signals.qdoc

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// Copyright (C) 2017 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
\page qtqml-syntax-signals.html
\meta {keywords} {qmltopic}
 
\title Signal and Handler Event System
\brief the event system in QML
 
Application and user interface components need to communicate with each other. For
example, a button needs to know that the user has clicked 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 \e signal is the event
and the signal is responded to through a \e {signal handler}. When a signal
is emitted, the corresponding signal handler is invoked. Placing logic such as
a script or other operations in the handler allows the component to respond to
the event.
 
\target qml-signals-and-handlers
\section1 Receiving signals with signal handlers
 
To receive a notification when a particular signal is emitted for a particular
object, the object definition should declare a signal handler named
\e on<Signal>, where \e <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.
 
For example, the \l [QtQuickControls]{Button} type from the
\l{Qt Quick Controls} module has a \c clicked signal, which
is emitted whenever the button is clicked. In this case, the signal handler for
receiving this signal should be \c onClicked. In the example below, whenever
the button is clicked, the \c onClicked handler is invoked, applying a random
color to the parent \l Rectangle:
 
\qml
import QtQuick
import QtQuick.Controls
 
Rectangle {
    id: rect
    width: 250; height: 250
 
    Button {
        anchors.bottom: parent.bottom
        anchors.horizontalCenter: parent.horizontalCenter
        text: "Change color!"
        onClicked: {
            rect.color = Qt.rgba(Math.random(), Math.random(), Math.random(), 1);
        }
    }
}
\endqml
 
\note Even though signal handlers look a bit like JavaScript functions, you
      should not call them directly. If you need to share code between signal
      handlers and other functionality, refactor it into a separate function.
      Otherwise always emit the signal if you want the signal handler to be
      called. There can be multiple handlers, in different scopes, for the
      same signal.
 
\section2 Property change signal handlers
 
A signal is automatically emitted when the value of a QML property changes.
This type of signal is a \e {property change signal} and signal handlers for
these signals are written in the form \e on<Property>Changed, where
\e <Property> is the name of the property, with the first letter capitalized.
 
For example, the \l MouseArea type has a \l {MouseArea::pressed}{pressed} property.
To receive a notification whenever this property changes, write a signal handler
named \c onPressedChanged:
 
\qml
import QtQuick
 
Rectangle {
    id: rect
    width: 100; height: 100
 
    TapHandler {
        onPressedChanged: console.log("taphandler pressed?", pressed)
    }
}
\endqml
 
Even though the \l TapHandler documentation does not document a signal handler
named \c onPressedChanged, the signal is implicitly provided by the fact that
the \c pressed property exists.
 
\section2 Signal parameters
 
Signals might have parameters. To access those, you should assign a function to the handler. Both
arrow functions and anonymous functions work.
 
For the following examples, consider a Status component with an errorOccurred signal (see
\l{Adding signals to custom QML types} for more information about how signals can be added to
QML components).
 
\qml
// Status.qml
import QtQuick
 
Item {
    id: myitem
 
    signal errorOccurred(message: string, line: int, column: int)
}
\endqml
 
\qml
Status {
    onErrorOccurred: (mgs, line, col) => console.log(`${line}:${col}: ${msg}`)
}
\endqml
 
\note The names of the formal parameters in the function do not have to match those in the
signal.
 
If you do not need to handle all parameters, it is possible to omit trailing ones:
\qml
Status {
    onErrorOccurred: message => console.log(message)
}
\endqml
 
It is not possible to leave out leading parameters you are interested in, however you can use some
placeholder name to indicate to readers that they are not important:
\qml
Status {
    onErrorOccurred: (_, _, col) => console.log(`Error happened at column ${col}`)
}
\endqml
 
\note Instead of using a function, it is possible, but discouraged, to use a plain code block. In
that case all signal parameters get injected into the scope of the block. However, this can make
code difficult to read as it's unclear where the parameters come from, and results in slower
lookups in the QML engine. Injecting parameters in this way is deprecated, and will cause runtime
warnings if the parameter is actually used.
 
\section3 Using the arguments special object
 
In JavaScript you can refer to the \c arguments special
object, which, when available, allows to access the values of the
arguments passed to a non-arrow function as an array-like object.
 
It is usually available in the body of a function or code block that
is assigned to a signal handler.
 
When a code block or an anonymous function is assigned to the signal
handler, the special \c arguments object will provide the arguments
that were passed over by the signal.
 
For example, both of the following will print \c{[object Arguments] world undefined}:
 
\qml
import QtQml
 
QtObject {
    id: root
 
    signal hello(message: string)
 
    onHello: { console.log(arguments, arguments[0], arguments[1]) }
 
    Component.onCompleted: root.hello("world")
}
\endqml
 
\qml
import QtQml
 
QtObject {
    id: root
 
    signal hello(message: string)
 
    onHello: function () { console.log(arguments, arguments[0], arguments[1]) }
 
    Component.onCompleted: root.hello("world")
}
\endqml
 
The behavior will differ when an arrow function is assigned to the signal handler.
Then, it will still be possible to access the \c arguments
special object, but it will be an empty array-like object.
 
For example, the following will print \c{[object Arguments] undefined undefined}:
 
\qml
import QtQml
 
QtObject {
    id: root
 
    signal hello(message: string)
 
    onHello: () => { console.log(arguments, arguments[0], arguments[1]) }
 
    Component.onCompleted: root.hello("world")
}
\endqml
 
The difference in behavior is due to the way the \c{arguments} special
object interacts with arrow functions but is consistent with the general
behavior for bindings.
 
By specification, an arrow function does not carry its own
\c arguments special object.
As an arrow function still borrows from its enclosing context, it can
borrow the \c arguments special object if one is available.
 
A binding provides its own scope on evaluation.
In particular, the retrieval of the underlying arrow function is
performed in the scope provided by the evaluation of the binding.
 
In the scope of the binding no argument is provided, such that an
empty \c arguments special object will be available and borrowed by
the arrow function on retrieval.
 
As a non-arrow function does provide the \c arguments special object
in its own scope it can refer to the arguments that were provided to
the underlying function itself, which are the forwarded arguments
provided by the signal.
 
Usage of the \c arguments special object should generally be avoided
in favor of the usage of named parameters, which are more explicit and
work consistently independently of the usage of an arrow or a non-arrow
function.
 
\section2 Using the Connections type
 
In some cases it may be desirable to access a signal outside of the object that
emits it. For these purposes, the \c QtQuick module provides the \l Connections
type for connecting to signals of arbitrary objects. A \l Connections object
can receive any signal from its specified \l {Connections::target}{target}.
 
For example, the \c onClicked handler in the earlier example could have been
received by the root \l Rectangle instead, by placing the \c onClicked handler
in a \l Connections object that has its \l {Connections::target}{target} set to
the \c button:
 
\qml
import QtQuick
import QtQuick.Controls
 
Rectangle {
    id: rect
    width: 250; height: 250
 
    Button {
        id: button
        anchors.bottom: parent.bottom
        anchors.horizontalCenter: parent.horizontalCenter
        text: "Change color!"
    }
 
    Connections {
        target: button
        function onClicked() {
            rect.color = Qt.rgba(Math.random(), Math.random(), Math.random(), 1);
        }
    }
}
\endqml
 
 
\section2 Attached signal handlers
 
An \l {Attached Properties and Attached Signal Handlers}{attached signal handler}
receives a signal from an \e {attaching type} rather than the object within which
the handler is declared.
 
For example, \l{Component::completed}{Component.onCompleted} is an attached
signal handler. It is often used to execute some JavaScript code when its
creation process is complete. Here is an example:
 
\qml
import QtQuick
 
Rectangle {
    width: 200; height: 200
    color: Qt.rgba(Qt.random(), Qt.random(), Qt.random(), 1)
 
    Component.onCompleted: {
        console.log("The rectangle's color is", color)
    }
}
\endqml
 
The \c onCompleted handler is not responding to a \c completed signal from
the \l Rectangle type. Instead, an object of the \c Component \e{attaching type}
with a \c completed signal has automatically been \e attached to the \l Rectangle
object by the QML engine. The engine emits this signal when the Rectangle object is
created, thus triggering the \c Component.onCompleted signal handler.
 
Attached signal handlers allow objects to be notified of particular signals that are
significant to each individual object. If there was no \c Component.onCompleted
attached signal handler, for example, an object could not receive this notification
without registering for some special signal from some special object.
The \e {attached signal handler} mechanism enables objects to receive particular
signals without extra code.
 
See \l {Attached properties and attached signal handlers} for more information on
attached signal handlers.
 
\section1 Adding signals to custom QML types
 
Signals can be added to custom QML types through the \c signal keyword.
 
The preferred syntax for defining a new signal is:
 
\tt{signal <name>[([<parameter name> : <type>[, ...]])]}
 
There is also an old syntax with the type before the name:
 
\tt{signal <name>[([<type> <parameter name>[, ...]])]}
 
You should use the new syntax.
 
A signal is emitted by invoking the signal as a method.
 
For example, the code below is defined in a file named \c SquareButton.qml. The
root \l Rectangle object has an \c activated signal, which is emitted whenever the
child \l TapHandler is \c tapped. In this particular example the activated signal
is emitted with the x and y coordinates of the mouse click:
 
\qml
// SquareButton.qml
import QtQuick
 
Rectangle {
    id: root
 
    signal activated(xPosition: real, yPosition: real)
    property point mouseXY
    property int side: 100
    width: side; height: side
 
    TapHandler {
        id: handler
        onTapped: root.activated(root.mouseXY.x, root.mouseXY.y)
        onPressedChanged: root.mouseXY = handler.point.position
    }
}
\endqml
 
Now any objects of the \c SquareButton can connect to the \c activated signal using an \c onActivated signal handler:
 
\qml
// myapplication.qml
SquareButton {
    onActivated: (xPosition, yPosition) => console.log(`Activated at {xPosition}, ${yPosition}`)
}
\endqml
 
See \l {Signal Attributes} for more details on writing signals for custom QML types.
 
 
\target qml-connect-signals-to-method
\section1 Connecting signals to methods and signals
 
Signal objects have a \c 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.
 
Below, the \c messageReceived signal is connected to three methods using the \c connect() method:
 
\qml
import QtQuick
 
Rectangle {
    id: relay
 
    signal messageReceived(person: string, notice: string)
 
    Component.onCompleted: {
        relay.messageReceived.connect(sendToPost)
        relay.messageReceived.connect(sendToTelegraph)
        relay.messageReceived.connect(sendToEmail)
        relay.messageReceived("Tom", "Happy Birthday")
    }
 
    function sendToPost(person: string, notice: string) {
        console.log(`Sending to post: ${person}, ${notice}`)
    }
    function sendToTelegraph(person: string, notice: string) {
        console.log(`Sending to telegraph: ${person}, ${notice}`)
    }
    function sendToEmail(person: string, notice: string) {
        console.log(`Sending to email: ${person}, ${notice}`)
    }
}
\endqml
 
In many cases it is sufficient to receive signals through signal handlers
rather than using the connect() function. However, using the \c connect
method allows a signal to be received by multiple methods as shown earlier,
which would not be possible with signal handlers as they must be uniquely
named. Also, the \c connect method is useful when connecting signals to
\l {Dynamic QML Object Creation from JavaScript}{dynamically created objects}.
 
There is a corresponding \c disconnect() method for removing connected signals:
 
\qml
Rectangle {
    id: relay
    //...
 
    function removeTelegraphSignal() {
        relay.messageReceived.disconnect(sendToTelegraph)
    }
}
\endqml
 
\section3 Signal to signal connect
 
By connecting signals to other signals, the \c connect() method can form different
signal chains.
 
\qml
import QtQuick
 
Rectangle {
    id: forwarder
    width: 100; height: 100
 
    signal send()
    onSend: console.log("Send clicked")
 
    TapHandler {
        id: mousearea
        anchors.fill: parent
        onTapped: console.log("Mouse clicked")
    }
 
    Component.onCompleted: {
        mousearea.tapped.connect(send)
    }
}
\endqml
 
 
Whenever the \l TapHandler's \c tapped signal is emitted, the \c send
signal will automatically be emitted as well.
 
\code
output:
    MouseArea clicked
    Send clicked
\endcode
 
\note Connections to function objects will stay alive as long as the sender of the signal is alive.
This behavior is analogous to the 3-argument version of QObject::connect() in C++.
 
\qml
Window {
    visible: true
    width: 400
    height: 400
 
    Item {
        id: item
        property color globalColor: "red"
 
        Button {
            text: "Change global color"
            onPressed: {
                item.globalColor = item.globalColor === Qt.color("red") ? "green" : "red"
            }
        }
 
        Button {
            x: 150
            text: "Clear rectangles"
            onPressed: repeater.model = 0
        }
 
        Repeater {
            id: repeater
            model: 5
            Rectangle {
                id: rect
                color: "red"
                width: 50
                height: 50
                x: (width + 2) * index + 2
                y: 100
                Component.onCompleted: {
                    if (index % 2 === 0) {
                        item.globalColorChanged.connect(() => {
                            color = item.globalColor
                        })
                    }
                }
            }
        }
    }
}
\endqml
 
In the contrived example above, the goal is to flip the color of every even rectangle to follow
some global color. To achieve this, for every even rectangle, a connection is made between the
globalColorChanged signal and a function to set the rectangle's color. This works as expected while
the rectangles are alive. However, once the clear button is pressed, the rectangles are gone but
the function handling the signal is still called every time the signal is emitted. This can be
seen by the error messages thrown by the function trying to run in the background when changing
the global color.
 
In the current setup, the connections would only be destroyed once the item holding
globalColor is destroyed. To prevent the connections from lingering on, they can be explicitly
disconnected when the rectangles are being destroyed.
 */