文章目录

• 1、ViewRoot 和 DecorView
• 2、 MeasureSpec
• 2.1MeasureSpec
• 2.2MeasureSpec和LayoutParams的对应关系
• 3、View的工作流程
• 3.1 Measure过程
• 3.1.1 view的测量过程
• 3.1.2 ViewGroup的测量过程
• 3.1.3 获取View宽高的时机
• 1、Activity/View # onWindowFocusChanged
• 2、view.post(runnable)
• 3、ViewTreeObserver
• 3.2Layout过程
• 3.3Draw过程
• 4、自定义View
• 4.1自定义view的分类
• 4.2 自定义view 注意点
• 4.3 例子
• 4.4 自定义view的思想

1、ViewRoot 和 DecorView

1. ViewRoot对应ViewRootImpl类，是连接WindowManager和DecorView的纽带。View的三大流程是通过ViewRoot完成的。 在ActivityThread中，当Activity对象被创建完毕时，会将DecorView添加到Window中，同时会创建ViewRootImpl，且ViewRootImpl和DecorView会建立关联。如下代码，WindowManagerGlobal的addView()方法：

public void addView(View view, ViewGroup.LayoutParams params,Display display, Window parentWindow) { ... root = new ViewRootImpl(view.getContext(), display); root.setView(view, wparams, panelParentView); ... }

2. View绘制流程从 performTraversals开始，经过Measure、layout、draw。流程图如下

3. DecorView是顶级View，是一个FrameLayout，上面是标题栏、下面是内容栏。内容栏就是setContengView的内容view，id是content。事件 经过DecorView 然后传给我们自己的View。
2、 MeasureSpec

MeasureSpec封装了从父级传递到子级的布局要求。系统把view的LayoutParams 根据 父容器施加的规则（父容器的SpecMode） 转换成 view的MeasureSpec，然后使用这个MeasureSpec确定view的测量宽高（不一定是最终宽高）。

2.1MeasureSpec 1.MeasureSpec—view的测量规格：高2位的SpecMode，低30位的SpecSize。
2.SpecMode的分类：
UNPECIFIED父容器对view不限制，要多大给多大，一般系统内部使用。
EXACTLY，父容器检测出view所需大小，view最终大小就是SpecSize的值。对应 LayoutParams中的matchParent、具体数值 两种模式。
AT_MOST，父容器制定了可用大小即SpecSize，view的大小不能大于这个值，具体要看view的具体实现。对应LayoutParams中的wrap_content。
2.2MeasureSpec和LayoutParams的对应关系

前面说了View的MeasureSpec是由LayoutParams和父容器的MeasureSpec共同决定。顶级view，即DecorView，是由窗口尺寸和自身LayoutParams决定。

1、DecorView，ViewRootImpl中measureHierarchy()方法（performTraversals中执行），代码如下，desiredWindowWidth、desiredWindowHeight是屏幕的尺寸。

private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp, final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) { ... childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width); childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height); performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); ... }

performMeasure()内部是调用mView.measure(childWidthMeasureSpec, childHeightMeasureSpec)，mView就是DecorVIew。继续看getRootMeasureSpec（）方法如下：

/** * Figures out the measure spec for the root view in a window based on it's * layout params. * * @param windowSize *The available width or height of the window * * @param rootDimension *The layout params for one dimension (width or height) of the *window. * * @return The measure spec to use to measure the root view. */ private static int getRootMeasureSpec(int windowSize, int rootDimension) { int measureSpec; switch (rootDimension) {case ViewGroup.LayoutParams.MATCH_PARENT: // Window can't resize. Force root view to be windowSize. measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY); break; case ViewGroup.LayoutParams.WRAP_CONTENT: // Window can resize. Set max size for root view. measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST); break; default: // Window wants to be an exact size. Force root view to be that size. measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY); break; } return measureSpec; }

DecorView的MeasureSpec就明确了，根据其LayoutParams：

• MATCH_PARENT：精确模式，就是窗口大小；
• WRAP_CONTENT：最大值模式，最大值不能超过窗口大小；
• 固定值（如100dp）：精确模式，就是LayoutParams的指定值。

2、普通View，测量过程从ViewGroup传递下来，看ViewGroup的measureChildWithMargins()方法：

/** * Ask one of the children of this view to measure itself, taking into * account both the MeasureSpec requirements for this view and its padding * and margins. The child must have MarginLayoutParams The heavy lifting is * done in getChildMeasureSpec. * * @param child The child to measure * @param parentWidthMeasureSpec The width requirements for this view * @param widthUsed Extra space that has been used up by the parent *horizontally (possibly by other children of the parent) * @param parentHeightMeasureSpec The height requirements for this view * @param heightUsed Extra space that has been used up by the parent *vertically (possibly by other children of the parent) */ protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) { final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams(); final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin + widthUsed, lp.width); final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin + heightUsed, lp.height); child.measure(childWidthMeasureSpec, childHeightMeasureSpec); }

即先获取child的MeasureSpec，再调child.measure()。可以看到，child的MeasureSpec是由父容器的??MeasureSpec、父容器的padding、child的LayoutParams、child的marging 共同决定。继续看getChildMeasureSpec()方法：

/** * Does the hard part of measureChildren: figuring out the MeasureSpec to * pass to a particular child. This method figures out the right MeasureSpec * for one dimension (height or width) of one child view. * * The goal is to combine information from our MeasureSpec with the * LayoutParams of the child to get the best possible results. For example, * if the this view knows its size (because its MeasureSpec has a mode of * EXACTLY), and the child has indicated in its LayoutParams that it wants * to be the same size as the parent, the parent should ask the child to * layout given an exact size. * * @param spec The requirements for this view * @param padding The padding of this view for the current dimension and *margins, if applicable * @param childDimension How big the child wants to be in the current *dimension * @return a MeasureSpec integer for the child */ public static int getChildMeasureSpec(int spec, int padding, int childDimension) { int specMode = MeasureSpec.getMode(spec); int specSize = MeasureSpec.getSize(spec); //padding,就是已被占用的空间，就是 父容器的padding+child的marging //size，是ViewGroup本身size减去已使用的空间，是ViewGroup能提供给child的最大值。 int size = Math.max(0, specSize - padding); int resultSize = 0; int resultMode = 0; switch (specMode) { // Parent has imposed an exact size on us case MeasureSpec.EXACTLY: if (childDimension >= 0) { resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size. So be it. resultSize = size; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size. It can't be // bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // Parent has imposed a maximum size on us case MeasureSpec.AT_MOST: if (childDimension >= 0) { // Child wants a specific size... so be it resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size, but our size is not fixed. // Constrain child to not be bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size. It can't be // bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // Parent asked to see how big we want to be case MeasureSpec.UNSPECIFIED: if (childDimension >= 0) { // Child wants a specific size... let him have it resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size... find out how big it should // be resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size; resultMode = MeasureSpec.UNSPECIFIED; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size.... find out how // big it should be resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size; resultMode = MeasureSpec.UNSPECIFIED; } break; } //noinspection ResourceType return MeasureSpec.makeMeasureSpec(resultSize, resultMode); }

可见，view的MeasureSpec由 viewParent的MeasureSpec和自身layoutParams确定。另外，child的可利用的尺寸是parent尺寸减去padding，上面代码已有注释，这很好理解。
梳理如下：

parentSpecMode /childLayoutParams	EXACTLY	AT_MOST	UNSPECIFIED
dp/px	EXACTLY childSize	EXACTLY childsize	EXACTLY childsize
match_parent	EXACTLY parentSize	AT_MOST parentSize	UNSPECIFIED 0
wrap_content	AT_MOST parentSize	AT_MOST parentSize	UNSPECIFIED 0

注意，parentSize是父容器可使用的大小。
更新，看到鸿洋公众号的文章关于UNSPECIFIED说明：

MeasureSpec.UNSPECIFIED是不是真的不常见？

在日常定制View时，确实很少会专门针对这个模式去做特殊处理，大多数情况下，都会把它当成MeasureSpec.AT_MOST一样看待，就比如最最常用的TextView，它在测量时也是不会区分UNSPECIFIED和AT_MOST的。

不过，虽说这个模式比较少直接接触到，但很多场景下，我们已经在不知不觉中用上了，比如RecyclerView的Item，如果Item的宽/高是wrap_content且列表可滚动的话，那么Item的宽/高的测量模式就会是UNSPECIFIED。
还有就是NestedScrollView和ScrollView，因为它们都是扩展自FrameLayout，所以它们的子View会测量两次，第一次测量时，子View的heightMeasureSpec的模式是写死为UNSPECIFIED的。
我们在自定义ViewGroup过程中，如果允许子View的尺寸比ViewGroup大的话，在测量子View时就可以把Mode指定为UNSPECIFIED。

看到ScrollView重写了measureChild方法，指定高度的mode是UNSPECIFIED

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3、View的工作流程

View的三大流程，measure、layout、draw。measure确定view的测量宽高，layout确定view的最终宽高和四个顶点位置，draw绘制到屏幕。

3.1 Measure过程 view的测量过程，由measure()方法完成。viewGroup测量自身后，还需调用child.measure()遍历测量子view。
3.1.1 view的测量过程

/** * * This is called to find out how big a view should be. The parent * supplies constraint information in the width and height parameters. *
* * * The actual measurement work of a view is performed in * {@link #onMeasure(int, int)}, called by this method. Therefore, only * {@link #onMeasure(int, int)} can and must be overridden by subclasses. *
* * * @param widthMeasureSpec Horizontal space requirements as imposed by the *parent * @param heightMeasureSpec Vertical space requirements as imposed by the *parent * * @see #onMeasure(int, int) */ public final void measure(int widthMeasureSpec, int heightMeasureSpec) { ... // measure ourselves, this should set the measured dimension flag back onMeasure(widthMeasureSpec, heightMeasureSpec); ... }

可见view的measure()方法是final，不可被子类重写。里面调用onMeasure()，实际真正的测量过程在onMeasure()中。所以只有onMeasure()可以且必须被子类重写。另外，参数widthMeasureSpec、heightMeasureSpec就是上一节最后的表格中的值。继续看onMeasure()：

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); }

从名字就可以看出，setMeasuredDimension()就是设置测量的尺寸，且在onMeasure()中必须被调用，否则在测量时会发送异常。getDefaultSize()获取默认的宽/高。所以View类中的onMeasure() 是设置默认的宽高。 继续看getDefaultSize()具体实现：

public static int getDefaultSize(int size, int measureSpec) { int result = size; int specMode = MeasureSpec.getMode(measureSpec); int specSize = MeasureSpec.getSize(measureSpec); switch (specMode) { case MeasureSpec.UNSPECIFIED: result = size; break; case MeasureSpec.AT_MOST: case MeasureSpec.EXACTLY: result = specSize; break; } return result; }

UNSPECIFIED，一般是系统使用，不需要关心。这里view大小直接取size，就是getSuggestedMinimumWidth()/getSuggestedMinimumHeight()，意思是 建议的 最小宽高。看下实现：

protected int getSuggestedMinimumWidth() { return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth()); }

没有背景，就取mMinWidth，就是xml中设置的minWidth属性值；有背景，取 mMinWidth 、背景的MinimumWidth 的较大值。drawable的getMinimumWidth()如下，有固有宽度就取固有宽度（如BitmapDrawable），没有就是0（如ShadeDrawable）。

public int getMinimumWidth() { final int intrinsicWidth = getIntrinsicWidth(); return intrinsicWidth > 0 ? intrinsicWidth : 0; }

AT_MOST、EXACTLY，直接取specSize，就是上一节最后的表格中的值，作为测量宽高。那这样取specSize是否合适呢？ 再来看一遍specSize的来源。

parentSpecMode /childLayoutParams	EXACTLY	AT_MOST	UNSPECIFIED
dp/px	1EXACTLY childSize	2EXACTLY childsize	EXACTLY childsize
match_parent	3EXACTLY parentSize	4AT_MOST parentSize	UNSPECIFIED 0
wrap_content	5AT_MOST parentSize	6AT_MOST parentSize	UNSPECIFIED 0

1、2的情况，具体dp值，取SpecSize没问题，因为是EXACTLY，就是给定的的尺寸。
3的情况，match_parent，取SpecSize，即parentSize，也没问题，因为是EXACTLY，也是确定的尺寸。
4的情况，match_parent，但父容器又是wrap_content，系统就给了AT_MOST+parentSize，限制最大尺寸为parentSize。而这里直接取specSize即parentSize，似乎也没问题。这个看一个例子一，如下，view是match_parent，可见view取得确实是parentSize。
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5、6的情况，wrapContent即AT_MOST+parentSize，取specSize也就是parentSize，所以和3、4一样都是parentSize，即View类 中 默认wrapContent等同于match_parent。
再看一个情况例子二，如下，View换成TextView（继承View），尺寸就不是parentSize了，而是内容尺寸，说明TextView在onMeasure中做了处理。

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继续看，例子三如下，同时有TextView、View，此时textView又是取parentSize（可用空间）：

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所以得出结论：
通常直接继承View的自定义View，在onMeasure()需要处理 ：
a、wrap_content的情况，否则wrap_content就等同于match_parent；
b、match_parent+父容器wrap_content的情况，否则就像例子一，父容器wrap_content是无效的，处理方式就是例子二中的textView。
总结就是，直接继承View的自定义View，需要处理AT_MOST时的宽高。
处理方式如下：

@Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { super.onMeasure(widthMeasureSpec, heightMeasureSpec); int widthSize = MeasureSpec.getSize(widthMeasureSpec); int widthMode = MeasureSpec.getMode(widthMeasureSpec); int heightSize = MeasureSpec.getSize(heightMeasureSpec); int heightMode = MeasureSpec.getMode(heightMeasureSpec); if (widthMode == MeasureSpec.AT_MOST && heightMode == MeasureSpec.AT_MOST) { setMeasuredDimension(mWidth, mHeight); } else if (widthMode == MeasureSpec.AT_MOST) { setMeasuredDimension(mWidth, heightSize); } else if (heightMode == MeasureSpec.AT_MOST) { setMeasuredDimension(widthSize, mHeight); } }

实际就是在 AT_MOST时 设置一个指定的尺寸mWidth、mHeight，其他情况沿用系统。至于mWidth、mHeight是多少，则要具体看你的view的逻辑了。例如TextView，可以参考其源码的实现。
3.1.2 ViewGroup的测量过程

ViewGroup需要完成自身的测量，还要遍历子view调用measure()方法进行测量。

ViewGroup是抽象类，没有重写onMeasure，因为无法做到统一，是让具体继承ViewGroup的子类重写自己的逻辑。但是提供一些方便的方法给子类调用。如measureChildren()、measureChild()、measureChildWithMargins()，上面第二节分析过measureChildWithMargins()，这里我们看下measureChildren()：

/** * Ask all of the children of this view to measure themselves, taking into * account both the MeasureSpec requirements for this view and its padding. * We skip children that are in the GONE state The heavy lifting is done in * getChildMeasureSpec. * * @param widthMeasureSpec The width requirements for this view * @param heightMeasureSpec The height requirements for this view */ protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) { final int size = mChildrenCount; final View[] children = mChildren; for (int i = 0; i < size; ++i) { final View child = children[i]; if ((child.mViewFlags & VISIBILITY_MASK) != GONE) { measureChild(child, widthMeasureSpec, heightMeasureSpec); } } }

就是遍历子view，调用measureChild()，继续看：

protected void measureChild(View child, int parentWidthMeasureSpec, int parentHeightMeasureSpec) { final LayoutParams lp = child.getLayoutParams(); final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft + mPaddingRight, lp.width); final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop + mPaddingBottom, lp.height); child.measure(childWidthMeasureSpec, childHeightMeasureSpec); }

通过getChildMeasureSpec()获取child的MeasureSpec，然后调用child.measure()，测量就传到child内部了，很好理解。measureChild()相比measureChildWithMargins() 没有考虑child的margin值。
上面说了，ViewGroup没有重写onMeasure，因为无法做到统一，让具体继承ViewGroup的子类重写自己的逻辑。具体看下LinearLayout的测量过程。

@Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { if (mOrientation == VERTICAL) { measureVertical(widthMeasureSpec, heightMeasureSpec); } else { measureHorizontal(widthMeasureSpec, heightMeasureSpec); } }

继续看measureVertical()：

void measureVertical(int widthMeasureSpec, int heightMeasureSpec) { ... //下面这句官方注释：看每个人多高，也记住最大宽度。想想这不就是计算竖向LinearLayout宽高的思路嘛！ // See how tall everyone is. Also remember max width. for (int i = 0; i < count; ++i) { ... final View child = getVirtualChildAt(i); ... final LayoutParams lp = (LayoutParams) child.getLayoutParams(); ... // Determine how big this child would like to be. If this or // previous children have given a weight, then we allow it to // use all available space (and we will shrink things later // if needed). final int usedHeight = totalWeight == 0 ? mTotalLength : 0; //这里测量child（里面就是measureChildWithMargins()） measureChildBeforeLayout(child, i, widthMeasureSpec, 0, heightMeasureSpec, usedHeight); final int childHeight = child.getMeasuredHeight(); ... final int totalLength = mTotalLength; //这里mTotalLength加上child的高度、margin，就是child高度累积。 mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin + lp.bottomMargin + getNextLocationOffset(child)); ... //这里记录最大宽度（包含margin） final int margin = lp.leftMargin + lp.rightMargin; final int measuredWidth = child.getMeasuredWidth() + margin; maxWidth = Math.max(maxWidth, measuredWidth); ... } //遍历完了：高度加上自身的上下padding // Add in our padding mTotalLength += mPaddingTop + mPaddingBottom; int heightSize = mTotalLength; // Check against our minimum height heightSize = Math.max(heightSize, getSuggestedMinimumHeight()); //这里很重要：调用resolveSizeAndState--决定 计算的高度（高度累加）和 LinearLayout的父容器约束的高度，取哪一个。 // Reconcile our calculated size with the heightMeasureSpec int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0); heightSize = heightSizeAndState & MEASURED_SIZE_MASK; ... //最大宽度加上左右margin maxWidth += mPaddingLeft + mPaddingRight; // Check against our minimum width maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth()); //设置最终的测量尺寸（宽也也同样调用resolveSizeAndState决定取哪个） setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState), heightSizeAndState); }

所以，简单概括就是：
1.先测量所有child；
2.根据child的情况获取自身宽高（累加高度、最大宽度）。
那么，是否就取 累加高度、最大宽度？再看下resolveSizeAndState():

/** * Utility to reconcile a desired size and state, with constraints imposed * by a MeasureSpec. Will take the desired size, unless a different size * is imposed by the constraints. The returned value is a compound integer, * with the resolved size in the {@link #MEASURED_SIZE_MASK} bits and * optionally the bit {@link #MEASURED_STATE_TOO_SMALL} set if the * resulting size is smaller than the size the view wants to be. * * @param size How big the view wants to be. --想要的尺寸 * @param measureSpec Constraints imposed by the parent. --父布局给的measureSpec * @param childMeasuredState Size information bit mask for the view's *children. * @return Size information bit mask as defined by *{@link #MEASURED_SIZE_MASK} and *{@link #MEASURED_STATE_TOO_SMALL}. */ public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) { final int specMode = MeasureSpec.getMode(measureSpec); final int specSize = MeasureSpec.getSize(measureSpec); final int result; switch (specMode) { case MeasureSpec.AT_MOST: //AT_MOST时，想要的尺寸大于约束的尺寸，就只能取 约束的尺寸。 if (specSize < size) { result = specSize | MEASURED_STATE_TOO_SMALL; } else { result = size; } break; case MeasureSpec.EXACTLY: //dp值、match_parent且父EXACTLY，就是SpecSize result = specSize; break; case MeasureSpec.UNSPECIFIED: default: result = size; } return result | (childMeasuredState & MEASURED_STATE_MASK); }

这个过程就是限制 AT_MOST时，即wrap_content（或match_parent且父wrap_content）时高度不能大于parent的剩余空间。
3.1.3 获取View宽高的时机

Measure过程完成，就可通过getMeasuredWidth()、getMeasuredHeight()获取测量宽高。但某些极端情况
需要多次Measure才能确定最终宽高。所以在onLayout方法中获取测量宽高是真正ok的。
我们知道，activity的onCreate中无法获取到view的宽高。实际onCreate、onStart、onResume都不能保证view已完成测量，所以可能获取的都是0。因为view的measure和activity生命周期不是同步的。

以下是保证可以获取view测量宽高的方法：
1、Activity/View # onWindowFocusChanged onWindowFocusChanged：View已初始化完毕，宽高已准备ok。 但会多次调用，获取焦点、失去焦点都回调用。(这个回调是ViewRootIml中分发到DecorView，接着到Activity、到各级View。)

@Override public void onWindowFocusChanged(boolean hasFocus) { super.onWindowFocusChanged(hasFocus); if (hasFocus) { int measuredWidth = scoreView.getMeasuredWidth(); int measuredHeight = scoreView.getMeasuredHeight(); } }

2、view.post(runnable) view.post可以把runnable放入消息队列，等待looper到此runnable是view已经初始化完成。v详细原理参考【Android源码解析】View.post()到底干了啥

@Override protected void onStart() { super.onStart(); scoreView.post(new Runnable() { @Override public void run() { int measuredWidth = scoreView.getMeasuredWidth(); int measuredHeight = scoreView.getMeasuredHeight(); } }); }

3、ViewTreeObserver ViewTreeObserver有很多回调，其中有个OnGlobalLayoutListener，当View树的状态发生改变或者View树内部view的可见性发生改变时 方法 onGlobalLayout()都会被调用。所以是会回调多次。 此时也可以获取view的宽高：

ViewTreeObserver observer = view.getViewTreeObserver(); observer.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() { @Override public void onGlobalLayout() { mDefaultControlLayout.getViewTreeObserver().removeGlobalOnLayoutListener(this); if (mIsGroupListAnimating) { mIsGroupListAnimationPending = true; } else { updateLayoutHeightInternal(animate); } } });

3.2Layout过程 layout()的作用是View用来确定view本身位置，内部调用onLayout()来确定子view的位置。 layout过程比measure过程简单很多。看View的layout方法：

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; //使用setFrame方法设置4个顶点，就确定位置了~ boolean changed = isLayoutModeOptical(mParent) ? setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b); if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) { //这里调用onLayout，是个空实现。ViewGroup中重写了，还是空实现，但加了abstract，即ViewGroup的子类必须重写onLayout确定子View的位置。 onLayout(changed, l, t, r, b); if (shouldDrawRoundScrollbar()) { if(mRoundScrollbarRenderer == null) { mRoundScrollbarRenderer = new RoundScrollbarRenderer(this); } } else { mRoundScrollbarRenderer = null; }mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED; ListenerInfo li = mListenerInfo; if (li != null && li.mOnLayoutChangeListeners != null) { ArrayList listenersCopy = (ArrayList)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); } } }... }

先是用setFrame方法设置4个顶点，就确定位置了，即mLeft、mTop、mBottom、mRight确定了。 然后调用onLayout，是个空实现。ViewGroup中重写了onLayout，还是空实现，但加了abstract，即ViewGroup的子类必须重写onLayout确定子View的位置。
那就看看LinearLayout的onLayout：

@Override protected void onLayout(boolean changed, int l, int t, int r, int b) { if (mOrientation == VERTICAL) { layoutVertical(l, t, r, b); } else { layoutHorizontal(l, t, r, b); } }

继续看layoutVertical()：

void layoutVertical(int left, int top, int right, int bottom) { final int paddingLeft = mPaddingLeft; int childTop; int childLeft; // Where right end of child should go final int width = right - left; int childRight = width - mPaddingRight; // Space available for child int childSpace = width - paddingLeft - mPaddingRight; final int count = getVirtualChildCount(); 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 + bottom - top - mTotalLength; break; // mTotalLength contains the padding already case Gravity.CENTER_VERTICAL: childTop = mPaddingTop + (bottom - top - mTotalLength) / 2; break; case Gravity.TOP: default: childTop = mPaddingTop; break; } //遍历子view for (int i = 0; i < count; i++) { final View child = getVirtualChildAt(i); if (child == null) { childTop += measureNullChild(i); } else if (child.getVisibility() != GONE) { //获取child的测量宽高 final int childWidth = child.getMeasuredWidth(); final int childHeight = child.getMeasuredHeight(); final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams(); int gravity = lp.gravity; if (gravity < 0) { gravity = minorGravity; } final int layoutDirection = getLayoutDirection(); final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection); switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) { case Gravity.CENTER_HORIZONTAL: childLeft = paddingLeft + ((childSpace - childWidth) / 2) + lp.leftMargin - lp.rightMargin; break; case Gravity.RIGHT: childLeft = childRight - childWidth - lp.rightMargin; break; case Gravity.LEFT: default: childLeft = paddingLeft + lp.leftMargin; break; }if (hasDividerBeforeChildAt(i)) { childTop += mDividerHeight; }childTop += lp.topMargin; //以上就是获取子view的左、上的位置，即宽高，然后调用setChildFrame setChildFrame(child, childLeft, childTop + getLocationOffset(child), childWidth, childHeight); //top位置加上高度和margin，就是下一个view的top childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child); i += getChildrenSkipCount(child, i); } } }

就是遍历子view，确认childLeft、childTop，调用setChildFrame确认子view的位置：

private void setChildFrame(View child, int left, int top, int width, int height) { //这里width、height就是 上面获取的 测量宽高 child.layout(left, top, left + width, top + height); }

【《Android开发艺术探索》|View的工作原理--《Android开发艺术探索》阅读笔记——第四章】也就是调用child的layout方法，这样就走child的layout过程了。
一个问题：getMeasuredWidth() 与 getWidth()有何区别？
答曰：一般情况，getMeasuredWidth() 与 getWidth()两者无区别。
先看，getWidth():

public final int getWidth() { return mRight - mLeft; }

在上面分析LinearLayout时，child.layout的参数中 mRight就是mLeft + measuredWidth，所以getWidth()就是measuredWidth。只不过是measuredWidth在测量过程产生，getWidth()在layout过程产生。 只要不重写view的layout()方法（也不需要重写）改变顶点位置就不会出现不同的情况，例如下面这个最终宽高比测量宽高大100。

public void layout(int l, int t, int r, int b) { super.layout(l,t,r+100,b+100); }

3.3Draw过程 draw过程：
1、画背景
2、画自己-- onDraw，自己实现
3、画子view-- dispatchDraw
4、画装饰

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; }

ViewGroup一般不用onDraw画自己，只需要画子View就可以了。但明确需要画自己的话，需要调用setViewNotDraw(false);
以上View的三大流程就分析完了。
4、自定义View

自定义view涉及view层次结构、事件分发、工作原理，有一定复杂度，但也是有章可循的。

4.1自定义view的分类

1. 继承View：重写onDraw，要处理wrap_content、padding。
2. 继承ViewGroup：重写onMeasure测量自己、子View，重写onLayout布局子View。
3. 继承特定View（如TextView）：扩展自己的功能。
4. 继承特定ViewGroup（如LinearLayout）：扩展自己的功能。

4.2 自定义view 注意点

1. 支持wrap_content：直接继承View或ViewGroup的，要在onMeasure中处理wrap_content的情况。
2. 支持padding：直接继承View在onDraw中处理；直接继承ViewGroup，在onMeasure、onLayout中处理padding和子view的margin。
3. 不要在View中使用handler，因为本身提供了post方法。
4. 在View#onDetachedFromWindow中停止动画或线程。
5. 处理好嵌套滑动。

4.3 例子 自定义ViewGroup实例：横向滑动HorizontalView
4.4 自定义view的思想 先掌握基本功，弹性滑动、滑动冲突、绘制原理等，然后选择自定义的类别，按照注意事项多做就可以了。
好了，今天就到这里，欢迎留言讨论~
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