已知系统内部是根据每个View的MeasureSpec来得到View得宽和高，那么我们是怎么获得到每个View对应的MeasureSpec呢？

DecorView

对于DecorView，其MeasureSpec由窗口的尺寸和其自身的LayoutParams来共同决定的。已知，ViewRootImpl类是连接WindowManager和DecorView的纽带。在ViewRootImpl中的measureHierarchy方法中有如下的代码，它展示了DecorView的MeasureSpec的创建过程：

childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);     //desireWindowWidth是屏幕的宽度
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

上面代码调用的getRootMeasureSpec()方法的代码如下

private static int getRootMeasureSpec(int windowSize, int rootDimension) {
      int measureSpec;
      switch (rootDimension){
      case ViewGroup.LayoutParams.MATCH_PARENT:
            mesureSpec = MeasureSpec.makeMeasureSpec(windowSize,MeasureSpec.EXACTLY);
            break;
      case ViewGroup.LayoutParams.WRAP_CONTENT:
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize,MeasureSpec.AT_MOST);
            break;
      default:
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension,MeasureSpec.EXACTLY);
            break;
      }
      return measureSpec;
}

普通View

普通View是指布局中的View，View的measure过程由ViewGroup传递而来，下面是ViewGroup的measureChildWidthMargins()方法

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=getHeightMeasureSpec(parentHeightMeasureSpec,mPaddingTop+mPaddingBottom+lp.topMargin+lp.bottomMargin+heightUsed,lp.height);
      child.measure(childWidthMeasureSepc, childHeightMeasureSpec);
}

在上面ViewGroup的的measureChildWithMargins()方法中，首先通过getChildMeasureSpec()方法获得子View的MeasureSpec，然后，调用child.measure()方法。那么ViewGroup中的getChildMeasureSpec()方法具体是什么样的呢？

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){
      case MeasureSpec.EXACTLY:
            if (childDimension >= 0){
                  resultSize = childDimension;
                  resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT){
                  resultSize = size;
                  resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT){
                  resultSize = size;
                  resultMode = MeasureSpec.AT_MOST;
            }
            break;
      case MeasureSpec.AT_MOST:
            if(childDimension >= 0){
                  resultSize = childDimension;
                  resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                  resultSize = size;
                  resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParam.WRAP_CONTENT){
                  resultSize = size;
                  resultMode = MeasureSpec.AT_MOST;
            }
            break;
      case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0){
                  resultSize = childDimension;
                  resultMode = MeasureSpec.EXACTLY
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                  resultSize = 0;
                  resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension ==== LayoutParams.UNSPECIFIED) {
                  result = 0;
                  resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
      }
      return MeasureSpec.makeMeasureSpec(resultSize,resultMode);
}