Qt 5.9 | Qt QML | QML Types | Binding QML Type

Qt 5.9.9 Reference Documentation

# Binding QML Type

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

Import Statement: | import QtQml 2.2

- List of all members, including inherited members

## Properties

- delayed : bool
- property : string
- target : Object
- value : any
- when : bool

## 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 QML 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
      }

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.