- Qt 5.5
- Qt Quick
- Qt Quick Examples - Threading
- Qt 5.5.1 Reference Documentation

# 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.js"

            onMessage: {
                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;
                }
            }
        }

Files:

- threading/threading.qml
- threading/threadedlistmodel/dataloader.js
- threading/threadedlistmodel/timedisplay.qml
- threading/workerscript/Spinner.qml
- threading/workerscript/workerscript.js
- threading/workerscript/workerscript.qml
- threading/main.cpp
- threading/threading.pro
- threading/threading.qmlproject
- threading/threading.qrc
- threading/threadedlistmodel/threadedlistmodel.qmlproject
- threading/workerscript/workerscript.qmlproject