QML 基本類型

QML 支持很多基本類型。

A 基本類型 是某種簡單值的引用，譬如 int 或 string 。這相比 QML 對象類型 , 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{} 對象或 size{} 對象。

Basic types can be used to refer to:

單個值 (如 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 引用值具有 width and height 屬性)

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.

支持的基本類型

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 語言提供的基本類型

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

bool	二進製 true/false 值
double	帶小數點的數字，以雙精度存儲
enumeration	命名枚舉值
int	整數 (如：0、10 或 -20)
list	QML 對象列錶
real	帶小數點的數字
string	自由形式文本字符串
url	資源定位器
var	一般特性類型

由 QML 模塊提供的基本類型

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	具有 X 和 Y 屬性的值
rect	具有 X、Y、寬度及高度屬性的值
size	具有寬度和高度屬性的值

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.

改變基本類型行為的特性

一些基本類型擁有特性：例如， font 類型擁有 pixelSize , family and bold properties. Unlike properties of 對象類型 , 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 對象類型 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.

另請參閱 QML 類型係統 .