# 类属性

`实例属性`属于一个特定类型的实例；`类属性`属于整个类型，在多个该类型的实例间共享。

`类属性`既可以定义为`存储属性`也可以定义为`计算属性`

`存储的类属性`必须在定义时提供初始化器。但是`类型属性`在首次访问时才初始化，同时`类型属性`的初始化是线程安全的。

## 定义类属性

使用关键字 `static` 来定义类型属性。在为类定义计算型类型属性时，可以改用关键字 `class` 来支持子类对父类的实现进行重写.

> 类，结构体和枚举都可以定义存储型类属性

```swift

class SomeStructure {
    static var storedTypeProperty = "Some value."
    class var computedTypeProperty: Int {
        return 1
    }
}
enum SomeEnumeration {
    static var storedTypeProperty = "Some value."
    static var computedTypeProperty: Int {
        return 6
    }
}
class SomeClass : SomeStructure{
    class override var computedTypeProperty: Int {
        return 107
    }
}

```

`类型属性`可以被子类继承，子类不可以定义与父类中名字相同的`类型属性`。\
当使用`class`定义的类型计算属性，可以被子类重写。

## 2. 类型属性可以定义属性观察器


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