QML supports built-in and custom value types.
A
value type
is one that is 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. Unlike an object type, a value type cannot be used to declare QML objects: it is not possible, for example, to declare an
int{}
object or a
size{}
对象。
Value types can be used to refer to:
width
and
height
properties)
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 language are listed below:
| bool | 二进制 true/false 值 |
| date | 日期值 |
| double | 带小数点的数字，以双精度存储 |
| enumeration | 命名枚举值 |
| int | Whole number, e.g. 0, 10, or -20 |
| list | QML 对象列表 |
| real | 带小数点的数字 |
| string | A free form text string |
| url | 资源定位器 |
| var | Generic property type |
| variant | Generic property type |
| void | Empty value type |
QML modules may extend the QML language with more value types. For example, the value types provided by the
QtQuick
module are listed below:
| color | ARGB color value |
| font | Font value with the properties of QFont. The font type refers to a font value with the properties of QFont |
| matrix4x4 | A matrix4x4 type is a 4-row and 4-column matrix |
| point | Value with x and y attributes |
| quaternion | A quaternion type has scalar, x, y, and z attributes |
| rect | Value with x, y, width and height attributes |
| size | Value with width and height attributes |
| vector2d | A vector2d type has x and y attributes |
| vector3d | Value with x, y, and z attributes |
| vector4d | A vector4d type has x, y, z and w attributes |
The Qt global object provides useful functions for manipulating values of value types.
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
type has
pixelSize
,
family
and
bold
properties. Unlike properties of
object types
, 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 .