概述

在Android中，View的onMeasure()方法用来对控件进行测量，确定控件的宽高。该方法的两个参数widthMeasureSpec和heightMeasureSpec由父View计算后传入子view的measure()方法，再由子view的measure()方法传入onMeasure()方法，本文将介绍MeasureSpec的创建规则

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)

LayoutParams

在开始源码分析前，我们要先介绍下LayoutParams，因为子view的MeasureSpec的创建需要用到子view的LayoutParams，LayoutParams有如下三种类型

• FILL_PARENT/MATCH_PARENT：填满父View
• WRAP_CONTENT：包裹内容
• 确定值：确定的值

源码分析

首先我们要先找到父View中调用子view的measure()方法的入口

首先先看View类的onMeasure方法如下，由于View是所有控件的基类，这里只是一个默认实现

我们应该看得是ViewGroup类型的类的onMeasure方法

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

查看源码后发现ViewGroup.java类中并未重写onMeasure()方法，且ViewGroup类是个抽象类，所以我们应该把目光投向ViewGroup的子类，比如AbsoluteLayout、LinearLayout、FrameLayout等

我们选其中一个看看，查看AbsoluteLayout的onMeasure()方法，可以知道如下的调用栈

可以看到measureChildren()和measureChild()是定义在ViewGroup中的，在子类中可以访问

AbsoluteLayout.onMeasure()->ViewGroup.measureChildren()->ViewGroup.measureChild()

接下来直接看measureChild()的源码

由下面的源码及其注释可以看到，子view的MeasureSpec是交由getChildMeasureSpec()方法来计算，终于是让我们找到了

    /*** Ask one of the children of this view to measure itself, taking into* account both the MeasureSpec requirements for this view and its padding.* The heavy lifting is done in getChildMeasureSpec.** @param child The child to measure* @param parentWidthMeasureSpec The width requirements for this view* @param parentHeightMeasureSpec The height requirements for this view*/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);}

接下来我们就看看getChildMeasureSpec()方法的源码，也是本文的重点内容

该方法的功能就是通过父view的MeasureSpec和子View的LayoutParams来计算出子View的MeasureSpec

父View的MeasureSpec有MeasureSpec.EXACTLY、MeasureSpec.AT_MOST和MeasureSpec.UNSPECIFIED三种类型，而子view的LayoutParams也有MATCH_PARENT、WRAP_CONTENT或者确定值三种类型，3*3得出9种情况，如下代码所示

    /*** 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);int size = Math.max(0, specSize - padding);int resultSize = 0;int resultMode = 0;switch (specMode) {// Parent has imposed an exact size on uscase 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 uscase MeasureSpec.AT_MOST:if (childDimension >= 0) {// Child wants a specific size... so be itresultSize = 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 becase MeasureSpec.UNSPECIFIED:if (childDimension >= 0) {// Child wants a specific size... let him have itresultSize = childDimension;resultMode = MeasureSpec.EXACTLY;} else if (childDimension == LayoutParams.MATCH_PARENT) {// Child wants to be our size... find out how big it should// beresultSize = 0;resultMode = MeasureSpec.UNSPECIFIED;} else if (childDimension == LayoutParams.WRAP_CONTENT) {// Child wants to determine its own size.... find out how// big it should beresultSize = 0;resultMode = MeasureSpec.UNSPECIFIED;}break;}return MeasureSpec.makeMeasureSpec(resultSize, resultMode);}

总结

子view的MeasureSpec创建规则如下表所示