/**
 * Registers a callback to be executed when the next frame is being drawn on RenderThread. This
 * callback will be executed on a RenderThread worker thread, and only used for the next frame
 * and thus it will only fire once.
 *
 * @param callback The callback to register.
 */
void registerRtFrameCallback(@NonNull FrameDrawingCallback callback) {
    if (mNextRtFrameCallbacks == null) {
        mNextRtFrameCallbacks = new ArrayList<>();
    }
    mNextRtFrameCallbacks.add(callback);
}

把 callback 加入到 mNextRtFrameCallbacks 中。在 updateRootDisplayList() 中调用 setFrameCallback() 向底层设置回调。