Property Binding
An object's property can be assigned a static value which stays constant until it is explicitly assigned a new value. However, to make the fullest use of QML and its built-in support for dynamic object behaviors, most QML objects use property bindings.
Property bindings are a core feature of QML that lets developers specify relationships between different object properties. When a property's dependencies change in value, the property is automatically updated according to the specified relationship.
Behind the scenes, the QML engine monitors the property's dependencies (that is, the variables in the binding expression). When a change is detected, the QML engine re-evaluates the binding expression and applies the new result to the property.
Overview
To create a property binding, a property is assigned a JavaScript expression that evaluates to the desired value. At its simplest, a binding may be a reference to another property. Take the following example, where the blue Rectangle's height is bound to the height of its parent:
Rectangle { width: 200; height: 200 Rectangle { width: 100 height: parent.height color: "blue" } }
Whenever the height of the parent rectangle changes, the height of the blue rectangle automatically updates to be of the same value.
A binding can contain any valid JavaScript expression or statement, as QML uses a standards compliant JavaScript engine. Bindings can access object properties, call methods and use built-in JavaScript objects such as Date
and Math
. Below are other possible bindings for the previous example:
height: parent.height / 2 height: Math.min(parent.width, parent.height) height: parent.height > 100 ? parent.height : parent.height/2 height: { if (parent.height > 100) return parent.height else return parent.height / 2 } height: someMethodThatReturnsHeight()
Below is a more complex example involving more objects and types:
Column { id: column width: 200 height: 200 Rectangle { id: topRect width: Math.max(bottomRect.width, parent.width/2) height: (parent.height / 3) + 10 color: "yellow" TextInput { id: myTextInput text: "Hello QML!" } } Rectangle { id: bottomRect width: 100 height: 50 color: myTextInput.text.length <= 10 ? "red" : "blue" } }
In the previous example,
-
topRect.width
depends onbottomRect.width
andcolumn.width
-
topRect.height
depends oncolumn.height
-
bottomRect.color
depends onmyTextInput.text.length
Syntactically, bindings are allowed to be of arbitrary complexity. However, if a binding is overly complex - such as involving multiple lines, or imperative loops - it could indicate that the binding is being used for more than describing property relationships. Complex bindings can reduce code performance, readability, and maintainability. It may be a good idea to redesign components that have complex bindings, or at least factor the binding out into a separate function.
Creating Property Bindings from JavaScript
A property with a binding is automatically updated as necessary. However, if the property is later assigned a static value from a JavaScript statement, the binding will be removed.
For example, the Rectangle below initially ensures that its height
is always twice its width
. However, when the space key is pressed, the current value of width*3
will be assigned to height
as a static value. After that, the height
will remain fixed at this value, even if the width
changes. The assignment of the static value removes the binding.
import QtQuick 2.0 Rectangle { width: 100 height: width * 2 focus: true Keys.onSpacePressed: { height = width * 3 } }
If the intention is to give the rectangle a fixed height and stop automatic updates, then this is not a problem. However, if the intention is to establish a new relationship between width
and height
, then the new binding expression must be wrapped in the Qt.binding() function instead:
import QtQuick 2.0 Rectangle { width: 100 height: width * 2 focus: true Keys.onSpacePressed: { height = Qt.binding(function() { return width * 3 }) } }
Now, after the space key is pressed, the rectangle's height will continue auto-updating to always be three times its width.
Using this
with Property Binding
When creating a property binding from JavaScript, the this
keyword can be used to refer to the object which receives the binding. This is helpful for resolving ambiguities with property names.
For example, the Component.onCompleted
handler below is defined within the scope of the Item. In this scope, width
refers to the Item's width, not the Rectangle's width. To bind the Rectangle's height
to its own width
, the binding expression must explicitly refer to this.width
(or alternatively, rect.width
):
Item { width: 500 height: 500 Rectangle { id: rect width: 100 color: "yellow" } Component.onCompleted: { rect.height = Qt.binding(function() { return this.width * 2 }) console.log("rect.height = " + rect.height) // prints 200, not 1000 } }
Note: The value of this
is not defined outside of property bindings. See JavaScript Environment Restrictions for details.
© The Qt Company Ltd
Licensed under the GNU Free Documentation License, Version 1.3.
https://doc.qt.io/qt-5.9/qtqml-syntax-propertybinding.html