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QML Signal and Handler Event SystemApplication and user interface components communicate with each other. For example, a button component needs to know that the user is clicking on it. The button may change colors to indicate its state or perform some logic. As well, application needs to know whether the user is clicking the button. The application may need to relay this clicking event to other applications. QML has a signal and handler mechanism, where the signal is the event and the component responds to the event through the handler. The signal is emitted and the handler is invoked. Placing logic such as scripts or other operations in the handler allows the component to respond to the event. Signals and HandlersSignals provide a way to notify other objects when an event has occurred. For example, the MouseArea clicked signal notifies other objects that the mouse has been clicked within the area. The syntax for defining a new signal is: signal <name>[([<type> <parameter name>[, ...]])] Attempting to declare two signals or methods with the same name in the same type block generates an error. However, a new signal may reuse the name of an existing signal on the type. (This should be done with caution, as the existing signal may be hidden and become inaccessible.) Here are various examples of signal declarations: If the signal has no parameters, the "()" brackets are optional. If parameters are used, the parameter types must be declared, as for the string and variant arguments of the perform signal. Adding a signal to an item automatically adds a signal handler as well. The signal hander is named on<SignalName>, with the first letter of the signal in uppercase. The previous signals have the following signal handlers: Further, each QML properties have a <property_name>Changed signal and its corresponding on<property_name>Changed signal handler. As a result, property changes may notify other components for any changes. To emit a signal, invoke it as a method. The signal handler binding is similar to a property binding and it is invoked when the signal is emitted. Use the defined argument names to access the respective arguments. Note that the Component.onCompleted is an attached signal handler; it is invoked when the Component initialization is complete. Connecting Signals to Methods and SignalsSignal objects have a connect() method to a connect a signal either to a method or another signal. When a signal is connected to a method, the method is automatically invoked whenever the signal is emitted. This mechanism enables a signal to be received by a method instead of a signal handler. The connect() method is appropriate when connecting a JavaScript method to a signal. There is a corresponding disconnect() method for removing connected signals. Signal to Signal ConnectBy connecting signals to other signals, the connect() method can form different signal chains. Whenever the MouseArea clicked signal is emitted, the send signal will automatically be emitted as well. output:
MouseArea clicked
Send clicked
Events from the Declarative RuntimeThere maybe cases where a signal comes from the declarative runtime. For example, it is possible to receive events from QML plugins. For more signal control, the connect() method and the Connections element may connect a signal from the runtime to another signal or method. For complete information on events from the runtime or creating signals from the runtime, read the The QML Engine and the C++ Types as QML Types articles. |
