QML 基本类型

QML 支持许多基本类型。

A 基本类型 is one that refers to a simple value, such as an int 或 string . This contrasts with a QML 对象类型 , which refers to an object with properties, signals, methods and so on. Unlike an object type, a basic type cannot be used to declare QML objects: it is not possible, for example, to declare an int{} object or a size{} 对象。

Basic types can be used to refer to:

A single value (e.g. int refers to a single number, var can refer to a single list of items)
A value that contains a simple set of property-value pairs (e.g. size refers to a value with width and height 属性)

When a variable or property holds a basic type and it is assigned to another variable or property, then a copy of the value is made. In JavaScript, this value is called a primitive value.

支持的基本类型

Some basic 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 basic types listed below may be used as a property type in a QML document, with the following exceptions:

list must be used in conjunction with a QML object type
enumeration cannot be used directly as the enumeration must be defined by a registered QML object type

由 QML 语言提供的基本类型

The basic types supported natively in the QML language are listed below:

bool	二进制 true/false 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	自由形式文本字符串
url	资源定位器
var	Generic property type

由 QML 模块提供的基本类型

QML modules may extend the QML language with more basic types. For example, the basic types provided by the QtQuick module are listed below:

date	日期值
point	Value with x and y attributes
rect	Value with x, y, width and height attributes
size	Value with width and height attributes

The Qt global object provides useful functions for manipulating values of basic types.

Currently only QML modules which are provided by Qt may provide their own basic types, however this may change in future releases of Qt QML. In order to use types provided by a particular QML module, clients must import that module in their QML documents.

Property Change Behavior for Basic Types

Some basic types have properties: for example, the font type has pixelSize , family and bold properties. Unlike properties of object types , properties of basic types do not provide their own property change signals. It is only possible to create a property change signal handler for the basic 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 basic 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 .