DecorView的测量过程

为何在Activity中获取DecorView的LayoutParams，返回为null??

View decorView = getWindow().getDecorView();
ViewGroup.LayoutParams layoutParams = decorView.getLayoutParams();

LogUtil.d("debug", "  decorView.layoutParams = " + layoutParams);

创建DecorView对象

      DecorView对象的创建详解

generateDecor()方法：

protected DecorView generateDecor() {
        return new DecorView(getContext(), -1);
    }

DecorView是Activity的顶级View，一般来说它内部包含标题栏和内容栏(layout.xml,即mContentParent)

接下来就是调用了generateLayout(mDecor)来初始化DecorView的结构。

这样一个PhoneWindow.DecorView对象就创建完成了。

 

DecorView添加到窗口  通过WindowManagerGlobal.addView方法

    //实例化ViewRootImpl对象
     root = new ViewRootImpl(view.getContext(), display);
     //顶层视图DecorView设置LayoutParams
     view.setLayoutParams(wparams);

对应于 ViewRootImpl 类，是连接 WindowManager 和 DecorView 的纽带。
ActivityThread 中当 activity 对象被创建好后，会将 DecorView 加入到 Window中同时完成 ViewRootImpl 的创建并建立和 DecorView 的联系。
root = new ViewRootImpl(view.getContext(), display);
root.setView(view, wparams, panelParentView);

 不理解？  这两个有什么区别？

 

DecorView的MeasureSpec的创建过程

在ViewRootImpl中的mearsureHierarchy方法中有一段代码展示了DecorView的MeasureSpec的创建过程：

       childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
       childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
       performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

看一下gerRootMeasureSpec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) {

        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;
    }

参考： Android开发艺术探索-View的工作原理

Android View系统解析(下)

Android View源码解读：浅谈DecorView与ViewRootImpl

decorView和window之间的层级及关系

Android View 测量流程(Measure)完全解析

DecorView对象的创建详解

View测量机制详解—从DecorView说起 - Loader's Blog - CSDN博客

 ViewRoot、DecorView、MeasureSpec和View的工作原理

Android里那些令人费解的命名（一）ViewRoot

View的工作原理