Qt Quick Examples - Threading#

This is a collection of QML multithreading examples.


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};

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.sync();   // updates the changes to the list


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";
                resultText.text = messageObject.result;

Example project @ code.qt.io