View 的绘制系列文章：
Android View 绘制流程之 DecorView 与 ViewRootImpl
Android View 的绘制流程之 Measure 过程详解 (一)
Android View 的绘制流程之 Layout 和 Draw 过程详解 (二)
Android View 的事件分发原理解析
在上一篇 Android View 的绘制流程之 Measure 过程详解 (一)，已经详细的分析了 DecorView 和其子 View 的测量过程，接下去就要开始讲 layout 和 draw 流程。下面开始进入分析：
DecorView Layout 阶段
在 ViewRootImpl 中，调用 performLayout 方法来确定 DecorView 在屏幕中的位置，下面看下具体的代码逻辑：
// ViewRootImpl
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 {
// 根据测量结果进行绘制
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); 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 instead
mHandlingLayoutInLayoutRequest = true; // Process fresh layout requests, then measure and layout
int 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'd
validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true);
if (validLayoutRequesters != null) {
final ArrayList<View> finalRequesters = validLayoutRequesters;
// Post second-pass requests to the next frame
getRunQueue().post(new Runnable() {
@Override
public 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;
}
当在 layout 绘制过程中，收到了关于重新 layout 的请求，会先判断这些请求里面哪些是有效的，如果是有效的，那么就必须先处理，清除 layout-request flags （View.PFLAG_FORCE_LAYOUT）这些标记，再做一次彻底的重绘工作：重新测量，layout。
那么对于 DecorView 来说，调用 layout 方法，就是对它自身进行布局，注意到传递的参数分别是 0，0， host.getMeasuredWidth, host.getMeasuredHeigh，它们分别代表了一个 View 的上下左右四个位置，显然，DecorView 的左上位置为 0，然后宽高为它的测量宽高，下面来看 layout 的具体代码：
// ViewGroup
public final void layout(int l, int t, int r, int b) {
if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {
if (mTransition != null) {
mTransition.layoutChange(this);
}
super.layout(l, t, r, b);
} else {
// record the fact that we noop'd it; request layout when transition finishes
mLayoutCalledWhileSuppressed = true;
}
}
由于 ViewGroup 的 layout 方法是 final 类型，子类不能重写，这里调用了父类的 View#layout 方法，下面看看该方法是如何操作的：
// 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;
// 设置界面的显示大小
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
// 发生了改变，或者是需要重新 layout，那么就会进入 onLayout 逻辑
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
// 开始
onLayout(changed, l, t, r, b); if (shouldDrawRoundScrollbar()) {
if(mRoundScrollbarRenderer == null) {
mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
}
} else {
mRoundScrollbarRenderer = null;
}
// 清除请求 layout 的标记
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 focus
clearFocusInternal(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);
}
}
调用了 setFrame 方法，并把四个位置信息传递进去，这个方法用于确定 View 的四个顶点的位置，看下具体的代码：
protected boolean setFrame(int left, int top, int right, int bottom) {
boolean changed = false;
if (DBG) {
Log.d(VIEW_LOG_TAG, this + " View.setFrame(" + left + "," + top + ","
+ right + "," + bottom + ")");
}
// 有一个不一样说明发生了改变
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
changed = true;
// Remember our drawn bit
int drawn = mPrivateFlags & PFLAG_DRAWN;
int oldWidth = mRight - mLeft;
int oldHeight = mBottom - mTop;
int newWidth = right - left;
int newHeight = bottom - top;
boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);
// Invalidate our old position
invalidate(sizeChanged);
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);
// 做好标记
mPrivateFlags |= PFLAG_HAS_BOUNDS;
// size 改变的回调
if (sizeChanged) {
sizeChange(newWidth, newHeight, oldWidth, oldHeight);
}
if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) {
// If we are visible, force the DRAWN bit to on so that
// this invalidate will go through (at least to our parent).
// This is because someone may have invalidated this view
// before this call to setFrame came in, thereby clearing
// the DRAWN bit.
mPrivateFlags |= PFLAG_DRAWN;
invalidate(sizeChanged);
// parent display list may need to be recreated based on a change in the bounds
// of any child
invalidateParentCaches();
}
// Reset drawn bit to original value (invalidate turns it off)
mPrivateFlags |= drawn;
mBackgroundSizeChanged = true;
mDefaultFocusHighlightSizeChanged = true;
if (mForegroundInfo != null) {
mForegroundInfo.mBoundsChanged = true;
}
notifySubtreeAccessibilityStateChangedIfNeeded();
}
return changed;
}
这里我们看到它对 mLeft、mTop、mRight、mBottom 这四个值进行了重新赋值，对于每一个View，包括 ViewGroup 来说，以上四个值保存了 Viwe 的位置信息，所以这四个值是最终宽高，也即是说，如果要得到 View 的位置信息，那么就应该在 layout 方法完成后调用 getLeft()、getTop() 等方法来取得最终宽高，如果是在此之前调用相应的方法，只能得到 0 的结果。当初始化完毕后，ViewGroup 的布局流程也就完成了。
赋值后，前后对比大小，如果发生了改变，就会调用了 sizeChange（）方法，最终会回调 onSizeChanged，标明 view 的尺寸发生了变化，第一次 laout 的时候也会调用。在自定义view 的时候，可以在这里获取控件的宽和高度。
private void sizeChange(int newWidth, int newHeight, int oldWidth, int oldHeight) {
onSizeChanged(newWidth, newHeight, oldWidth, oldHeight);
......
}
子 View Layout 流程
当 DecorView 确定好了自己的位置之后，开始调用 onLayout 来确定子 view 的位置。对于 onLayout 方法，View 和 ViewGroup 类是空实现，接下来看 FrameLayout 的实现：
// FrameLayout
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
可以看到，该方法调用了 layoutChildren 来确定子 view 的位置。
// FrameLaout
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
final int count = getChildCount();
// 获取 DecoverView 剩余的空间范围
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground(); final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground(); for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams(); final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight(); int childLeft;
int childTop;
// DEFAULT_CHILD_GRAVITY = Gravity.TOP | Gravity.START 默认是在左上角
int gravity = lp.gravity;
if (gravity == -1) {
gravity = DEFAULT_CHILD_GRAVITY;
} final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
// 确定左边起始点
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT:
default:
childLeft = parentLeft + lp.leftMargin;
}
// 确定上边起始点
switch (verticalGravity) {
case Gravity.TOP:
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
childTop = parentTop + lp.topMargin;
} child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
先梳理一下以上逻辑：
首先先获取父容器的 padding 值，得到 DecorView 的可用于显示的空间范围。
然后遍历其每一个子 View，根据子 View 的 layout_gravity 属性、子 View 的测量宽高、父容器的 padding 值、来确定子 View 的左上角的坐标位置
然后调用 child.layout 方法，参数是左上角坐标和自身宽高结合起来的，这样就可以确定子 View 的位置。
最终调用 layout 是 View#layout，前面已经分析过，就不在分析了。
可以看到子 View 的布局流程也很简单，如果子 View 是一个 ViewGroup，那么就会重复以上步骤，如果是一个 View，那么会直接调用 View#layout 方法，根据以上分析，在该方法内部会设置 view 的四个布局参数，接着调用 onLayout 方法 ：
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
此方法是一个空方法，也就是说需要子类去实现此方法，不同的 View 实现方式不同，这里就不分析了。
layout阶段的基本思想也是由根View开始，递归地完成整个控件树的布局（layout）工作。
对于宽高的获取这里在总结下：
这里需要注意一下，在非一般情况下，也就是通过人为设置，重写View的layout()强行设置，这种情况下，测量的值与最终的值是不一样的。
Layout 整体的流程图
上面分别从 View 和 ViewGroup 的角度讲解了布局流程，这里再以流程图的形式归纳一下整个 Layout 过程，便于加深记忆：
DecorView Draw 流程
Draw 的入口也是在 ViewRootImpl 中，执行 ViewRootImpl#performTraversals 中会执行 ViewRootIml#performDraw：
private void performDraw() {
...
//fullRedrawNeeded，它的作用是判断是否需要重新绘制全部视图
draw(fullRedrawNeeded);
...
}
然后会执行到 ViewRootImpl#draw：
private void draw(boolean fullRedrawNeeded) {
...
//获取mDirty，该值表示需要重绘的区域
final Rect dirty = mDirty;
if (mSurfaceHolder != null) {
// The app owns the surface, we won't draw.
dirty.setEmpty();
if (animating) {
if (mScroller != null) {
mScroller.abortAnimation();
}
disposeResizeBuffer();
}
return;
}
//如果fullRedrawNeeded为真，则把dirty区域置为整个屏幕，表示整个视图都需要绘制
//第一次绘制流程，需要绘制所有视图
if (fullRedrawNeeded) {
mAttachInfo.mIgnoreDirtyState = true;
dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
}
...
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
}
接着会执行到 ViewRootIml#drawSoftware，然后在 ViewRootIml#drawSoftware 会执行到 mView.draw(canvas)。
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
final Canvas canvas;
//锁定canvas区域，由dirty区域决定
//这个canvas就是我们想在上面绘制东西的画布
canvas = mSurface.lockCanvas(dirty);
...
//画布支持位图的密度，和手机分辨率相关
canvas.setDensity(mDensity);
...
if (!canvas.isOpaque() || yoff != 0 || xoff != 0) {
canvas.drawColor(0, PorterDuff.Mode.CLEAR);
}
...
canvas.translate(-xoff, -yoff);
...
//正式开始绘制
mView.draw(canvas);
...
//提交需要绘制的东西
surface.unlockCanvasAndPost(canvas);
}
mView.draw(canvas) 开始真正的绘制。此处 mView 就是 DecorView，先看 DecorView 中 Draw 的方法：
public void draw(Canvas canvas) {
super.draw(canvas);
if (mMenuBackground != null) {
mMenuBackground.draw(canvas);
}
}
里面会调用 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;
/*
* 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;
//绘制背景
if (!dirtyOpaque) {
drawBackground(canvas);
}
// 如果可以跳过2和5步
final int viewFlags = mViewFlags;
//判断是否有绘制衰退边缘的标示
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
// 如果没有绘制衰退边缘只需要3,4,6步
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(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);
// 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) {
final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;
if (drawTop) {
canvas.saveLayer(left, top, right, top + length, null, flags);
}
if (drawBottom) {
canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
}
if (drawLeft) {
canvas.saveLayer(left, top, left + length, bottom, null, flags);
}
if (drawRight) {
canvas.saveLayer(right - length, top, right, bottom, null, flags);
}
} 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);
// 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);
}
可以看到，draw过程比较复杂，但是逻辑十分清晰，而官方注释也清楚地说明了每一步的做法。我们首先来看一开始的标记位 dirtyOpaque，该标记位的作用是判断当前 View 是否是透明的，如果 View 是透明的，那么根据下面的逻辑可以看出，将不会执行一些步骤，比如绘制背景、绘制内容等。这样很容易理解，因为一个 View 既然是透明的，那就没必要绘制它了。接着是绘制流程的六个步骤，这里先小结这六个步骤分别是什么，然后再展开来讲。绘制流程的六个步骤：
对 View 的背景进行绘制
保存当前的图层信息(可跳过)
绘制 View 的内容
对 View 的子 View 进行绘制(如果有子 View )
绘制 View 的褪色的边缘，类似于阴影效果(可跳过)
绘制 View 的装饰（例如：滚动条）
其中第2步和第5步是可以跳过的，我们这里不做分析，我们重点来分析其它步骤。
ViewGroup子类默认情况下就是不执行 onDraw 方法的，在 ViewGroup 源码中的 initViewGroup() 方法中设置了一个标记，源码如下：
private void initViewGroup() {
// ViewGroup doesn't draw by default
if (!debugDraw()) {
setFlags(WILL_NOT_DRAW, DRAW_MASK);
}
......
}
看第二行注释也知道，ViewGroup 默认情况下是不会 draw 的。第四行调用 setFlags 方法设置标记 WILL_NOT_DRAW，在回到 View 中 draw 方法看第2行代码：
1 final int privateFlags = mPrivateFlags;
2 final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
3 (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
setFlags 方法就是对 View中mPrivateFlags 值进行相应改变，我们设置标记 WILL_NOT_DRAW 那么 dirtyOpaque 得到的值就为 true，从而 if (!dirtyOpaque) 不成立，也就不会执行onDraw 方法。
1. 绘制背景
View#drawBackground
private void drawBackground(Canvas canvas) {
//获取背景的Drawable，没有就不需要绘制
final Drawable background = mBackground;
if (background == null) {
return;
}
//确定背景Drawable边界
setBackgroundBounds();
...
//如果有偏移量先偏移画布再将drawable绘制上去
final int scrollX = mScrollX;
final int scrollY = mScrollY;
if ((scrollX | scrollY) == 0) {
background.draw(canvas);
} else {
canvas.translate(scrollX, scrollY);
//此处会执行各种Drawable对应的draw方法
background.draw(canvas);
//把画布的原点移回去，drawable在屏幕上的位置不动
canvas.translate(-scrollX, -scrollY);
}
}
3. 绘制 View 的内容
先跳过第 2 步，是因为不是所有的 View 都需绘制褪色边缘。DecorView#onDraw：
public void onDraw(Canvas c) {
super.onDraw(c);
mBackgroundFallback.draw(this, mContentRoot, c, mWindow.mContentParent,
mStatusColorViewState.view, mNavigationColorViewState.view);
}
View#onDraw
protected void onDraw(Canvas canvas) {
}
onDraw 是空实现，需要子 View 自己去绘制。对于DecorView 一般也没啥内容，除了需要背景颜色等，所以本身并需要绘制啥。
4. 绘制子View
DecorView 绘制完成后，开始绘制子 View，所以 ViewGroup 的绘制需要绘制子 View，直接看看 ViewGroup#dispatchDraw：
protected void dispatchDraw(Canvas canvas) {
boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
final int childrenCount = mChildrenCount;
final View[] children = mChildren;
int flags = mGroupFlags;
//ViewGroup是否有设置子View入场动画，如果有绑定到View
// 启动动画控制器
...
//指定修改区域
int clipSaveCount = 0;
final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
// 不让子view绘制在pandding里面，也就是去除padding
if (clipToPadding) {
clipSaveCount = canvas.save();
canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,
mScrollX + mRight - mLeft - mPaddingRight,
mScrollY + mBottom - mTop - mPaddingBottom);
}
...
for (int i = 0; i < childrenCount; i++) {
//先取mTransientViews中的View，mTransientViews中的View通过addTransientView添加，它们只是容器渲染的一个item
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;
}
}
int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i;
final View child = (preorderedList == null)
? children[childIndex] : preorderedList.get(childIndex);
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
...
}
ViewGroup#dispatchDraw 的流程是先启动第一次加到布局中的动画，然后确定绘制区域，遍历绘制 View，遍历 View 的时候优先绘制渲染的 mTransientViews，绘制 View 调用到ViewGroup#drawChild：
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
//View中有两个draw方法
//这个多参数的draw用于view绘制自身内容
return child.draw(canvas, this, drawingTime);
}
View#draw(canvas, this, drawingTime）
boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
boolean drawingWithRenderNode = mAttachInfo != null
&& mAttachInfo.mHardwareAccelerated
&& hardwareAcceleratedCanvas;
...
//主要判断是否有绘制缓存，如果有，直接使用缓存，如果没有，调用 draw(canvas)方法
if (!drawingWithDrawingCache) {
if (drawingWithRenderNode) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
((DisplayListCanvas) canvas).drawRenderNode(renderNode);
} else {
// Fast path for layouts with no backgrounds
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) {
// no layer paint, use temporary paint to draw bitmap
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 {
// use layer paint to draw the bitmap, merging the two alphas, but also restore
int layerPaintAlpha = mLayerPaint.getAlpha();
mLayerPaint.setAlpha((int) (alpha * layerPaintAlpha));
canvas.drawBitmap(cache, 0.0f, 0.0f, mLayerPaint);
mLayerPaint.setAlpha(layerPaintAlpha);
}
}
}
首先判断是否已经有缓存，即之前是否已经绘制过一次了，如果没有，则会调用 draw(canvas) 方法，开始正常的绘制，即上面所说的六个步骤，否则利用缓存来显示。
这一步也可以归纳为 ViewGroup 绘制过程，它对子 View 进行了绘制，而子 View 又会调用自身的 draw 方法来绘制自身，这样不断遍历子 View 及子 View 的不断对自身的绘制，从而使得 View 树完成绘制。
对于自定义 View ，如果需要绘制东西的话，直接重新 onDraw 就可以了。
6. 绘制装饰
public void onDrawForeground(Canvas canvas) {
//绘制滑动指示
onDrawScrollIndicators(canvas);
//绘制ScrollBar
onDrawScrollBars(canvas);
//获取前景色的Drawable，绘制到canvas上
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和5.绘制View的褪色边缘
当 horizontalEdges 或者 verticalEdges 有一个 true 的时候，表示需要绘制 View 的褪色边缘：
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
这时候先计算出是否需要绘制上下左右的褪色边缘和它的参数，然后保存视图层：
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) {
final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;
if (drawTop) {
canvas.saveLayer(left, top, right, top + length, null, flags);
}
if (drawBottom) {
canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
}
if (drawLeft) {
canvas.saveLayer(left, top, left + length, bottom, null, flags);
}
if (drawRight) {
canvas.saveLayer(right - length, top, right, bottom, null, flags);
}
} else {
scrollabilityCache.setFadeColor(solidColor);
}
绘制褪色边缘，恢复视图层 ：
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);
所谓的绘制装饰，就是指 View 除了背景、内容、子 View 的其余部分，例如滚动条等。
最后附上 View 的 draw 流程：
到此，View 的绘制流程就讲完了，下一篇会讲自定义 View。