QML 支持内置和自定义值类型。
A
值类型
is one that is conceptually passed by value rather than by reference, such as an
int
或
string
. This contrasts with
QML 对象类型
. Object types are passed by reference. If you assign an instance of an object type to two different properties, both properties carry the same value. Modifying the object is reflected in both properties. If you assign an instance of a value type to two different properties, the properties carry separate values. If you modify one of them, the other one stays the same. Value types are only conceptually passed by value since it must still be possible to interact with them as if they were JavaScript objects. To facilitate this, in reality they are passed as
Value Type References
when you access them from JavaScript code.
Unlike an object type, a value type cannot be used to declare QML objects: it is not possible, for example, to declare an
int{}
对象或
size{}
对象。
值类型可以用于引用:
width
and
height
特性)
When a variable or property holds a value type and it is assigned to another variable or property, then a copy of the value is made.
Some value types are supported by the engine by default and do not require an
import statement
to be used, while others do require the client to import the module which provides them. All of the value types listed below may be used as a
property
type in a QML document, with the following exceptions:
void
, which marks the absence of a value
list
must be used in conjunction with an object or value type as element
enumeration
cannot be used directly as the enumeration must be defined by a registered QML object type
The built-in value types supported natively in the QML 语言 are listed below:
二进制 true/false 值 | |
日期值 | |
带小数点的数字,以双精度存储 | |
命名枚举值 | |
整数 (如:0、10 或 -20) | |
QML 对象列表 | |
带小数点的数字 | |
自由形式文本字符串 | |
资源定位器 | |
一般特性类型 | |
一般特性类型 | |
空值类型 |
QML modules may extend the QML language with more value types.
For instance, the value types provided by the
QtQml
模块是:
具有 X 和 Y 属性的值 | |
具有 X、Y、宽度及高度属性的值 | |
具有宽度和高度属性的值 |
The value types provided by the
QtQuick
模块是:
ARGB color value | |
Font value with the properties of QFont. The font type refers to a font value with the properties of QFont | |
A matrix4x4 type is a 4-row and 4-column matrix | |
A quaternion type has scalar, x, y, and z attributes | |
A vector2d type has x and y attributes | |
Value with x, y, and z attributes | |
A vector4d type has x, y, z and w attributes |
The Qt 全局对象提供 useful functions for manipulating values of value types for the Qt Qml and Qt Quick 模块。
Other Qt modules will document their value types on their respective module pages.
You may define your own value types as described in 从 C++ 定义 QML 类型 . In order to use types provided by a particular QML module, clients must import that module in their QML documents.
Some value types have properties: for example, the
font
类型拥有
pixelSize
,
family
and
bold
properties. Unlike properties of
对象类型
, properties of value types do not provide their own property change signals. It is only possible to create a property change signal handler for the value type property itself:
Text { // invalid! onFont.pixelSizeChanged: doSomething() // also invalid! font { onPixelSizeChanged: doSomething() } // but this is ok onFontChanged: doSomething() }
Be aware, however, that a property change signal for a value type is emitted whenever any of its attributes have changed, as well as when the property itself changes. Take the following code, for example:
Text { onFontChanged: console.log("font changed") Text { id: otherText } focus: true // changing any of the font attributes, or reassigning the property // to a different font value, will invoke the onFontChanged handler Keys.onDigit1Pressed: font.pixelSize += 1 Keys.onDigit2Pressed: font.b = !font.b Keys.onDigit3Pressed: font = otherText.font }
In contrast, properties of an 对象类型 emit their own property change signals, and a property change signal handler for an object-type property is only invoked when the property is reassigned to a different object value.
另请参阅 QML 类型系统 .