QWebSocket and Synchronous Response/Request Cycle

ad7000

Hello,

I am writing a wrapper around a QWebSocket interface, so that I can provide the application with a simple to use API for making REST like calls and receiving request synchronously. I am using a callback system in which stored callbacks get executed and removed from a queue once a signal is emitted, but I am having trouble implementing the public methods so that they block until the signal is emitted without also blocking the main UI thread.

One such method is as follows:

void ApHorizon::fetchFieldOptions(QString tName, Utility::ApCallback callback)
{    
    ....
    // Some initialization
    ....

    m_messageQueue.append(payload);
    addCallback(tName, callback);

    emit messageQueueChanged(m_messageQueue);

    QEventLoop loop;
    auto handle = connect(this, &ApHorizon::serverMessageReceived, [tName, &loop](const QString type) {
        if (type == tName) loop.quit();
    });
    loop.exec();
    disconnect(handle);
}

That is, I want the method to block until a signal with a specific QString is received, and then continue after the fact, but at the same time without blocking the UI or other application functionality. I have tried moving the class instance to a separate thread but it seems the QEventLoop is working on the main thread no matter what thread I am in.

A humble thanks if you can provide some insight.

jsulm

@ad7000 A synchronous API means a blocking API. If you call such an API from your main thread then it will block. If you don't want this then don't use synchronous APIs.

ad7000

@jsulm Understood. I do want the API to be synchronous, but only within it's own thread, so that it can run in parallel with the main thread. This is what I am trying to figure out.

jsulm

@ad7000 Then use this in its own thread not in main thread.

ad7000

@jsulm I have tried having a QThread member variable in the class and moving the class instance to this thread in the class constructor, something like

MyClass::MyClass(QObject *parent) : QObject(parent) {
    moveToThread(&m_myClassThread);
    m_myClassThread.start();
}

I have done this for several of the classes that use the API class, as well as on the API class itself, but the UI still blocks. Any suggestions?

jsulm

@ad7000 My suggestion would be to post more code, else it is hard to say why your UI is blocking...

ad7000

@jsulm It's alright. I ended up going completely asynchronous by using a chained callback structure. Thanks anyways for the help!