协议[Protocols]

总结来看，协议为Swift提供了一种方便，灵活的对类型进行抽象描述的方式。

A protocol defines a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. The protocol can then be adopted by a class, structure, or enumeration to provide an actual implementation of those requirements. Any type that satisfies the requirements of a protocol is said to conform to that protocol.

Multiple protocols can be listed, and are separated by commas:

struct SomeStructure: FirstProtocol, AnotherProtocol {
    // structure definition goes here
}

If a class has a superclass, list the superclass name before any protocols it adopts, followed by a comma:

class SomeClass: SomeSuperclass, FirstProtocol, AnotherProtocol {
    // class definition goes here
}

Initializer Requirements

Protocols can require specific initializers to be implemented by conforming types. You can implement a protocol initializer requirement on a conforming class as either a designated initializer or a convenience initializer. In both cases, you must mark the initializer implementation with the required modifier:

protocol SomeProtocol {
    init(someParameter: Int)
}

class SomeClass: SomeProtocol {
    required init(someParameter: Int) {
        // initializer implementation goes here
    }
}

You don’t need to mark protocol initializer implementations with the required modifier on classes that are marked with the final modifier, because final classes can’t subclassed.

If a subclass overrides a designated initializer from a superclass, and also implements a matching initializer requirement from a protocol, mark the initializer implementation with both the required and overridemodifiers:

protocol SomeProtocol {
    init()
}


class SomeSuperClass {
    init() {
        // initializer implementation goes here
    }
}


class SomeSubClass: SomeSuperClass, SomeProtocol {
    // "required" from SomeProtocol conformance; "override" from SomeSuperClass
    required override init() {
        // initializer implementation goes here
    }
}

Protocol As Types

尽管协议不提供任何关于功能的具体实现，但是Swift中的协议也是可以作为一种类型进行使用的。