WorkerScript Element Reference

The WorkerScript element enables the use of threads in QML.

Detailed Description

The WorkerScript element enables the use of threads in QML.

Use WorkerScript to run operations in a new thread. This is useful for running operations in the background so that the main GUI thread is not blocked.

Messages can be passed between the new thread and the parent thread using sendMessage() and the onMessage() handler.

An example:

  1. import QtQuick 1.0
  3. Rectangle  {
  4.     width: 300; height: 300
  6.     Text  {
  7.         id: myText
  8.         text: 'Click anywhere'
  9.     }
  11.     WorkerScript  {
  12.         id: myWorker
  13.         source: "script.js"
  15.         onMessage: myText.text = messageObject.reply
  16.     }
  18.     MouseArea  {
  19.         anchors.fill: parent
  20.         onClicked: myWorker.sendMessage( { 'x': mouse.x, 'y': mouse.y })
  21.     }
  22. }

The above worker script specifies a JavaScript file, "script.js", that handles the operations to be performed in the new thread. Here is script.js:

  1. WorkerScript.onMessage = function(message)  {
  2.     // ... long-running operations and calculations are done here
  3.     WorkerScript.sendMessage( { 'reply': 'Mouse is at ' + message.x + ',' + message.y })
  4. }

When the user clicks anywhere within the rectangle, sendMessage() is called, triggering the WorkerScript.onMessage() handler in script.js. This in turn sends a reply message that is then received by the onMessage() handler of myWorker.


Since the WorkerScript.onMessage() function is run in a separate thread, the JavaScript file is evaluated in a context separate from the main QML engine. This means that unlike an ordinary JavaScript file that is imported into QML, the script.js in the above example cannot access the properties, methods or other attributes of the QML item, nor can it access any context properties set on the QML object through QDeclarativeContext.

Additionally, there are restrictions on the types of values that can be passed to and from the worker script. See the sendMessage() documentation for details.


Signal Handlers


Property Documentation

This holds the url of the JavaScript file that implements the WorkerScript.onMessage() handler for threaded operations.

Signal Handler Documentation

  • WorkerScript::onMessage ( jsobject msg )

This handler is called when a message msg is received from a worker script in another thread through a call to sendMessage().

Method Documentation

  • WorkerScript::sendMessage ( jsobject message )

Sends the given message to a worker script handler in another thread. The other worker script handler can receive this message through the onMessage() handler.

The message object may only contain values of the following types:

  • boolean, number, string
  • JavaScript objects and arrays
  • ListModel objects (any other type of QObject* is not allowed)

All objects and arrays are copied to the message. With the exception of ListModel objects, any modifications by the other thread to an object passed in message will not be reflected in the original object.

Notes provided by the Qt Community
  • 5

Votes: 3

Coverage: Qt library 4.7, 4.8

Picture of sigrid sigrid

Lab Rat
20 notes

This person works for Qt Development Frameworks. Nokia Certified Qt Developer

The QML engine and the rendering can only run in the GUI thread

Note that it is not possible to run the QML engine and the rendering in a non-GUI thread, they can only be run in the GUI thread.