Read The Fucking Source Code
引言
Android自定义控件涉及View的绘制分发流程
源码版本（Android Q — API 29）
本文涉及Android绘制流程
1. 顶层视角预览 draw
2. draw
2.1 View和ViewGroup的区别
View：View不执行dispatchDraw（子View的绘制）。
ViewGroup：ViewGroup执行dispatchDraw（子View的递归绘制）。
2.2 View和ViewGroup的汇总
2.2.1 View的 draw 过程（紫色代表可重写）
2.2.2 ViewGroup的 draw 过程（紫色代表可重写）
2.2.3 draw 各个过程的含义
2.3 draw自顶向下
2.3.1 最顶层（DecorView）分发的draw
2.3.1.1 我们来看ViewRootImpl中的performDraw()方法。
private void performDraw() {
//代码省略……
try {
//绘制分发
boolean canUseAsync = draw(fullRedrawNeeded);
if (usingAsyncReport && !canUseAsync) {
mAttachInfo.mThreadedRenderer.setFrameCompleteCallback(null);
usingAsyncReport = false;
}
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
//代码省略……
}
2.3.1.2 我们来看ViewRootImpl中的draw()方法。
private boolean draw(boolean fullRedrawNeeded) {
Surface surface = mSurface;
//代码省略……
//绘制分发
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
scalingRequired, dirty, surfaceInsets)) {
return false;
}
}
}
//代码省略……
}
2.3.1.3 我们来看ViewRootImpl中的drawSoftware()方法。
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty, Rect surfaceInsets) {
//代码省略……
//canvas获取
canvas = mSurface.lockCanvas(dirty);
//代码省略……
//绘制分发，终于到了DecorView的绘制分发
mView.draw(canvas);
//代码省略……
}
2.3.2 ViewGroup分发的draw
2.3.2.1 我们来看ViewGroup中的dispatchDraw()方法。
@Override
protected void dispatchDraw(Canvas canvas) {
//代码省略……
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) {
//遍历子View绘制
more |= drawChild(canvas, transientChild, drawingTime);
}
transientIndex++;
if (transientIndex >= transientCount) {
transientIndex = -1;
}
}
//代码省略……
}
2.3.2.2 我们来看ViewGroup中的drawChild()方法。
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
//递归遍历子View的绘制
return child.draw(canvas, this, drawingTime);
}
2.3.3 View分发的draw
2.3.3.1 我们来看View中的draw()方法。
@CallSuper
public void draw(Canvas canvas) {
//代码省略……
// Step 1, draw the background, if needed
int 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 content
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;
}
//代码省略……
// Step 3, draw the content
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;
//代码省略……
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);
}
}
2.3.3.2 View中的drawBackground()方法：绘制背景。
2.3.3.3 我们来看View中的onDraw()方法。
/**
* //需要具体控件自己实现，比如TextView
* Implement this to do your drawing.
*
* @param canvas the canvas on which the background will be drawn
*/
protected void onDraw(Canvas canvas) {
}
2.3.3.4 我们来看View中的dispatchDraw()方法。
/**
* //由draw调用以绘制子视图。这可能会被派生类重写，以便在绘制其子对象之前（但在绘制其自己的视图之后）获得控制权
* 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) {
}
2.3.3.5 View中的drawAutofilledHighlight()方法：绘制自动填充视图的高亮。
2.3.3.6 View中的ViewOverlay::dispatchDraw()方法：当覆盖层存在时，则绘制覆盖层。
2.3.3.7 我们来看View中的onDrawForeground()方法。
public void onDrawForeground(Canvas canvas) {
//绘制滚动指示器
onDrawScrollIndicators(canvas);
//绘制滚动条
onDrawScrollBars(canvas);
//绘制前景
final Drawable foreground = mForegroundInfo != null ? mForegroundInfo.mDrawable : null;
if (foreground != null) {
if (mForegroundInfo.mBoundsChanged) {
mForegroundInfo.mBoundsChanged = false;
final Rect selfBounds = mForegroundInfo.mSelfBounds;
final Rect overlayBounds = mForegroundInfo.mOverlayBounds;
if (mForegroundInfo.mInsidePadding) {
selfBounds.set(0, 0, getWidth(), getHeight());
} else {
selfBounds.set(getPaddingLeft(), getPaddingTop(),
getWidth() - getPaddingRight(), getHeight() - getPaddingBottom());
}
final int ld = getLayoutDirection();
Gravity.apply(mForegroundInfo.mGravity, foreground.getIntrinsicWidth(),
foreground.getIntrinsicHeight(), selfBounds, overlayBounds, ld);
foreground.setBounds(overlayBounds);
}
foreground.draw(canvas);
}
}
2.3.3.8 View中的drawDefaultFocusHighlight方法：绘制默认焦点的高亮。
2.3.3.9 View中的debugDrawFocus方法：实就是开发者选项中的显示布局边界的效果。
2.4 问题思考
ViewGroup 为什么无法通过 onDraw 方法绘制自定义内容？如果我希望重写 onDraw 来在 ViewGroup 进行一些绘制操作，怎样才能看到效果？
- ViewGroup 无法显示绘制内容是因为默认设置的标记位关闭执行了它的 drawBackground 和 onDraw 方法。ViewGroup 一般就是管理布局相关的事项，不处理自身的绘制有助于提高执行效率。
- 解决思路：调用 View.setWillNotDraw(false)；为 ViewGroup 设置背景图。
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