- Qt 5.15
- Qt Quick
- Qt Quick Examples - Threading
- Qt 5.15.19 Reference Documentation

# 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};
                     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