Swift Combine 之 Publisher的sink和assign

Subscriber（这篇文章主要讲assign和sink）

• Publisher中末尾提到了sink和assign的调用，现在具体看一下具体做了什么

sink操作符的代码调用

```
    let arr: [Int] = [1, 2, 100]
     
     /// `Sink` 将数组转换成一个数据流
    arr.publisher
         /// 过滤数据流中大于2的元素
         .filter{$0 > 2}
         /// 进行一次转换，转成String类型
         .compactMap{"\($0)"}
         /// 订阅数据源
         .sink { value in
             debugPrint("数据流: \(value)")
         }.store(in: &cancel)
 ```

Sink

• 之前讲解了Publisher的数据流发布的过程，sink相当于接收publisher发送的数据，然后执行receiveValue闭包，调用者可以进行自己的业务处理

• 先看一下sink操作符

  public func sink(
       receiveValue: @escaping (Output) -> Void
   ) -> AnyCancellable {
      /// 包装类`Sink`处理数据流，然后回调给receiveValue闭包
       let subscriber = Subscribers.Sink(
           receiveCompletion: { _ in },
           receiveValue: receiveValue
       )
       /// 订阅数据流
       subscribe(subscriber)
       /// GC 包装类
       return AnyCancellable(subscriber)
   }
  

• Sink包装类实现

• /// A simple subscriber that requests an unlimited number of values upon subscription.
     public final class Sink
         : Subscriber,
           Cancellable
     {
         /// 持有外部的闭包
         public var receiveValue: (Input) -> Void
  
         /// 完成闭包
         public var receiveCompletion: (Subscribers.Completion) -> Void
  
         /// 状态
         private var status = SubscriptionStatus.awaitingSubscription
  
         /// 初始化持有完成回调闭包和数据流回调闭包
         public init(
             receiveCompletion: @escaping (Subscribers.Completion) -> Void,
             receiveValue: @escaping ((Input) -> Void)
         ) {
             self.receiveCompletion = receiveCompletion
             self.receiveValue = receiveValue
         }
  
         /// `Subscription`协议方法实现
         public func receive(subscription: Subscription) {
  
             guard case .awaitingSubscription = status else {
        
                 subscription.cancel()
                 return
             }
             subscription.request(.unlimited)
         }
         /// `Subscribers` 协议实现，执行数据流回调
         public func receive(_ value: Input) -> Subscribers.Demand {
             /// 记录当前的数据流回调
             let receiveValue = self.receiveValue
             /// 触发数据流回调
             receiveValue(value)
             return .none
         }
         
         /// `Subscribers` 完成回调
         public func receive(completion: Subscribers.Completion) {
  
             let receiveCompletion = self.receiveCompletion
             self.receiveCompletion = { _ in }
  
             withExtendedLifetime(receiveValue) {
                 receiveValue = { _ in }
             }
             /// 执行完成回调
             receiveCompletion(completion)
         }
  
         /// 数据流取消（`cancel`）
         public func cancel() {
  
             guard case let .subscribed(subscription) = status else {
                 return
             }
  
             withExtendedLifetime((receiveValue, receiveCompletion)) {
                 receiveCompletion = { _ in }
                 receiveValue = { _ in }
             }
             subscription.cancel()
         }
     }
  

assign操作符代码调用

```
let arr: [Int] = [1, 2, 100]
     
      /// `Assign`使用keypath进行赋值
     arr.publisher
         .filter{$0 > 2}
         .compactMap{"\($0)"}
         /// keypath 赋值
         .assign(to: \.name, on: root).store(in: &cancel)
     
     debugPrint("root name: \(root.name)")
```

Assign

• 使用KeyPath来实现具体类的赋值操作，仅支持Class类型的具体类

• assign操作符

    public func assign(to keyPath: ReferenceWritableKeyPath,
                             on object: Root) -> AnyCancellable {
        /// 订阅类，`Assign`进行了包装，数据流流转到这里进行keypath赋值    
        let subscriber = Subscribers.Assign(object: object, keyPath: keyPath)
        /// 订阅数据流
        subscribe(subscriber)
       /// GC 处理
        return AnyCancellable(subscriber)
    }
  

• Assign包装类

• public final class Assign: Subscriber,
                                             Cancellable
     {
         public typealias Failure = Never
  
         public private(set) var object: Root?
  
         /// The key path that indicates the property to assign.
         public let keyPath: ReferenceWritableKeyPath
  
         private var status = SubscriptionStatus.awaitingSubscription
  
  
         /// 持有keypath的关联类和keypath的键
         public init(object: Root, keyPath: ReferenceWritableKeyPath) {
             self.object = object
             self.keyPath = keyPath
         }
  
         public func receive(subscription: Subscription) {
  
             guard case .awaitingSubscription = status else {
                 subscription.cancel()
                 return
             }
             status = .subscribed(subscription)
             subscription.request(.unlimited)
         }
  
         /// 数据流流转到这里，进行keypath赋值
         public func receive(_ value: Input) -> Subscribers.Demand {
  
             guard case .subscribed = status, let object = self.object else {
                 return .none
             }
             object[keyPath: keyPath] = value
             return .none
         }
  
         /// 完成回调
         public func receive(completion: Subscribers.Completion) {
             guard case .subscribed = status else {
                 return
             }
             terminateAndConsumeLock()
         }
  
         public func cancel() {
             guard case let .subscribed(subscription) = status else {
                 return
             }
             terminateAndConsumeLock()
             subscription.cancel()
         }
  
         private func terminateAndConsumeLock() {
             withExtendedLifetime(object) {
                 object = nil
             }
         }
     }