/**
 * Child container layer of {@code mSurface} with the same bounds as its parent, and cropped to
 * the surface insets. This surface is created only if a client requests it via {@link
 * #getBoundsLayer()}. By parenting to this bounds surface, child surfaces can ensure they do
 * not draw into the surface inset region set by the parent window.
 */
private SurfaceControl mBoundsLayer;

• mBoundsLayer 是子 layer 的容器，它的边界与父相同但调用 setBoundsLayerCrop() 做了裁剪。通过建立父子关系，可以确保子 surface 不会绘制到 insets 区域。
• SurfaceView 的 surface 就是与 mBoundsLayer 建立了父子关系。

public SurfaceControl getBoundsLayer() {
    if (mBoundsLayer == null) {
        mBoundsLayer = new SurfaceControl.Builder(mSurfaceSession)
                .setContainerLayer()
                .setName("Bounds for - " + getTitle().toString())
                .setParent(getRenderSurfaceControl())
                .setCallsite("ViewRootImpl.getBoundsLayer")
                .build();
        setBoundsLayerCrop();
        mTransaction.show(mBoundsLayer).apply();
    }
   return mBoundsLayer;
}

• 当调用 getBoundsLayer() 时创建。