private void updateRelativeZ(Transaction t) {
    final ViewRootImpl viewRoot = getViewRootImpl();
    if (viewRoot == null) {
        // We were just detached.
        return;
    }
    final SurfaceControl viewRootControl = viewRoot.getSurfaceControl();
    t.setRelativeLayer(mBackgroundControl, viewRootControl, Integer.MIN_VALUE);
    t.setRelativeLayer(mSurfaceControl, viewRootControl, mSubLayer);
}

设置相对层级，默认情况下 mSubLayer 是 -2，所以 SurfaceView layer 是窗口层级之下。