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Protocols.md

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####Protocols

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.

语法

protocol SomeProtocol {
  // protocol definition goes here
}

自定义类型要遵从某些protocol, 只需将protocol的名字置于类型名的后面用分号隔开,比如

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

Protocols as Types

  • protocol 本上就是一种类型(所以首字母要大写哦);
  • 可以作为函数中/方法中/ initializer 中的参数类型, 返回值类型;
  • 可以作为常量/变量/属性的类型;
  • 作为容器类中的元素.

NOTE: Types do not automatically adopt a protocol just by satisfying its requirements. They must always explicitly declare their adoption of the protocol.

Class-Only Protocols

protocol SomeClassOnlyProtocol: class, SomeInheritedProtocol {
  //class-only protocol definition goes here 
}

Class Implementations of Protocol Initializer Requirements

  • 类实现的 protocol方法需要在方法前添加required关键字(这样就能提示此类或者其子类 conform to 相应的方法.(说道这里, 那么 final 修饰的 class 就不必写 required 了. 因为它不能被继承.
  • 如果子类重写了父类的 designated initializer, 且同时也实现了遵循的 protocol 中的 initializer requirement同样的方法, 那么同时要使用required override来修饰.

NOTE: Use a class-only protocol when the behavior defined by that protocol's requirements assumes or requires that a conforming type has reference semantics rather than value semnatics.

Adding Protocol Conformance with an Extension

  • 如果类型实际上实现了某个 protocol 的所有 requirements, 但是没有声明其遵循了某个 protocol, 我们可以使用extension ComformingType: Protocol {} 来表明.

Protocol Inheritance

  • protocol能够多继承并且能够添加新的 requirements.
  • 我们可以限制 protocol 只适用于 class, 写法是在 protocol的继承列表中第一个位置添加 class 关键字.

Protocol Composition

Protocol compositions have the from protocol<SomeProtocol, AnotherProtocol>

  • 我们可以合并protocol requirement 为单个 requirement, 写法是 protocol由&连接起来. 注意这种并不是定义了一种新的 protocol 类型, 它仅仅是定义了一个temporary local protocol.

NOTE: Protocol compositions do not define a new, permanent protocol type. Rather, they define a temporary local protocol that has the combined requirements of all protocols in the composition.

** Checking for Protocol Conformance**

is and as:

  • The is operator returns true if an instance conforms to a protocol and returns false if it does not.
  • The as? version of the downcast operator returns an optional value of the protocol's type, and this value is nil if the instance does not conform to that protocol.
  • The as! version of the downcast operator forces the downcast to the protocol type and triggers a runtime error if the downcast does not successed.

Optional Protocol Requirements(这个只有在与 OC混编的时候才会有)

  • 我们通常使用optional前缀修饰符来指明protocol中的method或者property是optional的(同时我们必须在protocol前面加上@objc.
  • protocol 中的 optional requirement 中的 method 或者 property是 optional 的, 那么这个 method 或者property 本身的类型就自动变成一个 optional 的, 比如(Int) -> String 会变成 ((Int) -> String)?.

NOTE: Optional protocol requirements can only be specified if your protocol is amred with the @objc attribute. This attribute indicates that the protocol should be exposed to Objective-C code. Even if you are not interoperating with Objective-C, you need to mark your protocols with the @objc attribute if you want to specify optional requirements. Note that @objc protocols can be adopted only by classes that inherit from Objective-C classes or other @objc classes. They can't be adopted by structures or enumeratoins.

NOTE: Strictly speaking, you can write a custom class that conforms to CounterDataSource without implementing either protocol requirement. They are both optional, after all. Although technically allowed, this wouldn't make for a very good data source.

Protocol Extensions

Protocols可以用来拓展以便为 comforming types提供method 和 property 的实现.这就使得我们可以定义一些 protocol 的 behavior, 而不是将这些 behavior 分布到各个 comforming type或 global function 中去实现.

Protocol Default Implementations

我们可以利用 protocol extension 来为 protocol 的 method 或者 property 提供默认的实现. 如果 Comforming type 又实现了这些 methods 或者 properties, 则默认的实现会被替换.

注意, 我们可能注意到一旦 protocol 提供了Default Implementations, 意味着 Comforming type 可以不写 implementation, 但是它实际上是有Default Implementation 的, 这与前面的 optional protocol requirements是不同的.

Adding Constrains to Protocol Extensions

当然 extension有时也是有条件的, 不能谁都能 conform 的(那岂不是太随便了😀), 所以我们就可以为 extension 添加一些 constraints, 以下是写法:

extension Collection where Iterator.Element: TextRepresentable {
	var textualDescription: String {
		let itemsAsText = self.map { $0.textualDescription }
		return "[" + itemsAsText.joined(seperator: ", " + "]"
	}
}

NOTE: If a conforming type satisfies the requirements for multiple constrained extensions that provide implementations for the same method or property. Swift will use the implementation corresponding to the most specialized constraints.