 0; i--) {
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final View child = disappearingChildren.get(i);
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more |= drawChild(canvas, child, drawingTime);
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}
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}
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if (usingRenderNodeProperties) canvas.insertInorderBarrier();
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if (debugDraw()) {
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onDebugDraw(canvas);
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}
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if (clipToPadding) {
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canvas.restoreToCount(clipSaveCount);
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}
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// mGroupFlags might have been updated by drawChild()
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flags = mGroupFlags;
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if ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) {
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invalidate(true);
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}
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if ((flags & FLAG_ANIMATION_DONE) == 0 && (flags & FLAG_NOTIFY_ANIMATION_LISTENER) == 0 &&
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mLayoutAnimationController.isDone() && !more) {
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// We want to erase the drawing cache and notify the listener after the
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// next frame is drawn because one extra invalidate() is caused by
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// drawChild() after the animation is over
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mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER;
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final Runnable end = new Runnable() {
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@Override
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public void run() {
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notifyAnimationListener();
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}
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};
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post(end);
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}
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}
第一个for循环用于布局动画；第二个for循环绘制各子View，它内部调用了drawChild(canvas, child, drawingTime)：
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/**
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* Draw one child of this View Group. This method is responsible for getting
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* the canvas in the right state. This includes clipping, translating so
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* that the child's scrolled origin is at 0, 0, and applying any animation
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* transformations.
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*
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* @param canvas The canvas on which to draw the child
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* @param child Who to draw
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* @param drawingTime The time at which draw is occurring
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* @return True if an invalidate() was issued
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*/
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protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
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return child.draw(canvas, this, drawingTime);
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}
我们发现drawChild又调用了child.draw，即View中的draw函数，我们发现这个child.draw函数的参数和之前不一样，但是它内部还是调用了我们之前的View.draw(Canvas canvas),
来绘制自己的内容及递归分发绘制子View。
4. step6. 第六步 绘制装饰
onDrawForeground(canvas)
这里主要绘制滚动条、及各View的foreGround属性设置的图片，颜色值、水波纹等。
Ok,draw绘制流程就聊到这里，其实它与layout、measure都有一个共性，就是递归View树。
文章来源: blog.csdn.net，作者：冉航--小虾米，版权归原作者所有，如需转载，请联系作者。
原文链接：blog.csdn.net/gaoxiaoweiandy/article/details/103655089