Loader QML Type

Allows dynamic loading of a subtree from a URL or Component More...

• List of all members, including inherited members

Properties

• active : bool
• asynchronous : bool
• item : object
• progress : real
• source : url
• sourceComponent : Component
• status : enumeration

Signals

• loaded()

Methods

• object setSource(url source, object properties)

Detailed Description

Loader is used to dynamically load QML components.

Loader can load a QML file (using the source property) or a Component object (using the sourceComponent property). It is useful for delaying the creation of a component until it is required: for example, when a component should be created on demand, or when a component should not be created unnecessarily for performance reasons.

Here is a Loader that loads "Page1.qml" as a component when the MouseArea is clicked:

import QtQuick 2.0

Item {
    width: 200; height: 200

    Loader { id: pageLoader }

    MouseArea {
        anchors.fill: parent
        onClicked: pageLoader.source = "Page1.qml"
    }
}

The loaded object can be accessed using the item property.

If the source or sourceComponent changes, any previously instantiated items are destroyed. Setting source to an empty string or setting sourceComponent to undefined destroys the currently loaded object, freeing resources and leaving the Loader empty.

Loader sizing behavior

If the source component is not an Item type, Loader does not apply any special sizing rules. When used to load visual types, Loader applies the following sizing rules:

• If an explicit size is not specified for the Loader, the Loader is automatically resized to the size of the loaded item once the component is loaded.
• If the size of the Loader is specified explicitly by setting the width, height or by anchoring, the loaded item will be resized to the size of the Loader.

In both scenarios the size of the item and the Loader are identical. This ensures that anchoring to the Loader is equivalent to anchoring to the loaded item.

import QtQuick 2.0

Item {
    width: 200; height: 200

    Loader {
        // Explicitly set the size of the Loader to the parent item's size
        anchors.fill: parent
        sourceComponent: rect
    }

    Component {
        id: rect
        Rectangle {
            width: 50
            height: 50
            color: "red"
        }
    }
}

import QtQuick 2.0

Item {
    width: 200; height: 200

    Loader {
        // position the Loader in the center of the parent
        anchors.centerIn: parent
        sourceComponent: rect
    }

    Component {
        id: rect
        Rectangle {
            width: 50
            height: 50
            color: "red"
        }
    }
}

Receiving signals from loaded objects

Any signals emitted from the loaded object can be received using the Connections type. For example, the following application.qml loads MyItem.qml, and is able to receive the message signal from the loaded item through a Connections object:

import QtQuick 2.0

Item {
    width: 100; height: 100

    Loader {
       id: myLoader
       source: "MyItem.qml"
    }

    Connections {
        target: myLoader.item
        onMessage: console.log(msg)
    }
}

import QtQuick 2.0

Rectangle {
   id: myItem
   signal message(string msg)

   width: 100; height: 100

   MouseArea {
       anchors.fill: parent
       onClicked: myItem.message("clicked!")
   }
}

Alternatively, since MyItem.qml is loaded within the scope of the Loader, it could also directly call any function defined in the Loader or its parent Item.

Focus and key events

Loader is a focus scope. Its focus property must be set to true for any of its children to get the active focus. (See Keyboard Focus in Qt Quick for more details.) Any key events received in the loaded item should likely also be accepted so they are not propagated to the Loader.

For example, the following application.qml loads KeyReader.qml when the MouseArea is clicked. Notice the focus property is set to true for the Loader as well as the Item in the dynamically loaded object:

import QtQuick 2.0

Rectangle {
    width: 200; height: 200

    Loader {
        id: loader
        focus: true
    }

    MouseArea {
        anchors.fill: parent
        onClicked: loader.source = "KeyReader.qml"
    }

    Keys.onPressed: {
        console.log("Captured:", event.text);
    }
}

import QtQuick 2.0

Item {
    Item {
        focus: true
        Keys.onPressed: {
            console.log("Loaded item captured:", event.text);
            event.accepted = true;
        }
    }
}

Once KeyReader.qml is loaded, it accepts key events and sets event.accepted to true so that the event is not propagated to the parent Rectangle.

Since QtQuick 2.0, Loader can also load non-visual components.

Using a Loader within a view delegate

In some cases you may wish to use a Loader within a view delegate to improve delegate loading performance. This works well in most cases, but there is one important issue to be aware of related to the creation context of a Component.

In the following example, the index context property inserted by the ListView into delegateComponent's context will be inaccessible to Text, as the Loader will use the creation context of myComponent as the parent context when instantiating it, and index does not refer to anything within that context chain.

Item {
    width: 400
    height: 400

    Component {
        id: myComponent
        Text { text: index }    //fails
    }

    ListView {
        anchors.fill: parent
        model: 5
        delegate: Component {
            id: delegateComponent
            Loader {
                sourceComponent: myComponent
            }
        }
    }
}

In this situation we can either move the component inline,

        delegate: Component {
            Loader {
                sourceComponent: Component {
                    Text { text: index }    //okay
                }
            }
        }

into a separate file,

        delegate: Component {
            Loader {
                source: "MyComponent.qml" //okay
            }
        }

or explicitly set the required information as a property of the Loader (this works because the Loader sets itself as the context object for the component it is loading).

Item {
    width: 400
    height: 400

    Component {
        id: myComponent
        Text { text: modelIndex }    //okay
    }

    ListView {
        anchors.fill: parent
        model: 5
        delegate: Component {
            Loader {
                property int modelIndex: index
                sourceComponent: myComponent
            }
        }
    }
}

See also Dynamic Object Creation.

Property Documentation

Loader {
    source: "mycomponent.qml"
    asynchronous: true
    visible: status == Loader.Ready
}

Item {
    Component {
        id: redSquare
        Rectangle { color: "red"; width: 10; height: 10 }
    }

    Loader { sourceComponent: redSquare }
    Loader { sourceComponent: redSquare; x: 10 }
}

Signal Documentation

Method Documentation

// ExampleComponent.qml
import QtQuick 2.0
Rectangle {
    id: rect
    color: "red"
    width: 10
    height: 10

    Behavior on color {
        NumberAnimation {
            target: rect
            property: "width"
            to: (rect.width + 20)
            duration: 0
        }
    }
}

// example.qml
import QtQuick 2.0
Item {
    Loader {
        id: squareLoader
        onLoaded: console.log(squareLoader.item.width); // prints [10], not [30]
    }

    Component.onCompleted: {
        squareLoader.setSource("ExampleComponent.qml", { "color": "blue" });
        // will trigger the onLoaded code when complete.
    }
}