Swift Combine 之 Publisher数据流

Publisher

• 发布源协议，可以实现该协议来实现自己的数据源,
  • Subject继承自Publisher,提供了三套默认的内置实现类
• 容器包装类，具体实现交由Subscriber实现类来转发数据流
• 内置提供各种各样的操作符（函数式编程的仿函数，Swift语言的Operator)
  • allSatisfy
  • tryAllSatisfy
  • compactMap
  • contains
  • filter
  • tryFilter
  • throttle
  • ....等等
• Publisher接口

    public protocol Publisher {
    /// 数据输出流，相当于订阅者的数据输入流        
    associatedtype Output
    /// 数据发布，要么发布一个真实数据流，要么发布一个错误(或者可以选择丢弃错误，      Never忽略错误) 
    associatedtype Failure: Error
 
     /// 接收数据源输入流，并转发给订阅者
    func receive(subscriber: Subscriber) where Failure == Subscriber.Failure, Output ==       Subscriber.Input
  }

• 所有的操作符的流程都类似，追踪一个操作符的调用顺序和触发流程
• 定义一个数组的数据流 [1, 2, 10000].publisher, publisher是Sequence的一个扩展，内部使用Publishers.Sequence进行了包装成了一个可以被观察的数据流

 extension Sequence {
     public var publisher: Publishers.Sequence {
         return .init(sequence: self)
     }
- `Publishers.Sequence` 内部实现
 /// 序列流继承于`Publisher` 
 public struct Sequence: Publisher {
         /// 输出源
         public typealias Output = Elements.Element
 
         public let sequence: Elements
 
         public init(sequence: Elements) {
             self.sequence = sequence
         }
         /// 实现 `Publisher`协议方法`receive`
         public func receive(subscriber: Downstream)
             where Failure == Downstream.Failure,
                   Elements.Element == Downstream.Input
         {
             /// 包装类`Inner`实现了`Subscription`协议，内部持有当前收到的`subscriber`, 进行转发
             let inner = Inner(downstream: subscriber, sequence: sequence)
 
             /// 判断是否序列是否到末尾，如果序列结束发送完成事件并取消序列，数据流完成，反之，持续接收数据流
             if inner.isExhausted {
                 subscriber.receive(subscription: Subscriptions.empty)
                 subscriber.receive(completion: .finished)
                 inner.cancel()
             } else {
                 /// 内部会调用`Subscription`协议的 `request`方法 
                 subscriber.receive(subscription: inner)
             }
         }
     }

• Inner作为一个私有类，单独实现了序列数据流的内部数据源的流转（每一个操作符都有一套内部特有的Inner实现类）,源码我进行了部分简化

  private final class Inner
         : Subscription
         where Downstream.Input == Elements.Element,
               Downstream.Failure == Failure
     {
         
         typealias Iterator = Elements.Iterator
         typealias Element = Elements.Element
 
         private var sequence: Elements?
         private var downstream: Downstream?
         private var iterator: Iterator
         private var next: Element?
         private var pendingDemand = Subscribers.Demand.none
 
         /// 初始化持有的`downstream`数据流，方便后续数据流转
         fileprivate init(downstream: Downstream, sequence: Elements) {
             self.sequence = sequence
             self.downstream = downstream
             self.iterator = sequence.makeIterator()
             next = iterator.next()
         }
         
         func request(_ demand: Subscribers.Demand) {
             guard downstream != nil else {
                 return
             }
 
             while let downstream = self.downstream, pendingDemand > 0 {
                 if let current = self.next {
                     /// 迭代数据流，依次进行数据的转发，交给订阅者接收
                     let additionalDemand = downstream.receive(current)
                 }
 
                 if next == nil {
                     self.downstream = nil
                     self.sequence = nil
                     /// 序列结束，发送完成事件
                     downstream.receive(completion: .finished)
                     return
                 }
             }
         }
     }

• 上面定义了数据流源,并发出了数据，等待订阅者监听数据流，sink和assign操作符可以进行订阅,后续会列出sink和assign的源码

代码演示片段

         class Root: NSObject {
             var name: String = ""
         }
         
         let root = Root()
         
         let arr: [Int] = [1, 2, 100]
         
         /// 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)"}
             /// `Sink`订阅数据源
             .sink { value in
                 debugPrint("数据流: \(value)")
             }.store(in: &cancel)
         
          /// 使用keypath进行赋值
         arr.publisher
             .filter{$0 > 2}
             .compactMap{"\($0)"}
             /// `Assign` keypath 赋值
             .assign(to: \.name, on: root).store(in: &cancel)
         
         debugPrint("root name: \(root.name)")
 
        /// 控制台输出
        "数据流: 100"
        "root name: 100"