QML Loader Element

The Loader item allows dynamically loading an Item-based subtree from a URL or Component. More...

Inherits Item

This element was introduced in Qt 4.7.

• List of all members, including inherited members

Properties

• item : Item
• progress : real
• source : url
• sourceComponent : Component
• status : enumeration

Signals

• onLoaded

Detailed Description

Loader is used to dynamically load visual QML components. It 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 1.0

 Item {
     width: 200; height: 200

     Loader { id: pageLoader }

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

The loaded item 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 item, freeing resources and leaving the Loader empty.

Loader sizing behavior

Loader is like any other visual item and must be positioned and sized accordingly to become visible.

• 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 1.0

 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 1.0

 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 items

Any signals emitted from the loaded item can be received using the Connections element. 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 1.0

 Item {
     width: 100; height: 100

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

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

 import QtQuick 1.0

 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 the focus documentation page 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 1.0

 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 1.0

 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.

See also Dynamic Object Creation.

Property Documentation

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

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

Signal Documentation