结构关系

ViewRootImpl单独是一个普通类，不是视图类。没有继承View。

DecorView是PhoneWindow的一个内部类，是一个视图类继承FrageLayout(ViewGroup).

ViewRootImpl.javapublic final class ViewRootImpl implements ViewParent,        View.AttachInfo.Callbacks, HardwareRenderer.HardwareDrawCallbacks {    //...    ...}PhoneWindow.javapublic class PhoneWindow extends Window implements MenuBuilder.Callback {        private final class DecorView extends FrameLayout implements RootViewSurfaceTaker {      //...      ...    }   }Window.javapublic abstract class Window {    //    ...}

// This is the view in which the window contents are placed. It is either    // mDecor itself, or a child of mDecor where the contents go.    private ViewGroup mContentParent;    private void installDecor() {        if (mDecor == null) {            mDecor = generateDecor();            mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);            mDecor.setIsRootNamespace(true);            if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {                mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);            }        }        if (mContentParent == null) {            mContentParent = generateLayout(mDecor);            ...        }    }