Defining Object Types through QML Documents

One of the core features of QML is that it enables QML object types to be easily defined in a lightweight manner through QML documents to suit the needs of individual QML applications. The standard Qt Quick module provides various types like Rectangle, Text and Image for building a QML application; beyond these, you can easily define your own QML types to be reused within your application. This ability to create your own types forms the building blocks of any QML application.

Defining an Object Type with a QML File

Naming Custom QML Object Types

To create an object type, a QML document should be placed into a text file named as <TypeName>.qml where <TypeName> is the desired name of the type. The type name has the following requirements:

• It must be comprised of alphanumeric characters or underscores.
• It must begin with an uppercase letter.

This document is then automatically recognized by the engine as a definition of a QML type. Additionally, a type defined in this manner is automatically made available to other QML files within the same local directory as the engine searches within the immediate directory when resolving QML type names.

Custom QML Type Definition

For example, below is a document that declares a Rectangle with a child MouseArea. The document has been saved to file named SquareButton.qml:

// SquareButton.qml
import QtQuick 2.0

Rectangle {
    property int side: 100
    width: side; height: side
    color: "red"

    MouseArea {
        anchors.fill: parent
        onClicked: console.log("Button clicked!")
    }
}

Since the file is named SquareButton.qml, this can now be used as a type named SquareButton by any other QML file within the same directory. For example, if there was a myapplication.qml file in the same directory, it could refer to the SquareButton type:

// myapplication.qml
import QtQuick 2.0

SquareButton {}

This creates a 100 x 100 red Rectangle with an inner MouseArea, as defined in SquareButton.qml. When this myapplication.qml document is loaded by the engine, it loads the SquareButton.qml document as a component and instantiates it to create a SquareButton object.

The SquareButton type encapsulates the tree of QML objects declared in SquareButton.qml. When the QML engine instantiates a SquareButton object from this type, it is instantiating an object from the Rectangle tree declared in SquareButton.qml.

Inline Components

Sometimes, it can be inconvenient to create a new file for a type, for instance when reusing a small delegate in multiple views. If you don't actually need to expose the type, but only need to create an instance, Component is an option. But if you want to declare properties with the component types, or if you want to use it in multiple files, Component is not an option. In that case, you can use inline components. Inline components declare a new component inside of a file. The syntax for that is

component <component name> : BaseType {
    // declare properties and bindings here
}

Inside the file which declares the inline component, the type can be referenced simply by its name.

// Images.qml
import QtQuick 2.15

Item {
    component LabeledImage: Column {
        property alias source: image.source
        property alias caption: text.text

        Image {
            id: image
            width: 50
            height: 50
        }
        Text {
            id: text
            font.bold: true
        }
    }

    Row {
        LabeledImage {
            id: before
            source: "before.png"
            caption: "Before"
        }
        LabeledImage {
            id: after
            source: "after.png"
            caption: "After"
        }
    }
    property LabeledImage selectedImage: before
}

In other files, it has to be prefixed with the name of its containing component.

// LabeledImageBox.qml
import QtQuick 2.15

Rectangle {
    property alias caption: image.caption
    property alias source: image.source
    border.width: 2
    border.color: "black"
    Images.LabeledImage {
        id: image
    }
}

// A.qml
import QtQuick 2.15

Item {
    id: root
    property string message: "From A"
    component MyInlineComponent : Item {
        Component.onCompleted: console.log(root.message)
    }
}
// B.qml
import QtQuick 2.15

Item {
    A.MyInlineComponent {}
}

Importing Types Defined Outside the Current Directory

If SquareButton.qml was not in the same directory as myapplication.qml, the SquareButton type would need to be specifically made available through an import statement in myapplication.qml. It could be imported from a relative path on the file system, or as an installed module; see module for more details.

Accessible Attributes of Custom Types

The root object definition in a .qml file defines the attributes that are available for a QML type. All properties, signals and methods that belong to this root object - whether they are custom declared, or come from the QML type of the root object - are externally accessible and can be read and modified for objects of this type.

For example, the root object type in the SquareButton.qml file above is Rectangle. This means any properties defined by the Rectangle type can be modified for a SquareButton object. The code below defines three SquareButton objects with customized values for some of the properties of the root Rectangle object of the SquareButton type:

// application.qml
import QtQuick 2.0

Column {
    SquareButton { side: 50 }
    SquareButton { x: 50; color: "blue" }
    SquareButton { radius: 10 }
}

The attributes that are accessible to objects of the custom QML type include any custom properties, methods and signals that have additionally been defined for an object. For example, suppose the Rectangle in SquareButton.qml had been defined as follows, with additional properties, methods and signals:

// SquareButton.qml
import QtQuick 2.0

Rectangle {
    id: root

    property bool pressed: mouseArea.pressed

    signal buttonClicked(real xPos, real yPos)

    function randomizeColor() {
        root.color = Qt.rgba(Math.random(), Math.random(), Math.random(), 1)
    }

    property int side: 100
    width: side; height: side
    color: "red"

    MouseArea {
        id: mouseArea
        anchors.fill: parent
        onClicked: (mouse)=> root.buttonClicked(mouse.x, mouse.y)
    }
}

Any SquareButton object could make use of the pressed property, buttonClicked signal and randomizeColor() method that have been added to the root Rectangle:

// application.qml
import QtQuick 2.0

SquareButton {
    id: squareButton

    onButtonClicked: (xPos, yPos)=> {
        console.log("Clicked", xPos, yPos)
        randomizeColor()
    }

    Text { text: squareButton.pressed ? "Down" : "Up" }
}

Note that any of the id values defined in SquareButton.qml are not accessible to SquareButton objects, as id values are only accessible from within the component scope in which a component is declared. The SquareButton object definition above cannot refer to mouseArea in order to refer to the MouseArea child, and if it had an id of root rather than squareButton, this would not conflict with the id of the same value for the root object defined in SquareButton.qml as the two would be declared within separate scopes.

Pragmas

You can prepend global instructions to a QML document using the pragma keyword. The following pragmas are supported:

Singleton

pragma Singleton declares the component defined in the QML document as singleton. Singletons are created only once per QML engine. In order to use a QML-declared singleton you also have to register it with its module. See qt_target_qml_sources for how to do this with CMake.

ListPropertyAssignBehavior

With this pragma you can define how assignments to list properties shall be handled in components defined in the QML document. By default, assigning to a list property appends to the list. You can explicitly request this behavior using the value Append. Alternatively, you can request the contents of list properties to always be replaced using Replace, or replaced if the property is not the default property using ReplaceIfNotDefault. For example:

pragma ListPropertyAssignBehavior: ReplaceIfNotDefault

The same declaration can also be given for C++-defined types. See QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_APPEND, QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE, and QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE_IF_NOT_DEFAULT

ComponentBehavior

With this pragma you can restrict components defined in this file to only create objects within their original context. This holds for inline components as well as Component elements explicitly or implicitly created as properties. If a component is bound to its context, you can safely use IDs from the rest of the file within the component. Otherwise, the engine and the QML tooling cannot know in advance what type, if any, such IDs will resolve to at run time.

In order to bind the components to their context specify the Bound argument:

pragma ComponentBehavior: Bound

The default is Unbound. You can also specify it explicitly. In a future version of Qt the default will change to Bound.

Delegate components bound to their context don't receive their own private contexts on instantiation. This means that model data can only be passed via required properties in this case. Passing model data via context properties will not work. This concerns delegates to e.g. Instantiator, Repeater, ListView, TableView, GridView, TreeView and in general anything that uses DelegateModel internally.

For example, the following will not work:

pragma ComponentBehavior: Bound
import QtQuick

ListView {
    delegate: Rectangle {
        color: model.myColor
    }
}

The delegate property of ListView is a component. Therefore, a Component is implicitly created around the Rectangle here. That component is bound to its context. It doesn't receive the context property model provided by ListView. To make it work, you'd have to write it this way:

pragma ComponentBehavior: Bound
import QtQuick

ListView {
    delegate: Rectangle {
        required property color myColor
        color: myColor
    }
}

You can nest components in a QML file. The pragma holds for all components in the file, no matter how deeply nested.

FunctionSignatureBehavior

With this pragma you can change the way type annotations on functions are handled. By default the interpreter and JIT ignore type annotations, but the QML script compiler enforces them when compiling to C++.

Specifying Enforce as value makes sure the type annotations are always enforced. The resulting type coercions increase the overhead of calling typed JavaScript functions.

Specifying Ignore as value makes the QML script compiler ignore any JavaScript functions when compiling the document to C++. This means less code is compiled to C++ ahead of time, and more code has to be interpreted or JIT-compiled.

ValueTypeBehavior

With this pragma you can change the way value types and sequences are handled.

Value types and sequences are generally treated as references. This means, if you retrieve a value type instance from a property into a local value, and then change the local value, the original property is also changed. Furthermore, if you write the original property explicitly, the local value is also updated. This behavior is rather unintuitive in many places, and you should not rely on it. The Copy and Reference values for the ValueTypeBehavior pragma are experimental options to change this behavior. You should not use them. Specifying Copy causes all value types to be treated as actual copies. Specifying Reference explicitly states the default behavior.

Rather than using Copy you should explicitly re-load references to value types and sequences any time they can have been affected by side effects. Side effects can happen whenever you call a function or imperatively set a property. qmllint provides guidance on this. For example, in the following code the variable f is affected by side effects after writing width. This is because there may be a binding in a derived type or in a Binding element that updates font when width is changed.

import QtQuick
Text {
    function a() : real {
        var f = font;
        width = f.pixelSize;
        return f.pointSize;
    }
}

In order to address this, you can avoid holding f across the write operation on width:

import QtQuick
Text {
    function a() : real {
        var f = font;
        width = f.pixelSize;
        f = font;
        return f.pointSize;
    }
}

This, in turn can be shortened to:

import QtQuick
Text {
    function a() : real {
        width = font.pixelSize;
        return font.pointSize;
    }
}

You might assume that re-retrieving the font property is costly, but actually the QML engine automatically refreshes value type references each time you read from them. So this is not more expensive than the first version, but a clearer way to express the same operations.