前面已经介绍了视图的测量（onMeasure）和视图的摆放（onLayout），这两者结合可以实现自定义ViewGroup，瀑布流布局就是一个典型的自定义ViewGroup，接下来将要讲解自定义View，自定义View的核心实现方法是onDraw，本文主要目的是带领大家一起分析源码，最终找到视图的
绘制步骤。
onDraw是自定义View主要实现的方法，在源码中，执行onDraw的入口是ViewRootImpl类的performDraw方法。
ViewRootImpl.java
private void performDraw() {
//...（省略）
boolean canUseAsync = draw(fullRedrawNeeded);
//...（省略）
}
在performDraw方法中的核心方法是draw，我们进入draw方法，如下
private boolean draw(boolean fullRedrawNeeded) {
Surface surface = mSurface;
//...（省略）
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
scalingRequired, dirty, surfaceInsets)) {
return false;
}
//...（省略）
return useAsyncReport;
}
draw方法里有一个drawSoftware方法，参数surface我将它理解成一个画板，或者是一个预览界面，就像摄像头拍照前的预览界面。
继续往下走，进入drawSoftware方法
/**
* @return true if drawing was successful, false if an error occurred
*/
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty, Rect surfaceInsets) {
// Draw with software renderer.
final Canvas canvas;
//...（省略）
canvas = mSurface.lockCanvas(dirty);
//...（省略）
mView.draw(canvas);
//...（省略）
return true;
}
Canvas是一个放在画板上的画布，通过Surface获取一张画布
canvas = mSurface.lockCanvas(dirty);
就像相机点击拍照按钮时生成的一张图像，图像是放在一张白纸或者底片上的，这个白纸或者底片就相当于一张画布（Canvas），那么图像是怎么绘制到白纸或者底片上的呢？请进入draw(canvas)方法一探究竟。
/**
* Manually render this view (and all of its children) to the given Canvas.
* The view must have already done a full layout before this function is
* called. When implementing a view, implement
* {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.
* If you do need to override this method, call the superclass version.
*
* @param canvas The Canvas to which the View is rendered.
*/
@CallSuper
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;
/*
* 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 needed
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) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
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);
// Step 7, draw the default focus highlight
drawDefaultFocusHighlight(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' layers
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;
// clip the fade length if top and bottom fades overlap
// overlapping fades produce odd-looking artifacts
if (verticalEdges && (top + length > bottom - length)) {
length = (bottom - top) / 2;
}
// also clip horizontal fades if necessary
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);
if (debugDraw()) {
debugDrawFocus(canvas);
}
}
以上代码中有这样一段注释，如下：
/*
* 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)
*/
中文翻译如下：
绘制遍历执行几个绘制步骤，这些步骤必须按适当的顺序执行：
1、对View的背景进行绘制
2、保存当前的图层信息
3、绘制View的内容
4、对View的子View进行绘制(如果有子View)
5、绘制View的褪色的边缘，类似于阴影效果
6、绘制View的装饰（例如滚动条）
以上6个步骤就是视图绘制的6大步骤，请谨记。
下面开始简单介绍下这几个步骤
【第一步】 对View的背景进行绘制
if (!dirtyOpaque) {
drawBackground(canvas);
}
【第二步】 保存当前的图层信息
源码中，有这样一段注释
// skip step 2 & 5 if possible (common case)
意思是说：如果有可能，将跳过第二步和第五步。
【第三步】 绘制View的内容
if (!dirtyOpaque) onDraw(canvas);
这里的onDraw方法，这是我们上层自定义View需要重写的方法。
【第四步】 对View的子View进行绘制(如果有子View)
dispatchDraw(canvas);
如果有必要，可以在上层重写dispatchDraw(canvas)，对子View进行绘制。
【第五步】 绘制View的褪色的边缘，类似于阴影效果
源码中，有这样一段注释
// skip step 2 & 5 if possible (common case)
意思是说：如果有可能，将跳过第二步和第五步。
【第六步】 绘制View的装饰（例如滚动条）
//绘制装饰（前景，滚动条）
onDrawForeground(canvas);
【第七步】 绘制默认焦点突出显示
drawDefaultFocusHighlight(canvas);
其实，在源码中，引出了第七步，以上源码注释中的步骤可能被Google程序员漏写了。
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