官方对于 extension 使用的解释Swift Extensions doc
1. 与 OC 分类相似，但是没有名字区别
Extensions are similar to categories in Objective-C. (Unlike Objective-C categories, Swift extensions don’t have names.)
2. 使用的几个场景
Extensions in Swift can:
- Add computed instance properties and computed type properties
- Define instance methods and type methods
- Provide new initializers
- Define subscripts
- Define and use new nested types
- Make an existing type conform to a protocol
2.1 添加计算属性
tips
Extensions can add new computed properties, but they can’t add stored properties, or add property observers to existing properties.
允许添加计算属性，不允许添加存储属性，或者添加已声明属性的观察服务。
/// 添加计算属性
extension Double {
var m: Double {
return self
}
var km: Double {
return self / 1000
}
var cm: Double {
return self * 1000
}
}
let result = 50 * 1.0.cm + 10.m // 10.5.m
2.2 定义对象方法/常量方法
/// 添加方法
class ExtensionDemo {
}
/// 对象添加方法
extension ExtensionDemo {
/// 添加静态方法
static func staticMethod() -> Void {
}
/// 添加实例方法
func instanceMethod() -> Void {
}
}
/// 常量添加方法
extension Int {
func repetitions(task: () -> Void) {
for _ in 0..<self {
task()
}
}
/// 可以改变自身 s
mutating func square(x: Int, y: Int) -> Int {
self = x * y
return self
}
}
func methodDemo() -> Void {
ExtensionDemo.staticMethod()
ExtensionDemo().instanceMethod()
10.repetitions {
print("1")
}
}
2.3支持添加新的构造方法
/// 添加构造方法
extension Int {
init(x: Double, y: Double) {
self.init(x * y)
}
}
2.4 支持 Subscripts
/// 支持 Subscripts
extension Int {
subscript(index: Int) -> Int {
return 0
}
}
let index = 100[0]
2.5 Define and use new nested types
Extensions can add new nested types to existing classes, structures, and enumerations:
classes,
structures,
enumerations:
/// 支持嵌套类型
enum ExtensionEnumDemo {
case first
case second
}
/// 枚举支持内嵌对象
extension ExtensionEnumDemo {
class ExtensionSub {
}
}
/// 对象支持内嵌对象
extension ExtensionDemo {
class ExtensionSub {
}
}
/// 结构体支持内嵌对象
extension Int {
class ExtensionSub {
}
enum Types {
case first
case second
}
}
2.6 Make an existing type conform to a protocol 已有类型支持协议
/// 支持协议
extension UIViewController : UITableViewDelegate {
func cellForRow(at indexPath: IndexPath) -> UITableViewCell? {
return nil
}
}
