内容

• 特性
• 详细描述
• 绑定到不可访问特性
• 条件绑定

Binding QML 类型

使能够创建任意特性绑定。 更多...

导入语句：

import QtQml 2.2

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特性

• delayed : bool
• property : string
• restoreMode : enumeration
• target : Object
• value : any
• 当 : bool

详细描述

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

绑定到不可访问特性

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 }
					

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

条件绑定

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.

另请参阅 Qt QML .

特性文档编制

[since 5.8] 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 。例如：

Item {
    id: item
    property rect rectangle: Qt.rect(0, 0, 200, 200)
}
Binding {
    target: item
    property: "rectangle.x"
    value: 100
}
					

[since 5.14] restoreMode : enumeration

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

可能的值包括：

• 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.

默认值为 Binding.RestoreBindingOrValue .

注意： This property exists for backwards compatibility with earlier versions of Qt. Don't use it in new code.

该特性在 Qt 5.14 引入。

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.

当 : 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
}
					

By default, any binding or value that was set perviously is restored when the binding becomes inactive. You can customize the restoration behavior using the restoreMode 特性。

另请参阅 restoreMode .