ListModel QML Type

The following example shows a ListModel containing three elements, with the roles "name" and "cost".

import QtQuick 2.0

ListModel {
    id: fruitModel

    ListElement {
        name: "Apple"
        cost: 2.45
    }
    ListElement {
        name: "Orange"
        cost: 3.25
    }
    ListElement {
        name: "Banana"
        cost: 1.95
    }
}

Roles (properties) in each element must begin with a lower-case letter and should be common to all elements in a model. The ListElement documentation provides more guidelines for how elements should be defined.

Since the example model contains an id property, it can be referenced by views, such as the ListView in this example:

import QtQuick 2.0

Rectangle {
    width: 200; height: 200

    ListModel {
        id: fruitModel
        ...
    }

    Component {
        id: fruitDelegate
        Row {
            spacing: 10
            Text { text: name }
            Text { text: '$' + cost }
        }
    }

    ListView {
        anchors.fill: parent
        model: fruitModel
        delegate: fruitDelegate
    }
}

It is possible for roles to contain list data. In the following example we create a list of fruit attributes:

ListModel {
    id: fruitModel

    ListElement {
        name: "Apple"
        cost: 2.45
        attributes: [
            ListElement { description: "Core" },
            ListElement { description: "Deciduous" }
        ]
    }
    ListElement {
        name: "Orange"
        cost: 3.25
        attributes: [
            ListElement { description: "Citrus" }
        ]
    }
    ListElement {
        name: "Banana"
        cost: 1.95
        attributes: [
            ListElement { description: "Tropical" },
            ListElement { description: "Seedless" }
        ]
    }
}

The delegate displays all the fruit attributes:

Component {
    id: fruitDelegate
    Item {
        width: 200; height: 50
        Text { id: nameField; text: name }
        Text { text: '$' + cost; anchors.left: nameField.right }
        Row {
            anchors.top: nameField.bottom
            spacing: 5
            Text { text: "Attributes:" }
            Repeater {
                model: attributes
                Text { text: description }
            }
        }
    }
}

The content of a ListModel may be created and modified using the clear(), append(), set(), insert() and setProperty() methods. For example:

    Component {
        id: fruitDelegate
        Item {
            width: 200; height: 50
            Text { text: name }
            Text { text: '$' + cost; anchors.right: parent.right }

            // Double the price when clicked.
            MouseArea {
                anchors.fill: parent
                onClicked: fruitModel.setProperty(index, "cost", cost * 2)
            }
        }
    }

Note that when creating content dynamically the set of available properties cannot be changed once set. Whatever properties are first added to the model are the only permitted properties in the model.

ListModel can be used together with WorkerScript access a list model from multiple threads. This is useful if list modifications are synchronous and take some time: the list operations can be moved to a different thread to avoid blocking of the main GUI thread.

Here is an example that uses WorkerScript to periodically append the current time to a list model:

        Timer {
            id: timer
            interval: 2000; repeat: true
            running: true
            triggeredOnStart: true

            onTriggered: {
                var msg = {'action': 'appendCurrentTime', 'model': listModel};
                worker.sendMessage(msg);
            }
        }

The included file, dataloader.mjs, looks like this:

WorkerScript.onMessage = function(msg) {
    if (msg.action == 'appendCurrentTime') {
        var data = {'time': new Date().toTimeString()};
        msg.model.append(data);
        msg.model.sync();   // updates the changes to the list
    }
}

The timer in the main example sends messages to the worker script by calling WorkerScript::sendMessage(). When this message is received, WorkerScript.onMessage() is invoked in dataloader.mjs, which appends the current time to the list model.

Note the call to sync() from the external thread. You must call sync() or else the changes made to the list from that thread will not be reflected in the list model in the main thread.

See also Data Models, Qt Quick Examples - Threading, Qt QML

The number of data entries in the model.

By default, the type of a role is fixed the first time the role is used. For example, if you create a role called "data" and assign a number to it, you can no longer assign a string to the "data" role. However, when the dynamicRoles property is enabled, the type of a given role is not fixed and can be different between elements.

The dynamicRoles property must be set before any data is added to the ListModel, and must be set from the main thread.

A ListModel that has data statically defined (via the ListElement QML syntax) cannot have the dynamicRoles property enabled.

There is a significant performance cost to using a ListModel with dynamic roles enabled. The cost varies from platform to platform but is typically somewhere between 4-6x slower than using static role types.

Due to the performance cost of using dynamic roles, they are disabled by default.

Adds a new item to the end of the list model, with the values in dict.

fruitModel.append({"cost": 5.95, "name":"Pizza"})

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Deletes all content from the model.

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Returns the item at index in the list model. This allows the item data to be accessed or modified from JavaScript:

Component.onCompleted: {
    fruitModel.append({"cost": 5.95, "name":"Jackfruit"});
    console.log(fruitModel.get(0).cost);
    fruitModel.get(0).cost = 10.95;
}

The index must be an element in the list.

Note that properties of the returned object that are themselves objects will also be models, and this get() method is used to access elements:

fruitModel.append(..., "attributes":
    [{"name":"spikes","value":"7mm"},
     {"name":"color","value":"green"}]);
fruitModel.get(0).attributes.get(1).value; // == "green"

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Adds a new item to the list model at position index, with the values in dict.

fruitModel.insert(2, {"cost": 5.95, "name":"Pizza"})

The index must be to an existing item in the list, or one past the end of the list (equivalent to append).

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Moves n items from one position to another.

The from and to ranges must exist; for example, to move the first 3 items to the end of the list:

fruitModel.move(0, fruitModel.count - 3, 3)

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Deletes count number of items at index from the model.

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Changes the item at index in the list model with the values in dict. Properties not appearing in dict are left unchanged.

fruitModel.set(3, {"cost": 5.95, "name":"Pizza"})

If index is equal to count() then a new item is appended to the list. Otherwise, index must be an element in the list.

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Changes the property of the item at index in the list model to value.

fruitModel.setProperty(3, "cost", 5.95)

The index must be an element in the list.

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Writes any unsaved changes to the list model after it has been modified from a worker script.