status_t SurfaceComposerClient::Transaction::apply(bool synchronous) {
    if (mStatus != NO_ERROR) {
        return mStatus;
    }

    sp<ISurfaceComposer> sf(ComposerService::getComposerService());

    bool hasListenerCallbacks = !mListenerCallbacks.empty();
    std::vector<ListenerCallbacks> listenerCallbacks;
    // For every listener with registered callbacks
    for (const auto& [listener, callbackInfo] : mListenerCallbacks) {
        auto& [callbackIds, surfaceControls] = callbackInfo;
        if (callbackIds.empty()) {
            continue;
        }

        if (surfaceControls.empty()) {
            listenerCallbacks.emplace_back(IInterface::asBinder(listener), std::move(callbackIds));
        } else {
            // If the listener has any SurfaceControls set on this Transaction update the surface
            // state
            for (const auto& surfaceControl : surfaceControls) {
                layer_state_t* s = getLayerState(surfaceControl);
                if (!s) {
                    ALOGE("failed to get layer state");
                    continue;
                }
                std::vector<CallbackId> callbacks(callbackIds.begin(), callbackIds.end());
                s->what |= layer_state_t::eHasListenerCallbacksChanged;
                s->listeners.emplace_back(IInterface::asBinder(listener), callbacks);
            }
        }
    }

• 遍历 Transaction::mListenerCallbacks。
• 如果 surfaceControls 是空的，把 callbackIds 存到 listenerCallbacks，ListenerCallbacks 向量。
• 否则，遍历 surfaceControls，向 layer_state_t 的 listeners 写入 ListenerCallbacks。给每一个 layer_state_t 都写入 callbackIds 不是冗余的吗？没有副作用吗？
• CallbackId 是在 Transaction::addTransactionCallback() 设置的。
• surfaceControls 是在 Transaction::registerSurfaceControlForCallback() 中设置的。

    cacheBuffers();

Transaction::cacheBuffers() 缓存 buffer，为了减少 GraphicBuffer 跨进程传递成本。

    Vector<ComposerState> composerStates;
    Vector<DisplayState> displayStates;
    uint32_t flags = 0;

    mForceSynchronous |= synchronous;

    for (auto const& kv : mComposerStates){
        composerStates.add(kv.second);
    }

    displayStates = std::move(mDisplayStates);

    if (mForceSynchronous) {
        flags |= ISurfaceComposer::eSynchronous;
    }
    if (mAnimation) {
        flags |= ISurfaceComposer::eAnimation;
    }

    // If both mEarlyWakeupStart and mEarlyWakeupEnd are set
    // it is equivalent for none
    if (mEarlyWakeupStart && !mEarlyWakeupEnd) {
        flags |= ISurfaceComposer::eEarlyWakeupStart;
    }
    if (mEarlyWakeupEnd && !mEarlyWakeupStart) {
        flags |= ISurfaceComposer::eEarlyWakeupEnd;
    }

    sp<IBinder> applyToken = mApplyToken
            ? mApplyToken
            : IInterface::asBinder(TransactionCompletedListener::getIInstance());

拷贝 composerStates 和 displayStates，并根据属性操作 flags。

    sf->setTransactionState(mFrameTimelineInfo, composerStates, displayStates, flags, applyToken,
                            mInputWindowCommands, mDesiredPresentTime, mIsAutoTimestamp,
                            {} /*uncacheBuffer - only set in doUncacheBufferTransaction*/,
                            hasListenerCallbacks, listenerCallbacks, mId);
    mId = generateId();

    // Clear the current states and flags
    clear();

    mStatus = NO_ERROR;
    return NO_ERROR;
}

调用 setTransactionState() 调用 SurfaceFlinger::setTransactionState()。最后调用 Transaction::clear()。