layer_state_t* SurfaceComposerClient::Transaction::getLayerState(const sp<SurfaceControl>&sc) {
auto handle = sc->getLayerStateHandle();

    if (mComposerStates.count(handle) == 0) {
        // we don't have it, add an initialized layer_state to our list
        ComposerState s;

        s.state.surface = handle;
        s.state.layerId = sc->getLayerId();

        mComposerStates[handle] = s;
    }

    return &(mComposerStates[handle].state);
}

• mComposerStates 是 Transaction 的一个字段，它是 <IBinder, ComposerState> 映射，IBinder 是 SurfaceControl 的 handle。
• 如果 mComposerStates 中没有 sc 对应的 layer_state_t，则新建一个并设置 surface 和 layerId。