void SurfaceFlinger::applyTransactionState(const FrameTimelineInfo& frameTimelineInfo,
                                           const Vector<ComposerState>& states,
                                           Vector<DisplayState>& displays, uint32_t flags,
                                           const InputWindowCommands& inputWindowCommands,
                                           const int64_t desiredPresentTime, bool isAutoTimestamp,
                                           const client_cache_t& uncacheBuffer,
                                           const int64_t postTime, uint32_t permissions,
                                           bool hasListenerCallbacks,
                                           const std::vector<ListenerCallbacks>& listenerCallbacks,
                                           int originPid, int originUid, uint64_t transactionId) {
    uint32_t transactionFlags = 0;
    for (DisplayState& display : displays) {
        display.sanitize(permissions);
        transactionFlags |= setDisplayStateLocked(display);
    }

遍历 DisplayState。

• 根据 permissions 移除无权限的 flag。
• 调用 setDisplayStateLocked() 设置 DisplayState。

void SurfaceFlinger::applyTransactionState(...
    ...
    // start and end registration for listeners w/ no surface so they can get their callback.  Note
    // that listeners with SurfaceControls will start registration during setClientStateLocked
    // below.
    for (const auto& listener : listenerCallbacks) {
        mTransactionCallbackInvoker.startRegistration(listener);
        mTransactionCallbackInvoker.endRegistration(listener);
    }

    std::unordered_set<ListenerCallbacks, ListenerCallbacksHash> listenerCallbacksWithSurfaces;
    uint32_t clientStateFlags = 0;
    for (const ComposerState& state : states) {
        clientStateFlags |=
                setClientStateLocked(frameTimelineInfo, state, desiredPresentTime, isAutoTimestamp,
                                     postTime, permissions, listenerCallbacksWithSurfaces);
        if ((flags & eAnimation) && state.state.surface) {
            if (const auto layer = fromHandle(state.state.surface).promote(); layer) {
                mScheduler->recordLayerHistory(layer.get(),
                                               isAutoTimestamp ? 0 : desiredPresentTime,
                                               LayerHistory::LayerUpdateType::AnimationTX);
            }
        }
    }

    for (const auto& listenerCallback : listenerCallbacksWithSurfaces) {
        mTransactionCallbackInvoker.endRegistration(listenerCallback);
    }

    // If the state doesn't require a traversal and there are callbacks, send them now
    if (!(clientStateFlags & eTraversalNeeded) && hasListenerCallbacks) {
        mTransactionCallbackInvoker.sendCallbacks();
    }
    transactionFlags |= clientStateFlags;

• 遍历 listenerCallbacks，调用 TransactionCallbackInvoker 的 startRegistration() 和 endRegistration()。
• 遍历 ComposerState，调用 SurfaceFlinger::setClientStateLocked()。
• 如果是动画事务，调用 recordLayerHistory() 记录 layer。
• listenerCallbacksWithSurfaces 是在 setClientStateLocked() 设置的，
• 如果不需要 traversal，则调用 sendCallbacks() 发送回调。

void SurfaceFlinger::applyTransactionState(...
    ...
    if (permissions & Permission::ACCESS_SURFACE_FLINGER) {
        transactionFlags |= addInputWindowCommands(inputWindowCommands);
    } else if (!inputWindowCommands.empty()) {
        ALOGE("Only privileged callers are allowed to send input commands.");
    }

    if (uncacheBuffer.isValid()) {
        ClientCache::getInstance().erase(uncacheBuffer);
    }

• 处理 inputWindowCommands。
• 如果 uncacheBuffer 可用，说明有缓存失效，则从 ClientCache 中移除。

    // If a synchronous transaction is explicitly requested without any changes, force a transaction
    // anyway. This can be used as a flush mechanism for previous async transactions.
    // Empty animation transaction can be used to simulate back-pressure, so also force a
    // transaction for empty animation transactions.
    if (transactionFlags == 0 &&
            ((flags & eSynchronous) || (flags & eAnimation))) {
        transactionFlags = eTransactionNeeded;
    }

    if (transactionFlags) {
        if (mInterceptor->isEnabled()) {
            mInterceptor->saveTransaction(states, mCurrentState.displays, displays, flags,
                                          originPid, originUid, transactionId);
        }

        // We are on the main thread, we are about to preform a traversal. Clear the traversal bit
        // so we don't have to wake up again next frame to preform an unnecessary traversal.
        if (transactionFlags & eTraversalNeeded) {
            transactionFlags = transactionFlags & (~eTraversalNeeded);
            mForceTraversal = true;
        }
        if (transactionFlags) {
            setTransactionFlags(transactionFlags);
        }

        if (flags & eAnimation) {
            mAnimTransactionPending = true;
        }
    }
}

• 如果设置了 eSynchronous 或 eAnimation，则设置 eTransactionNeeded。
• saveTransaction() 再看。
• applyTransactionState() 可能在主线程调用，在 onInitializeDisplays()。为了避免主线程 traversal，先移除 eTraversalNeeded，并设置 mForceTraversal 为 true。