其实总结一句话，就是在各种环境下添加约束。
一、switch语句中
enum Person {
case man(name: String, age: Int)
case women(name: String, age: Int)
}
------
let p = Person.man(name: "loong", age: 30)
switch p {
case let .man(name, age) where age > 25:
print("peron: \(name), age > 25")
default:
print("default")
}
------console
peron: loong, age > 25
二、for-in语句中
let arrayOne = [1, 2, 3, 4, 5]
let dictionary = [1: "v1", 2: "v2"]
for i in arrayOne where dictionary[i] != nil {
print("i: \(i)")
}
---console
i: 1
i: 2
三、do-catch语句中
enum ExceptionError:Error{
case httpCode(Int)
}
func throwError() throws {
throw ExceptionError.httpCode(500)
}
------
do {
try throwError()
} catch ExceptionError.httpCode(let httpCode) where httpCode >= 500 {
print("server error")
}
四、Protocol约束
protocol SampleProtocol { }
extension SampleProtocol where Self: UILabel {
// 只给遵守SampleProtocol协议的UILabel添加了拓展
func getString() -> String {
return self.text ?? ""
}
}
extension UIView: SampleProtocol { }
------
// 如果是UIView的话，调用不了getString扩展方法
let label = UILabel(frame: CGRect.zero)
label.text = "hello, world"
let text = label.getString()
print("label text: \(text)")
---console
label text: hello, world
五、泛型约束
类型约束：指定类型参数必须继承自特定的类、遵守某个协议或协议组合。
func addMember<T>(a: T) where T: SampleProtocol {
}
// 简约写法
func addValue<T: AnyObject>(a: T) {
}
