Managing asynchronous communication: how to examine response received in another thread

Question

I have a dispatcher thread and a listener thread. When I dispatch a command, I want to wait for response before I send follow up command. Moreover I need to examine the respond before I can proceed with 2nd command, the least of which is to confirm the response is received and everything is okay. My pseudo code is below:

void MainWindow::downloadData()
{
    dispatcher->getInfo(); // sends command

    // QString response = receiver->response() // idealy I would like to check response but since its async, i can't really do that!

    dipatcher->askData(); // the 2nd command and so forth

}

Is there any elegant way to solve this issue? The only way I can think of is if I use the same thread and all calls are blocking but that's not necessarily a good solution.

In Qt, I could use signals and slots to connect them in cascading manner so when the first signal is triggered it initiates the whole sequence of operation (each slot emitting a new signal) but seems rather dirty as well.

Answer 1

One of the most robust ways to handle asynchronous events and process a chains/graphs of actions upon these events are FSMs. Qt provides a basis for implementing FSMs with its Qt-State machine framework. I'd suggest to go this way. Unfortunately all the examples provided by Qt for FSM are dealing with GUIs and animations.

The advantage of FSM approach is, FSMs can be represented both as graphs and as tables. The first option is great for understanding, the second for validation, that there are no endless loops and "dead" ends.

I've built on basis of Qt-FSM framework own framework for defining FSMs in a domain specific language. I use it for controlling a complex machine having couple of sensors actors all working asynchronously. Using DSL helps me to implement in higher abstraction - in the abstraction level of FSM-graphs.