遍历View树performTraversals（）执行过程
view树遍历概述
还是回到ViewRoot.java，我们直接看performTraversals（），该函数就是android系统View树遍历工作的核心。一眼看去，发现这个函数挺长的，但是逻辑是非常清晰的，其执行过程可简单概括为根据之前所有设置好的状态，判断是否需要计算视图大小（measure）、是否需要重新安置视图的位置（layout），以及是否需要重绘（draw）视图，可以用以下图来表示该流程。
private void performTraversals() {
// cache mView since it is used so much below...
//1 处理mAttachInfo的初始化，并根据resize、visibility改变的情况，给相应的变量赋值。
final View host = mView;
final View.AttachInfo attachInfo = mAttachInfo;
final int viewVisibility = getHostVisibility();
boolean viewVisibilityChanged = mViewVisibility != viewVisibility
|| mNewSurfaceNeeded;
float appScale = mAttachInfo.mApplicationScale;
WindowManager.LayoutParams params = null;
if (mWindowAttributesChanged) {
mWindowAttributesChanged = false;
surfaceChanged = true;
params = lp;
}
Rect frame = mWinFrame;
if (mFirst) {
// For the very first time, tell the view hierarchy that it
// is attached to the window. Note that at this point the surface
// object is not initialized to its backing store, but soon it
// will be (assuming the window is visible).
attachInfo.mSurface = mSurface;
attachInfo.mUse32BitDrawingCache = PixelFormat.formatHasAlpha(lp.format) ||
lp.format == PixelFormat.RGBX_8888;
attachInfo.mHasWindowFocus = false;
attachInfo.mWindowVisibility = viewVisibility;
......
}
//2 如果mLayoutRequested判断为true，那么说明需要重新layout，不过在此之前那必须重新measure。
if (mLayoutRequested) {
// Execute enqueued actions on every layout in case a view that was detached
// enqueued an action after being detached
getRunQueue().executeActions(attachInfo.mHandler);
if (mFirst) {
......
}
}
//3 判断是否有子视图的属性发生变化，ViewRoot需要获取这些变化。
if (attachInfo.mRecomputeGlobalAttributes) {
......
}
if (mFirst || attachInfo.mViewVisibilityChanged) {
......
}
//4 根据上面得到的变量数值，确定我们的view需要多大尺寸才能装下。于是就得measure了，有viewgroup的weight属性还得再做些处理
// Ask host how big it wants to be
host.measure(childWidthMeasureSpec, childHeightMeasureSpec);
mLayoutRequested = true;
}
}
//5 measure完毕，接下来可以layout了。
final boolean didLayout = mLayoutRequested;
boolean triggerGlobalLayoutListener = didLayout
|| attachInfo.mRecomputeGlobalAttributes;
if (didLayout) {
host.layout(0, 0, host.mMeasuredWidth, host.mMeasuredHeight);
}
//6 如果mFirst为true，那么会进行view获取焦点的动作。
if (mFirst) {
mRealFocusedView = mView.findFocus();
}
boolean cancelDraw = attachInfo.mTreeObserver.dispatchOnPreDraw();
//7 终于，来到最后一步，前面的工作可以说都是铺垫，都是为了draw而准备的。
if (!cancelDraw && !newSurface) {
mFullRedrawNeeded = false;
draw(fullRedrawNeeded);
}
下面我们就来会会view那三部曲吧。
计算视图大小（measure）的过程
整个view视图的Measure过程就是一个量体裁衣，按需分配的过程。看一下以下的递归过程：
从上图可以看出，measure过程始于ViewRoot的host.measure（），调的就是view类的measure（）函数，该函数然后回调onMeasure。如果host对象是一个ViewGroup实例，一般会重载onMeasure，如果没有的话，则会执行view类中默认的onMeasure。合理的情况是编程人员重载onMeasure并逐一对里面的子view进行measure。我们可以看一下view的measure方法：
/**
* 该方法在需要确定view所需尺寸大小时调用，父视图会提供宽和高的属性约束。
* 具体视图完全可以在onMeasure中改变这些。
* @see #onMeasure(int, int)
*/
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) {
// 首先清除dimension的设值
mPrivateFlags &= ~MEASURED_DIMENSION_SET;
// measure 自己, 并设置dimension
onMeasure(widthMeasureSpec, heightMeasureSpec);
// 抛出未设值flag的异常
if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {
throw new IllegalStateException("onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
mPrivateFlags |= LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
}
这里强烈建议去看看viewGroup实例FrameLayout和LinearLayout的onMeasure方法，一定会有所感悟的，尤其是LinerLayout的。这样对于viewGroup的专用标签pading和weight也会有新的体会。
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="100dip"
>
<TextView
android:layout_width="fill_parent"
android:layout_height="20dip"
android:layout_weight="2"
android:text="@string/hello"
/>
<ListView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="#ff00ff00"
></ListView>
</LinearLayout>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="100dip"
>
<TextView
android:layout_width="fill_parent"
android:layout_height="60dip"
android:layout_weight="2"
android:text="@string/hello"
/>
<ListView
android:layout_width="fill_parent"
android:layout_height="0dip"
android:layout_weight="2"
android:background="#ff00ff00"
></ListView>
</LinearLayout>
请问以上两布局有无不同，能否自行画出？
布局（layout）过程
执行完measure过程，也就是说各个view的大小尺寸已经登记在案，现在它们要确定的是自己应该置身于何处，也就是摆放在哪里。好吧，这个就是layout的职责所在，让父视图按照子视图的大小及布局参数，将子视图放置在合适的位置上。
同样需要看下以下流程图：
public void layout(int l, int t, int r, int b) {
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
//调用setFrame()函数给当前视图设置参数中指定的位置
boolean changed = setFrame(l, t, r, b);
if (changed || (mPrivateFlags & LAYOUT_REQUIRED) == LAYOUT_REQUIRED) {
if (ViewDebug.TRACE_HIERARCHY) {
ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_LAYOUT);
}
//回调onLayout()函数
onLayout(changed, l, t, r, b);
mPrivateFlags &= ~LAYOUT_REQUIRED;
//4.0新增监听可捕获layout变化
if (mOnLayoutChangeListeners != null) {
ArrayList<OnLayoutChangeListener> listenersCopy =
(ArrayList<OnLayoutChangeListener>) 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);
}
}
}
//layout完成，清楚标签
mPrivateFlags &= ~FORCE_LAYOUT;
}
view中的该layout函数流程大概如下：
1，调用setFrame()将位置参数保存，这些参数会保存到view内部变量（mLeft,mTop,mRight,mButtom）。如果有数值的变化那么会调用invalidate()重绘那些区域。
2，回调onLayout()，View中定义的onLayout（）函数默认什么都不做，View系统提供onLayout函数的目的是为了使系统包含有子视图的父视图能够在onLayout()函数对子视图进行位置分配，正因为这样，如果是viewGroup类型，就必须重载onLayout()，由此ViewGroup的onLayout为abstract也就很好解释了。
3，清楚mPrivateFlags中的LAYOUT_REQUIRED标记，因为layout的操作已经完成了。
为了对layout过程有更深的体会，有必要针对特定的一种viewGroup进行分析，我们还是把魔爪伸向linearLayout，看看它的onMeasure，onMeasure中会根据mOrientation变量，进行不同的layout，我们看一种就行了：
void layoutVertical() {
final int paddingLeft = mPaddingLeft;
int childTop;
int childLeft;
// 获得子视图可以用的宽度，顺便也把子视图左边沿的位置计算出来。
final int width = mRight - mLeft;
int childRight = width - mPaddingRight;
// Space available for child
int childSpace = width - paddingLeft - mPaddingRight;
final int count = getVirtualChildCount();
//根据父视图中的gravity属性，决定子视图的起始位置。
final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
switch (majorGravity) {
case Gravity.BOTTOM:
// mTotalLength contains the padding already
childTop = mPaddingTop + mBottom - mTop - mTotalLength;
break;
// mTotalLength contains the padding already
case Gravity.CENTER_VERTICAL:
childTop = mPaddingTop + (mBottom - mTop - mTotalLength) / 2;
break;
case Gravity.TOP:
default:
childTop = mPaddingTop;
break;
}
//遍历所有子视图,为它们分配位置.setChildFrame()结果还是会调用child.layout()为子视图设置布局位置.
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
......
childTop += lp.topMargin;
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
}
绘制（draw）过程
这是见证奇迹的时刻，draw过程就是要把view对象绘制到屏幕上，如果它是一个viewGroup，则需要递归绘制它所有的子视图。视图中有哪些元素是需要绘制的呢？
1，view背景。所有view都会有一个背景，可以是一个颜色值，也可以是一张背景图片
2，视图本身内容。比如TextView的文字
3，渐变边框。就是一个shader对象，让视图看起来更具有层次感
4，滚动条。
按照惯例，还是先看一下绘制的总体流程吧：
眼尖的同学应该发现了，上面这张图比前面的measure和layout多了一步：draw（）。在performTraversals（）函数中调用的是viewRoot的draw（）函数，在该函数中进行一系列的前端处理后，再调用host.draw()。
一般情况下，View对象不应该重载draw（）函数，因此，host.draw（）就是view.draw()。该函数内部过程也就是View系统绘制过程的核心过程，该函数中会依次绘制前面所说哦四种元素，其中绘制视图本身的具体实现就是回调onDraw()函数，应用程序一般也会重载onDraw()函数以绘制所设计的View的真正界面内容。
Google源码的注释太nice了，我加任何说辞都显得多余，就不画蛇添足了：
public void draw(Canvas canvas) {
if (ViewDebug.TRACE_HIERARCHY) {
ViewDebug.trace(this, ViewDebug.HierarchyTraceType.DRAW);
}
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & DIRTY_MASK) == DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~DIRTY_MASK) | 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) {
final Drawable background = mBGDrawable;
if (background != null) {
final int scrollX = mScrollX;
final int scrollY = mScrollY;
if (mBackgroundSizeChanged) {
background.setBounds(0, 0, mRight - mLeft, mBottom - mTop);
mBackgroundSizeChanged = false;
}
if ((scrollX | scrollY) == 0) {
background.draw(canvas);
} else {
canvas.translate(scrollX, scrollY);
background.draw(canvas);
canvas.translate(-scrollX, -scrollY);
}
}
}
// 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);
// Step 6, draw decorations (scrollbars)
onDrawScrollBars(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;
}
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);
}
} 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);
canvas.drawRect(left, top, right, top + length, p);
}
。。。。。
canvas.restoreToCount(saveCount);
// Step 6, draw decorations (scrollbars)
onDrawScrollBars(canvas);
}
绘制完界面内容后，如果该视图还包含子视图，则调用dispatchDraw()函数，实际上起作用的还是viewGroup的dispatchDraw()函数。需要说明的是应用程序不应该再重载ViewGroup中该方法，因为它已经有了默认而且标准的view系统流程。dispatchDraw()内部for循环调用drawChild()分别绘制每一个子视图，而drawChild()内部又会调用draw（）函数完成子视图的内部绘制工作。
protected void dispatchDraw(Canvas canvas) {
final int count = mChildrenCount;
final View[] children = mChildren;
int flags = mGroupFlags;
//1 判断mGroupFlags是否设有FLAG_RUN_ANIMATION标识并且不为0.该layout动画指的是加载或移除子视图时候呈现的动画.
if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {
final boolean cache = (mGroupFlags & FLAG_ANIMATION_CACHE) == FLAG_ANIMATION_CACHE;
final boolean buildCache = !isHardwareAccelerated();//硬件加速,4.0加入.
for (int i = 0; i < count; i++) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {
final LayoutParams params = child.getLayoutParams();
attachLayoutAnimationParameters(child, params, i, count);
bindLayoutAnimation(child);
}
}
//2 处理padding属性,如果该viewGroup有设置.
int saveCount = 0;
final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
if (clipToPadding) {
saveCount = canvas.save();
canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,
mScrollX + mRight - mLeft - mPaddingRight,
mScrollY + mBottom - mTop - mPaddingBottom);
}
//3 开始绘制子视图动画之前先清除flag.
// We will draw our child's animation, let's reset the flag
mPrivateFlags &= ~DRAW_ANIMATION;
mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED;
boolean more = false;
final long drawingTime = getDrawingTime();
//4 使用佛如循环,使viewGroup的子视图逐个调用drawChild函数.
if ((flags & FLAG_USE_CHILD_DRAWING_ORDER) == 0) {
for (int i = 0; i < count; i++) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
} else {
for (int i = 0; i < count; i++) {
final View child = children[getChildDrawingOrder(count, i)];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
}
//5 Draw any disappearing views that have animations
if (mDisappearingChildren != null) {
final ArrayList<View> disappearingChildren = mDisappearingChildren;
final int disappearingCount = disappearingChildren.size() - 1;
// Go backwards -- we may delete as animations finish
for (int i = disappearingCount; i >= 0; i--) {
final View child = disappearingChildren.get(i);
more |= drawChild(canvas, child, drawingTime);
}
}
if (clipToPadding) {
canvas.restoreToCount(saveCount);
}
// mGroupFlags might have been updated by drawChild()
flags = mGroupFlags;
if ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) {
invalidate(true);
}
}
整个view的工作流程那是相当复杂的，这需要静下心来，加以时日细细揣摩才能略有体会，本人写有小Demo一个，就当抛砖引玉：ClickMe ！
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