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<td >Qt 5.12</td><td ><a href="qtquick-index.html">Qt Quick</a></td><td >Qt Quick Examples - Threading</td></tr></table><table class="buildversion"><tr>
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<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#running-the-example">Running the Example</a></li>
<li class="level1"><a href="#threaded-listmodel">Threaded ListModel</a></li>
<li class="level1"><a href="#workerscript">WorkerScript</a></li>
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<h1 class="title">Qt Quick Examples - Threading</h1>
<span class="subtitle"></span>
<!-- $$$threading-brief -->
<p>This is a collection of QML multithreading examples.</p>
<!-- @@@threading -->
<!-- $$$threading-description -->
<div class="descr"> <a name="details"></a>
<p class="centerAlign"><img src="images/qml-threading-example.png" alt="" /></p><p><i>Threading</i> is a collection of QML multithreading examples.</p>
<a name="running-the-example"></a>
<h2 id="running-the-example">Running the Example</h2>
<p>To run the example from Qt Creator, open the <b>Welcome</b> mode and select the example from <b>Examples</b>. For more information, visit Building and Running an Example.</p>
<a name="threaded-listmodel"></a>
<h2 id="threaded-listmodel">Threaded ListModel</h2>
<p><i>Threaded ListModel</i> contains a <a href="qml-qtquick-listview.html">ListView</a> 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:</p>
<pre class="qml">

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

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

</pre>
<p>Inside the worker thread, the ListModel is synchronized once the data is finished loading:</p>
<pre class="cpp">

  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
      }
  }

</pre>
<a name="workerscript"></a>
<h2 id="workerscript">WorkerScript</h2>
<p><i>WorkerScript</i> contains an example of using a <a href="qtquick-threading-example.html#workerscript">WorkerScript</a> 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 <a href="qtquick-threading-example.html#workerscript">WorkerScript</a> in another thread, the UI is not blocked during this time.</p>
<p>When the UI needs another value, a request is sent to the <a href="qtquick-threading-example.html#workerscript">WorkerScript</a>:</p>
<pre class="qml">

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

</pre>
<p>The workerscript then is free to take a really long time to calculate it:</p>
<pre class="cpp">

  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} );
  }

</pre>
<p>When it's done, the result returns to the main scene via the <a href="qtquick-threading-example.html#workerscript">WorkerScript</a> type:</p>
<pre class="qml">

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

          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;
              }
          }
      }

</pre>
<p>Files:</p>
<ul>
<li><a href="qtquick-threading-main-cpp.html">threading/main.cpp</a></li>
<li><a href="qtquick-threading-threadedlistmodel-threadedlistmodel-qmlproject.html">threading/threadedlistmodel/threadedlistmodel.qmlproject</a></li>
<li><a href="qtquick-threading-threadedlistmodel-timedisplay-qml.html">threading/threadedlistmodel/timedisplay.qml</a></li>
<li><a href="qtquick-threading-threading-pro.html">threading/threading.pro</a></li>
<li><a href="qtquick-threading-threading-qml.html">threading/threading.qml</a></li>
<li><a href="qtquick-threading-threading-qmlproject.html">threading/threading.qmlproject</a></li>
<li><a href="qtquick-threading-threading-qrc.html">threading/threading.qrc</a></li>
<li><a href="qtquick-threading-workerscript-spinner-qml.html">threading/workerscript/Spinner.qml</a></li>
<li><a href="qtquick-threading-workerscript-workerscript-qml.html">threading/workerscript/workerscript.qml</a></li>
<li><a href="qtquick-threading-workerscript-workerscript-qmlproject.html">threading/workerscript/workerscript.qmlproject</a></li>
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