Swift static characteristics Protocol on extension

Experimental sample code

//声明一个协议
protocol SharedString {
    func methodForOverride() -> Void
    func methodWithoutOverride() -> Void
}

//扩展协议，实现默认实现
//注意方法methodWithoutOverride 中调用了 methodForOverride 方法，这里有一个上写文切换的问题
extension SharedString {
    func methodForOverride() -> Void {
        print("��")
    }

    func methodWithoutOverride() -> Void {
        print("��")
        methodForOverride()
        print("��")
    }
}

//在String类的扩展中遵守并实现协议
extension String:SharedString {
    func methodForOverride() -> Void {
        print(self)
    }
}

//创建字符串并切换上下文到SharedString协议
let Shared:SharedString = "hello"

Shared.methodForOverride()
Shared.methodWithoutOverride()

Execution of different code conversion obtained are summarized

Case number	Agreement statement method	Protocol extensions in the default implementation	Follow the protocol class rewrite	Following the experimental protocol property (switch contexts)	Result of the method execution method
1	YES	NO	YES	NO	Follow the protocol class method overridden
2	YES	YES	YES	YES	Follow the protocol class method overridden
3	YES	NO	YES	YES	Follow the protocol class method overridden
4	NO	NO	YES	NO	Follow the protocol class method overridden
5	YES	YES	YES	NO	Follow the protocol class method overridden
6	NO	YES	NO	YES	The default extension protocol implementation
7	NO	YES	NO	NO	The default extension protocol implementation
8	YES	YES	NO	YES	The default extension protocol implementation
9	YES	YES	NO	NO	The default extension protocol implementation
10	NO	YES	YES	YES	The default extension protocol implementation
11	NO	YES	YES	NO	1 and 2 are methods of protocol extension. The method, overridden in class to follow, a method call, the method follows the protocol class is rewritten. Method 2 is not rewritten, but in the agreement called Tier 2 method, this time, when the calling method 1 method 2 is still the default protocol extensions to achieve
12	NO	NO	YES	YES	ERROR
13	NO	NO	NO	YES	ERROR
14	NO	NO	NO	NO	ERROR
15	YES	NO	NO	NO	ERROR
16	YES	NO	NO	YES	goal conversions

The method of rewriting protocol extensions defined in the protocol extensions to retain the static characteristics of the original method is rewritten version of cartridge, you can obtain different versions by switching contexts, while not rewriting the method, which can ensure invariance.

We can follow a principle, to those who want to be overridden method defined in the agreement, those who do not want to define and implement the replication method on protocol extensions.