Qt Quick Examples - Threading

Threading is a collection of QML multithreading examples.

Running the Example

To run the example from Qt Creator, open the Welcome mode and select the example from Examples. For more information, visit Building and Running an Example.

Threaded ListModel

Threaded ListModel contains a ListView and a ListModel. The ListModel object is updated asynchronously in another thread, and the results propagate back to the main thread. A timer requests updates from the worker thread periodically:

        Timer {
            id: timer
            interval: 2000; repeat: true
            running: true
            triggeredOnStart: true

            onTriggered: {
                var msg = {'action': 'appendCurrentTime', 'model': listModel};
                worker.sendMessage(msg);
            }
        }

Inside the worker thread, the ListModel is synchronized once the data is finished loading:

WorkerScript.onMessage = function(msg) {
    if (msg.action == 'appendCurrentTime') {
        var data = {'time': new Date().toTimeString()};
        msg.model.append(data);
        msg.model.sync();   // updates the changes to the list
    }
}

WorkerScript

WorkerScript contains an example of using a WorkerScript to offload expensive calculations into another thread. This keeps the UI from being blocked. This example calculates numbers in Pascal's Triangle, and not in a very optimal way, so it will often take several seconds to complete the calculation. By doing this in a WorkerScript in another thread, the UI is not blocked during this time.

When the UI needs another value, a request is sent to the WorkerScript:

        Spinner {
            id: rowSpinner
            label: "Row"
            onValueChanged: {
                resultText.text = "Loading...";
                myWorker.sendMessage( { row: rowSpinner.value, column: columnSpinner.value } );
            }
        }

The workerscript then is free to take a really long time to calculate it:

WorkerScript.onMessage = function(message) {
    //Calculate result (may take a while, using a naive algorithm)
    var calculatedResult = triangle(message.row, message.column);
    //Send result back to main thread
    WorkerScript.sendMessage( { row: message.row,
                                column: message.column,
                                result: calculatedResult} );
}

When it's done, the result returns to the main scene via the WorkerScript type:

    WorkerScript {
        id: myWorker
        source: "workerscript.mjs"

        onMessage: (messageObject) => {
            if (messageObject.row == rowSpinner.value && messageObject.column == columnSpinner.value){ //Not an old result
                if (messageObject.result == -1)
                    resultText.text = "Column must be <= Row";
                else
                    resultText.text = messageObject.result;
            }
        }
    }

Example project @ code.qt.io