Qt is based on the QObject class, which provides a signal-slot system. This means that QML also uses this system to handle signals and slots. In this article, we will explore how to use the signal-slot system in QML.

Basic Signals

Every QML item has two basic signals:

• completed(): emitted when the object is created
• destruction(): emitted when the object is destroyed

Let's create a simple example to demonstrate how to use signals and slots in QML.

SignalItem.qml

import QtQuick 2.12

QtObject {
 signal signalItemCreated(string name)

 Component.onCompleted: {
 signalItemCreated("SignalItem")
 }
}

main.qml

import QtQuick 2.13
import QtQuick.Window 2.13

Window {
 visible: true
 width: 640
 height: 480
 title: qsTr("Hello World")

 Component.onCompleted: {
 console.log("Window created.")
 }

 SignalItem {
 id: mySignalItem
 onSignalItemCreated: {
 console.log(name + " created.")
 }
 }
}

In this example, we define a SignalItem QML item that emits a signalItemCreated signal when it is created. We also create a main.qml file that contains an instance of the SignalItem and sets up a handler for the signalItemCreated signal.

Connecting Signals to Slots

To connect signals from one object to slots in another object, we can use the Connections element.

main.qml

import QtQuick 2.13
import QtQuick.Window 2.13

Window {
 visible: true
 width: 640
 height: 480
 title: qsTr("Hello World")

 Component.onCompleted: {
 console.log("Window created.")
 }

 Connections {
 target: mySignalItem

 onSignalItemCreated: {
 console.log("SignalItem created.")
 }
 }

 SignalItem {
 id: mySignalItem
 }
}

In this example, we connect the signalItemCreated signal from the mySignalItem object to a slot that prints a message to the console.

Limitations of Connections

However, there are some limitations when using the Connections element. For example:

• We cannot define multiple handlers for the same signal
• The target object must be in scope for the connections to work

To overcome these limitations, we can use the C++-style approach to connect signals and slots.

Using QObject's Connect and Disconnect Methods

We can use the connect and disconnect methods provided by the QObject class to connect signals from one object to slots in another object.

main.qml

import QtQuick 2.13
import QtQuick.Window 2.13

Window {
 visible: true
 width: 640
 height: 480
 title: qsTr("Hello World")

 Component.onCompleted: {
 console.log("Window created.")

 // Connect the signalItemCreated signal to the signalItemCreatedHandler slot.
 mySignalItem.signalItemCreated.connect(signalItemCreatedHandler)
 }

 function signalItemCreatedHandler(){
 console.log("SignalItem created.")
 }

 SignalItem {
 id: mySignalItem
 }
}

In this example, we use the connect method to connect the signalItemCreated signal from the mySignalItem object to a slot that prints a message to the console.

By using the signal-slot system in QML, we can decouple the interaction between objects and make our code more modular and reusable.