Combine核心概念
Combine主要有3大核心概念：Publisher，Operator和Subscriber：
Publisher是数据的提供者，它提供了最原始的数据，不管这个数据是从什么地方获取的。如果把pipline想象成一条包子生产线，那么Publisher就表示食材Subscriber是数据的接收者，它要求接收的数据必须是处理好的，同样把pipline想象成一条包子生产线，则Subscriber就是成品包子，而不是中间产物(菜馅等)Operator是中间处理过程，它上下联通Publisher和Subscriber，对Publisher输出地数据进行处理，然后返回成品数据给Subscriber
Publisher
public protocol Publisher {
/// The kind of values published by this publisher.
associatedtype Output
/// The kind of errors this publisher might publish.
///
/// Use `Never` if this `Publisher` does not publish errors.
associatedtype Failure : Error
/// Attaches the specified subscriber to this publisher.
///
/// Implementations of ``Publisher`` must implement this method.
///
/// The provided implementation of ``Publisher/subscribe(_:)-4u8kn``calls this method.
///
/// - Parameter subscriber: The subscriber to attach to this ``Publisher``, after which it can receive values.
func receive<S>(subscriber: S) where S : Subscriber, Self.Failure == S.Failure, Self.Output == S.Input
}
从上边的代码，我们可以分析出3点重要信息：
Publisher是一个协议，我们后续用到的所有publishers都实现了这个协议Publisher/receive(subscriber:)是该协议的核心方法，它接受的参数subscriber需要实现Subscriber协议，这就是Operator和Subscriber能够连接Publisher原因Self.Failure == S.Failure, Self.Output == S.Input这个条件限制了Publisher输出的数据类型必须跟Subscriber输入的数据类型保持一致
Publisher/receive(subscriber:) 并没有限制Subscriber的数量，因此 Publisher 可以接受多个订阅。
Subscriber
public protocol Subscriber : CustomCombineIdentifierConvertible {
/// The kind of values this subscriber receives.
associatedtype Input
/// The kind of errors this subscriber might receive.
///
/// Use `Never` if this `Subscriber` cannot receive errors.
associatedtype Failure : Error
/// Tells the subscriber that it has successfully subscribed to the publisher and may request items.
///
/// Use the received ``Subscription`` to request items from the publisher.
/// - Parameter subscription: A subscription that represents the connection between publisher and subscriber.
func receive(subscription: Subscription)
/// Tells the subscriber that the publisher has produced an element.
///
/// - Parameter input: The published element.
/// - Returns: A `Subscribers.Demand` instance indicating how many more elements the subscriber expects to receive.
func receive(_ input: Self.Input) -> Subscribers.Demand
/// Tells the subscriber that the publisher has completed publishing, either normally or with an error.
///
/// - Parameter completion: A ``Subscribers/Completion`` case indicating whether publishing completed normally or with an error.
func receive(completion: Subscribers.Completion<Self.Failure>)
}
我们仔细分析上边的代码，同样会得到以下几个重要信息：
Subscriber实现了CustomCombineIdentifierConvertible协议，用于标记唯一身份Subscriber同样是一个协议Subscriber/receive(subscription:)该方法由Subscriber实现，但是由Publisher来调用，Publisher调用了该方法后会传递一个实现了Subscription协议的实例，Subscriber使用该实例发送request请求数据Subscriber实现了Subscriber/receive(_:)协议，Publisher调用该方法发送数据Subscriber实现了Subscriber/receive(completion:)协议，Publisher调用该方法发送结束事件(.finished或.failure)Subscriber只接收输入数据
****在真实的开发中，我们用到最多的Subscriber是assign和sink。
Operator
常用的Operator：
["scan", "tryScan", "map/tryMap", "flatMap","setFailureType",
"compactMap/tryCompactMap", "filter/tryFilter", "removeDuplicates",
"replace", "collect", "ignoreOutput", "reduce","max", "min", "count",
"first", "last", "drop", "prepend", "dropFirst", "prefix", "output",
"combineLatest","merge", "zip", "allSatisfy", "contains", "catch",
"assertNoFailure", "retry", "mapError", "switchToLatest", "debounce",
"delay", "measureInterval", "throttle", "timeout", "encode", "decode",
"share", "multicast","breakpoint", "breakpointOnError", "handleEvents",
"print", "receive", "subscribe"]
在Combine中，Operator是函数：
extension Publishers {
/// A publisher that transforms all elements from the upstream publisher with a provided closure.
public struct Map<Upstream, Output> : Publisher where Upstream : Publisher {
/// The kind of errors this publisher might publish.
///
/// Use `Never` if this `Publisher` does not publish errors.
public typealias Failure = Upstream.Failure
/// The publisher from which this publisher receives elements.
public let upstream: Upstream
/// The closure that transforms elements from the upstream publisher.
public let transform: (Upstream.Output) -> Output
public init(upstream: Upstream, transform: @escaping (Upstream.Output) -> Output)
/// Attaches the specified subscriber to this publisher.
///
/// Implementations of ``Publisher`` must implement this method.
///
/// The provided implementation of ``Publisher/subscribe(_:)-4u8kn``calls this method.
///
/// - Parameter subscriber: The subscriber to attach to this ``Publisher``, after which it can receive values.
public func receive<S>(subscriber: S) where Output == S.Input, S : Subscriber, Upstream.Failure == S.Failure
}
}
从上边的代码中，我们分析出以下几个重要信息：
结构体
Map实现了Publisher协议，因此它本身就是一个Publisherinit(upstream: Upstream, transform: @escaping (Upstream.Output) -> Output)从它的初始化化函数来看，它需要传入一个upstream: Upstream,这个upstream就是一个Publisher,还需要传入一个闭包，闭包的参数是上游publisher的输出数据
Map接收一个Publisher作为输入，然后等待该publisher输出数据，然后把该数据映射成其他数据类型。
Subscription
Subscription也是一个不容忽视的概念，是它连接了Publisher和Subscriber，我们看看它的代码：
public protocol Subscription : Cancellable, CustomCombineIdentifierConvertible {
/// Tells a publisher that it may send more values to the subscriber.
func request(_ demand: Subscribers.Demand)
}
同样，基于上边的代码，我们分析出以下几个信息：
它是一个协议，实现该协议的实例必须实现协议要求的方法
它继承了
Cancellable协议，因此实现了Subscription协议的实例自然就可以取消pipline使用
request函数发送请求
事件过程
Publisher收到订阅Publisher调用Subscriber的Subscriber/receive(subscription:)方法返回一个subscription实例Subscriber使用subscription发送requestPublisher调用Subscriber的Subscriber/receive(_:)方法发送数据Publisher调用Subscriber的Subscriber/receive(completion:)方法发送完成事件