Android View绘制流程（二）

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。



[java] view
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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：

[java] view
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/**

* 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。

[java] view
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<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：



[java] view
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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：

[java] view
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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：

[java] view
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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。

[java] view
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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);

}

}