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BindingThe Binding element allows arbitrary property bindings to be created. More... Inherits QtObject PropertiesDetailed DescriptionBinding to an inaccessible propertySometimes it is necessary to bind to a property of an object that wasn't directly instantiated by QML - generally a property of a class exported to QML by C++. In these cases, regular property binding doesn't work. Binding allows you to bind any value to any property. For example, imagine a C++ application that maps an "app.enteredText" property into QML. You could use Binding to update the enteredText property like this. TextEdit { id: myTextField; text: "Please type here..." }
Binding { target: app; property: "enteredText"; value: myTextField.text }
Whenever the text in the TextEdit is updated, the C++ property will be updated also. "Single-branch" conditional bindingIn some circumstances you may want to control the value of a property only when a certain condition is true (and relinquish control in all other cirumstances). This often isn't possible to accomplish with a direct binding, as you need to supply values for all possible branches. // warning: "Unable to assign [undefined] to double value" value: if (mouse.pressed) mouse.mouseX The above example will produce a warning whenever we release the mouse, as the value of the binding is undefined when the mouse isn't pressed. We can use the Binding element to rewrite the above code and avoid the warning. Binding on value {
when: mouse.pressed
value: mouse.mouseX
}
The Binding element will also restore any previously set direct bindings on the property. In that sense, it functions much like a simplified State. // this is equivilant to the above Binding State { name: "pressed" when: mouse.pressed PropertyChanges { target: obj value: mouse.mouseX } } If the binding target or binding property is changed, the bound value is immediately pushed onto the new target. See also QtQml. Property DocumentationThe property to be updated. 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. 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. |
