Binding QML Type

Enables the arbitrary creation of property bindings. More...

Import Statement: import QtQml 2.15


Detailed Description

In QML, property bindings result in a dependency between the properties of different objects.

Binding to an Inaccessible Property

Sometimes it is necessary to bind an object's property to that of another object that isn't directly instantiated by QML, such as a property of a class exported to QML by C++. You can use the Binding type to establish this dependency; binding any value to any object's property.

For example, in a C++ application that maps an "app.enteredText" property into QML, you can use Binding to update the enteredText property.

TextEdit { id: myTextField; text: "Please type here..." }
Binding { target: app; property: "enteredText"; value: myTextField.text }

When text changes, the C++ property enteredText will update automatically.

Conditional Bindings

In some cases you may want to modify the value of a property when a certain condition is met but leave it unmodified otherwise. Often, it's not possible to do this with direct bindings, as you have to supply values for all possible branches.

For example, the code snippet below results in a warning whenever you release the mouse. This is because the value of the binding is undefined when the mouse isn't pressed.

// produces warning: "Unable to assign [undefined] to double value"
value: if (mouse.pressed) mouse.mouseX

The Binding type can prevent this warning.

Binding on value {
    when: mouse.pressed
    value: mouse.mouseX

The Binding type restores any previously set direct bindings on the property.

See also Qt QML.

Property Documentation

delayed : bool

This property holds whether the binding should be delayed.

A delayed binding will not immediately update the target, but rather wait until the event queue has been cleared. This can be used as an optimization, or to prevent intermediary values from being assigned.

Binding {
    target: contactName; property: 'text'
    value: givenName + " " + familyName; when: list.ListView.isCurrentItem
    delayed: true

This property was introduced in Qt 5.8.

property : string

The property to be updated.

This can be a group property if the expression results in accessing a property of a value type. For example:

Item {
    id: item

    property rect rectangle: Qt.rect(0, 0, 200, 200)

Binding {
    target: item
    property: "rectangle.x"
    value: 100

restoreMode : enumeration

This property can be used to describe if and how the original value should be restored when the binding is disabled.

The possible values are:

  • Binding.RestoreNone The original value is not restored at all
  • Binding.RestoreBinding The original value is restored if it was another binding. In that case the old binding is in effect again.
  • Binding.RestoreValue The original value is restored if it was a plain value rather than a binding.
  • Binding.RestoreBindingOrValue The original value is always restored.

Warning: The default value is Binding.RestoreBinding. This will change in Qt 6.0 to Binding.RestoreBindingOrValue.

If you rely on any specific behavior regarding the restoration of plain values when bindings get disabled you should migrate to explicitly set the restoreMode.

Reliance on a restoreMode that doesn't restore the previous binding or value for a specific property results in a run-time warning.

This property was introduced in Qt 5.14.

target : Object

The object to be updated.

value : any

The value to be set on the target object and property. This can be a constant (which isn't very useful), or a bound expression.

when : bool

This property holds when the binding is active. This should be set to an expression that evaluates to true when you want the binding to be active.

Binding {
    target: contactName; property: 'text'
    value: name; when: list.ListView.isCurrentItem

When the binding becomes inactive again, any direct bindings that were previously set on the property will be restored.

Note: By default, a previously set literal value is not restored when the Binding becomes inactive. Rather, the last value set by the now inactive Binding is retained. You can customize the restoration behavior for literal values as well as bindings using the restoreMode property. The default will change in Qt 6.0.

See also restoreMode.

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