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 : QtObject
• 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

 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

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

 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

 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"
       }
   }
 }

If the source component is not an Item type, Loader does not apply any special sizing rules.

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

 Item {
     width: 100; height: 100

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

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

 import QtQuick

 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

 Rectangle {
     width: 200; height: 200

     Loader {
         id: loader
         focus: true
     }

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

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

 import QtQuick

 Item {
     Item {
         focus: true
         Keys.onPressed: (event)=> {
             console.log("KeyReader 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.
     }
 }