bool BufferStateLayer::setTransactionCompletedListeners(
        const std::vector<sp<CallbackHandle>>& handles) {
    // If there is no handle, we will not send a callback so reset mReleasePreviousBuffer and return
    if (handles.empty()) {
        mReleasePreviousBuffer = false;
        return false;
    }

• 如果 handles 是空的，返回 false。

    const bool willPresent = willPresentCurrentTransaction();

    for (const auto& handle : handles) {
        // If this transaction set a buffer on this layer, release its previous buffer
        handle->releasePreviousBuffer = mReleasePreviousBuffer;

        // If this layer will be presented in this frame
        if (willPresent) {
            // If this transaction set an acquire fence on this layer, set its acquire time
            handle->acquireTime = mCallbackHandleAcquireTime;
            handle->frameNumber = mDrawingState.frameNumber;

            // Notify the transaction completed thread that there is a pending latched callback
            // handle
            mFlinger->getTransactionCallbackInvoker().registerPendingCallbackHandle(handle);

            // Store so latched time and release fence can be set
            mDrawingState.callbackHandles.push_back(handle);

        } else { // If this layer will NOT need to be relatched and presented this frame
            // Notify the transaction completed thread this handle is done
            mFlinger->getTransactionCallbackInvoker().registerUnpresentedCallbackHandle(handle);
        }
    }

    mReleasePreviousBuffer = false;
    mCallbackHandleAcquireTime = -1;

    return willPresent;
}

• BufferStateLayer::willPresentCurrentTransaction() 返回当前 layer 会不会显示。
• 在 BufferStateLayer::setBuffer() 中会设置 mReleasePreviousBuffer 为 true。
• 如果 willPresent 是 true，则设置 acquireTime 和 frameNumber。
  • mCallbackHandleAcquireTime 是在 BufferStateLayer::setAcquireFence() 设置的。
  • 调用 TransactionCallbackInvoker::registerPendingCallbackHandle() 如果从 mCompletedTransactions 可以找到相关 TransactionStats，则 mPendingTransactions 中的计数加一。
  • 把 handle 写入 mDrawingState.callbackHandles。
• 如果 willPresent 是 false，调用 TransactionCallbackInvoker::registerUnpresentedCallbackHandle()。
• 重置 mReleasePreviousBuffer 和 mCallbackHandleAcquireTime。
• 返回 willPresent。