view的测量
根据（二）中分析，我们知道view的测量、布局、绘制分别调用了performMeasure、performLayout、perfromDraw方法；
源码如下：
总结UI详细测量、布局和绘制步骤如下：
1、测量：
View.measure--->View.onMeasure--->View.setMeasuredDimension--->View.setMeasureDimensionRaw
2、布局：
View.layout ---> View.onLayout
3、绘制：
ViewRootImpl.draw(fullRedrawNeeded)--->ViewRootImpl.drawSoftware--->view.draw(Canvas)
下面我们依次分析具体的测量、布局和绘制实现逻辑
测量：
/frameworks/base/core/java/android/view/View.java
View的测量会执行measure(int widthMeasureSpec, int heightMeasureSpec)方法，至于measure怎么执行先不探讨，我们先了解一下其中的两个参数，其实他们都是int类型的MeasureSpec。MeasureSpec可以说是View测量过程的前提，所以我们很有必要先来了解一下MeasureSpec。
MeasureSpec 工作原理
MeasureSpec 代表一个32位的int值，高2位代表SpecMode,低30位代表SpecSize。
SpecMode是指测量模式，SpecSize是指在某种测量模式下的大小。
MeasureSpec是View中的一个静态内部类。
我们可以把MeasureSpec理解为测量规则，而这个测量规则是由测量模式和和该模式下的测量大小共同组成的。
使用方式如下：
int MeasureSpec = MeasureSpec.makeMeasureSpec(specSize,SpecMode);
确定了View测量规则后，我们也可以通过测量规则获取测量模式和该模式下的测量大小。
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
SpecMode有三类：
UNSPECIFIED
父容器不对View有任何限制，要多大给多大，这般情况一般用于系统内部，表示一种测量状态，如ScrollView测量子View时用的就是这个。
EXACTLY
父容器已经检测出View所需要的大小，这个时候View的最终大小就是SpecSize所测定的值，它对应于LayoutParams中的match_parent和具体的数值（如40dp,60dp）这两种模式。
AT_MOST
父容器指定了一个可用大小即SpecSize,View的大小不能大于这个值，具体是什么值要看不同View的具体实现。它对应于LayoutParams中的wrap_content.
下面我们再详细分解普通View如何测量
普通View的MeasureSpec的创建过程
MeasureSpec很重要，上文中我们也了解了MeasureSpec的工作原理，那如何获取MeasureSpec呢？下面就结合源码来分析MeasureSpec的创建过程。
/frameworks/base/core/java/android/view/ViewGroup.java
先来看下ViewGroup中的measureChild方法
在这个方法中，先获取了子View的布局参数，然后通过getChildMeasureSpec方法分别得到子View的宽高测量规则，即childWidthMeasureSpec和childHeightMeasureSpec，最后调用子View的measure方法，至此测量过程就由父View传递到了子View.。MeasureSpec确定后就可以在onMeasure方法确定View的测量宽高了。
下面重点分析一下getChildMeasureSpec，源码如下：
/**
* 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 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);
}
此方法比较清晰，它主要用来通过父View的MeasureSpec和子View的LayoutParams来确定子View的MeasureSpec的，即普通View的MeasureSpec创建过程。
接着我们再了解一下DecorView的MeaureSpace是如何创建的
普通View的MeasureSpec的创建过程阐述了怎样通过父View的MeasureSpec和子View的LayoutParams来确定子View的MeasureSpec。那顶级View,即DecorView的MeasureSpec创建过程又是怎样的呢？ViewRootImp的measureHierarchy方法中有如下代码：
接着来看getRootMeasureSpec方法
从上述源码，我们可以得出如下规则，具体根据它的LayoutParams来划分：
LayoutParams.MATCH_PARENT:精确模式 其大小就为屏幕的尺寸大小
ViewGroup.LayoutParams.WRAP_CONTENT：最大模式，大小不定，但是不能超过屏幕的大小
具体数值（如40dp）:精确模式，大小为LayoutParamas指定的大小。
参考博文：https://www.cnblogs.com/xyhuangjinfu/p/5435201.html
