参考从ViewRootImpl类分析View绘制的流程
一、测量measure
从performTraversals方法我们可以看到，在执行performMeasure测量之前要通过getRootMeasureSpec方法获得顶层视图DecorView的测量规格，跟踪代码进入getRootMeasureSpec（）方法：
/**
* 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) {
//匹配父容器时，测量模式为MeasureSpec.EXACTLY，
//测量大小直接为屏幕的大小，也就是充满真个屏幕
case ViewGroup.LayoutParams.MATCH_PARENT:
// Window can't resize. Force root view to be windowSize.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize,
MeasureSpec.EXACTLY);
break;
//包裹内容时，测量模式为MeasureSpec.AT_MOST，
//测量大小直接为屏幕大小，也就是充满真个屏幕
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize,
MeasureSpec.AT_MOST);
break;
//其他情况时，测量模式为MeasureSpec.EXACTLY，
//测量大小为DecorView顶层视图布局设置的大小。
default:
// Window wants to be an exact size. Force root view to be that size.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension,
MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
分析：该方法主要作用是在整个窗口的基础上计算出root view（顶层视图DecorView）的测量规格，该方法的两个参数分别表示：
windowSize：当前手机窗口的有效宽和高，一般都是除了通知栏的屏幕宽和高
rootDimension 根布局DecorView请求的宽和高，由前面的博客我们知道是MATCH_PARENT
由 《从setContentView方法分析Android加载布局流程》可知，我们的DecorView根布局宽和高都是MATCH_PARENT，因此DecorView根布局的测量模式就是MeasureSpec.EXACTLY，测量大小一般都是整个屏幕大小，所以一般我们的Activity窗口都是全屏的。因此上面代码走第一个分支，通过调用MeasureSpec.makeMeasureSpec方法将DecorView的测量模式和测量大小封装成DecorView的测量规格。
由于performMeasure（）方法调用了 View中measure（）方法俩进行测量，并且DecorView（继承自FrameLayout）的父类是ViewGroup，祖父类是View。因此我们从View的成员函数measure开始分析整个测量过程。
int mOldWidthMeasureSpec = Integer.MIN_VALUE;
int mOldHeightMeasureSpec = Integer.MIN_VALUE;
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
..................
//如果上一次的测量规格和这次不一样，则条件满足，重新测量视图View的大小
if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) {
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT)
== PFLAG_FORCE_LAYOUT ? -1 : mMeasureCache.indexOfKey(key);
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set
// the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {
long value = mMeasureCache.valueAt(cacheIndex);
// Casting a long to int drops the high 32 bits, no mask needed
setMeasuredDimensionRaw((int) (value >> 32), (int) value);
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
}
分析：
1.代码第10行：判断当前视图View是否需要重新测量，当上一次视图View测量的规格和本次视图View测量规格不一样时，就说明视图View的大小有改变，因此需要重新测量。
2.代码第23行：调用了onMeasure方法进行测量，说明View主要的测量逻辑是在该方法中实现。
3.代码第35-36行：保存本次视图View的测量规格到mOldWidthMeasureSpec和mOldHeightMeasureSpec以便下次测量条件的判断是否需要重新测量。
跟踪代码，进入View类的 onMeasure方法
/**
*
* Measure the view and its content to determine the measured width and the
* measured height. This method is invoked by {@link #measure(int, int)} and
* should be overriden by subclasses to provide accurate and efficient
* measurement of their contents.
*
*
*
* CONTRACT: When overriding this method, you
* must call {@link #setMeasuredDimension(int, int)} to store the
* measured width and height of this view. Failure to do so will trigger an
* IllegalStateException, thrown by
* {@link #measure(int, int)}. Calling the superclass'
* {@link #onMeasure(int, int)} is a valid use.
*
*
*
* The base class implementation of measure defaults to the background size,
* unless a larger size is allowed by the MeasureSpec. Subclasses should
* override {@link #onMeasure(int, int)} to provide better measurements of
* their content.
*
*
*
* If this method is overridden, it is the subclass's responsibility to make
* sure the measured height and width are at least the view's minimum height
* and width ({@link #getSuggestedMinimumHeight()} and
* {@link #getSuggestedMinimumWidth()}).
*
*
* @param widthMeasureSpec horizontal space requirements
* as imposed by the parent.
* The requirements are encoded with
* {@link android.view.View.MeasureSpec}.
* @param heightMeasureSpec vertical space requirements
* as imposed by the parent.
* The requirements are encoded with
* {@link android.view.View.MeasureSpec}.
*
* @see #getMeasuredWidth()
* @see #getMeasuredHeight()
* @see #setMeasuredDimension(int, int)
* @see #getSuggestedMinimumHeight()
* @see #getSuggestedMinimumWidth()
* @see android.view.View.MeasureSpec#getMode(int)
* @see android.view.View.MeasureSpec#getSize(int)
*/
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(
getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
分析：
该方法的实现也很简单，直接调用setMeasuredDimension方法完成视图View的测量。我们知道，Android中所有的视图组件都是继承自View实现的。因此该方法提供了一个默认测量视图View大小的实现。
言外之意，如果你不想你自己的View使用默认实现来测量View的宽高的话，你可以在子类中重写onMeasure方法来自定义测量方法。我们先来看看默认测量宽高的实现。跟踪代码进入getDefaultSize方法：
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