A component is an instantiable QML definition, typically contained in a .qml file. For instance, a Button component may be defined in Button.qml. The QML runtime may instantiate this Button component to create Button objects. Alternatively, a component may be defined inside a Component element.

Moreover, the Button definition may also contain other components. A Button component could use a Text element for its label and other components to implement its functions. Compounding components to form new components (and effectively new interfaces) is the emphasis in QML.

Defining New Components

Any snippet of QML code may become a component, by placing the code in a QML file (extension is .qml). A complete Button component that responds to user input may be in a Button.qml file.

//contents of Button.qml
import QtQuick 1.0

Rectangle {
    id: button
    width: 145; height: 60
    color: "blue"
    smooth: true; radius: 9
    property alias text: label.text
    border {color: "#B9C5D0"; width: 1}

    gradient: Gradient {
        GradientStop {color: "#CFF7FF"; position: 0.0}
        GradientStop {color: "#99C0E5"; position: 0.57}
        GradientStop {color: "#719FCB"; position: 0.9}
    }

    Text {
        id: label
        anchors.centerIn: parent
        text: "Click Me!"
        font.pointSize: 12
        color: "blue"
    }

    MouseArea {
        anchors.fill: parent
        onClicked: console.log(text + " clicked")
    }
}

Alternatively, a Component element may encapsulate a QML object to form a component.

Rectangle {
    Component {
        id: inlinecomponent
        Rectangle {
            id: display
            width: 50; height: 50
            color: "blue"
        }
    }
}

Loading a Component

The initialization of inline components is different from loading a component from a .qml file.

Importing a Component

A component defined in a .qml file is directly usable by declaring the name of the component. For example, a button defined in Button.qml is created by declaring a Button. The button is defined in the Defining New Components section.

import QtQuick 1.0

Rectangle {
    width: 175; height: 350
    color: "lightgrey"

    Column {
        anchors.centerIn: parent
        spacing: 15
        Button {}
        Button {text: "Me Too!"}
        Button {text: "Me Three!"}
    }
}

Note that the component name, Button, matches the QML filename, Button.qml. Also, the first character is in upper case. Matching the names allow components in the same directory to be in the direct import path of the application.

For flexibility, a qmldir file is for dictating which additional components, plugins, or directories should be imported. By using a qmldir file, component names do not need to match the filenames. The qmldir file should, however, be in an imported path.

Button ./Button.qml
FocusButton ./focusbutton.qml

Loading an Inline Component

A consequence of inline components is that initialization may be deferred or delayed. A component may be created during a MouseArea event or by using a Loader element. The component can create an object, which is addressable in a similar way as an id property. Thus, the created object may have its bindings set and read like a normal QML object.

    Component {
        id: inlinecomponent
        Rectangle {
            id: display
            width: 50; height: 50
            color: "blue"
        }
    }
    MouseArea {
        anchors.fill: parent
        onClicked: {
            inlinecomponent.createObject(parent)

            var second = inlinecomponent.createObject(parent)

            var third = inlinecomponent.createObject(parent)
            third.x = second.width + 10
            third.color = "red"
        }
    }

Component Properties

Initializing a component, either from a .qml file or initializing an inline component, have several properties to facilitate component execution. Specifically, there are attached properties and attached signal handlers for setting properties during the lifetime of a component.

The Component.onCompleted attached signal handler is called when the component completes initialization. It is useful for executing any commands after component initialization. Similarly, the Component.onDestruction signal handler executes when the component finishes destruction.

Top-Level Component

Choosing the top-level or the root object of components is an important design aspect because the top-level object dictates which properties are accessible outside the component. Some elements are not visual elements and will not have visual properties exposed outside the component. Likewise, some elements add functionality that are not available to visual elements.

Consider the Button component from the Defining New Components section; it's top-level object is a Rectangle. When imported, the Button component will possess the Rectangle's properties, methods, signals, and any custom properties.

Rectangle {
    //...
    width: 145; height: 60
    color: "blue"
    smooth: true; radius: 9
    property alias text: label.text
    //...
}

The Button's text alias is accessible from outside the component as well as the Rectangle's visual properties and signals such as x, y, anchors, and states.

Alternatively, we may choose a FocusScope as our top-level object. The FocusScope element manage keyboard focus for its children which is beneficial for certain types of interfaces. However, since FocusScopes are not visual elements, the visual properties of its child need to be exposed.

//contents of focusbutton.qml
import QtQuick 1.0

FocusScope {

    //FocusScope needs to bind to visual properties of the children
    property alias color: button.color
    x: button.x; y: button.y
    width: button.width; height: button.height

    Rectangle {
        id: button
        width: 145; height: 60
        color: "blue"
        smooth: true; radius: 9
        property alias text: label.text
        border {color: "#B9C5D0"; width: 1}

        gradient: Gradient {
            GradientStop {color: "#CFF7FF"; position: 0.0}
            GradientStop {color: "#99C0E5"; position: 0.57}
            GradientStop {color: "#719FCB"; position: 0.9}
        }

        Text {
            id: label
            anchors.centerIn: parent
            text: "Click Me!"
            font.pointSize: 12
            color: "blue"
        }

        MouseArea {
            anchors.fill: parent
            onClicked: console.log(text + " clicked")
        }
    }
}