Connecting Objects to Signals

To receive a notification when a particular signal is emitted for a particular object, the object definition should declare a signal handler named on<Signal> where <Signal> is the name of the signal, with the first letter capitalized. The signal handler should contain the JavaScript code to be executed when the signal handler is invoked.

QML types have predefined signals that are emitted when users interact with the application. For example, the Mouse Area type has a clicked signal that is emitted whenever the mouse is clicked within the area. Since the signal name is clicked, the signal handler for receiving this signal is named onClicked.

A signal is automatically emitted when the value of a QML property changes. This type of signal is a property change signal and signal handlers for these signals are written in the form on<Property>Changed, where <Property> is the name of the property, with the first letter capitalized.

For example, the Mouse Area type has a pressed property. To receive a notification whenever this property changes, you would use a signal handler called onPressedChanged.

For more information about signals and signal handlers, see Signal and Handler Event System.

To access a signal outside of the object that emits it, you can use the Connections type for connecting to signals of arbitrary objects. A connections object can receive any signal from its specified target. You can connect objects to signals that are available to them in the Connections view.

To connect objects to signals:

  1. In the Connections view, select the Connections tab.
  2. Select the (Add) button to add a connection.
  3. Double-click the value in the Target column to add the object to connect to a signal.
  4. Double-click the value in the Signal Handler column to select the signal that the connection will listen to from a list of all signals available for the object.
  5. Double-click the value in the Actions column to specify the action to perform when the signal is emitted. You use JavaScript to specify the actions.

Right-click a connection and select Open Connection Editor in the context menu to specify the connection in Connection Editor.

Adding Signal Handlers

If a signal handler that you need is not listed in the Signal Handler column, you can add it:

  1. Right-click a component in the Navigator or Form Editor view and select Add New Signal Handler in the context menu.
  2. In the Signal field, select the signal to handle.

    "Implement Signal Handler dialog"

  3. Select the radio buttons to filter the list to only display frequently used signals or property changes.
  4. Select OK.

The added signal handler is automatically exported as a property.

Adding Actions and Assignments

You use the Connection Editor to create the JavaScript expressions for actions and assignments. An action connects an object to a signal, whereas an assignment fetches property values from another object.

For more information about the logical operators that you can use to construct conditional expressions, see Summary of Logical Operators.

To create JavaScript expressions for actions:

  1. Select Action as the type of the connections object.
  2. Select the object to connect to a signal.
  3. Select the action to perform when the signal is emitted.

To create JavaScript expressions for assignments:

  1. Select Assignment as the type of the connections object.
  2. Select the target object for the property assignment.
  3. Select the property of the target object to assign a value to.
  4. Select the source object for the property assignment.
  5. Select the property of the source object to fetch the value from.

For an example of connecting the clicked signal of a button to a state, see Log In UI - Part 3.

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