发布者 Publisher
在Combine中，Publisher是观察者模式中的Observable，并且可以通过组合变换（利用Operator）重新生成新的Publisher。
/// # Creating Your Own Publishers
///
/// Rather than implementing the `Publisher` protocol yourself, you can create your own publisher by using one of several types provided by the Combine framework:
///
/// - Use a concrete subclass of ``Subject``, such as ``PassthroughSubject``, to publish values on-demand by calling its ``Subject/send(_:)`` method.
/// - Use a ``CurrentValueSubject`` to publish whenever you update the subject’s underlying value.
/// - Add the `@Published` annotation to a property of one of your own types. In doing so, the property gains a publisher that emits an event whenever the property’s value changes. See the ``Published`` type for an example of this approach.
@available(macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
public protocol Publisher<Output, Failure> {
/// 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
}
在Publisher定义中，Output代表数据流中的输出值，值的更新可能是同步或者异步。Failure代表可能产生的错误。Publisher最核心的是定义了值与可能的错误。
Publisher通过receive(subscriber:)接受订阅，并且要求Subscriber的值和错误的类型一致来保证类型安全。
官方例子：
extension NotificationCenter {
struct Publisher: Combine.Publisher {
typealias Output = Notification
typealias Failure = Never
init(center: NotificationCenter, name: Notification.Name, object: Any? = nil)
}
}
例子中，Publisher提供的值是Notification类型，而且永远不会产生错误（Never）。这个扩展很方便的将任何Notification转换成Publisher，便于我们将应用改造成Reactive。
Just
只发送一次消息就结束
let justPublisher = Just("一次消息")
Empty
不提供任何值更新，并且可以选择立即正常结束。
func emptyFunc() {
// 只发送一次消息，不带任何信息
let emptyPublisher = Empty(completeImmediately: true, outputType: (Any).self, failureType: Never.self)
let emptyPublisher2 = Empty<Any, Never>(completeImmediately: false)
}
Once
Once可以提供两种数据流之一：
- 发送一次值更新，然后立即正常结束（和Just一样）
- 立即因错误而终止
没有找到对应的API，还在翻阅资料中……
Fail
- 发送一次值更新，然后立即因错误而终止
- 立即因错误而终止
enum NetWorkError: Error, Equatable {
case message(String, Any?)
case noNetWork
static func == (lhs: NetWorkError, rhs: NetWorkError) -> Bool {
lhs.errorDescription == rhs.errorDescription
}
}
extension NetWorkError: LocalizedError {
var errorDescription: String? {
return "\(self)"
}
}
let failPublisher = Fail(outputType: Any.self, failure: NetWorkError.message("我是错误", nil))
Sequence
将给定的一个序列按序通知到订阅者。
let sequencePublisher = [1, 2, 3].publisher
Future
Future需要提供执行具体操作的closure，这个操作可以是异步，最终返回一个Result。
所以Future要么发送一个值正常结束，要么因错误而终止。在将一些一步操作转换为Publisher时非常有用，特别是网络请求。
let future = Future<Data?, Never> { promise in
URLSession.shared.dataTask(with: URL(string: "https://api")!) { data, response, error in
promise(.success(data))
}.resume()
}
Deferred
deferred初始化需要提供一个closure，只有在Subscriber订阅的时候才会生成指定的Publisher，并且每个Subscriber获取到的Publisher都是全新的
let deferredPublisher = Deferred {
return Just(arc4random())
}
延时发布消息
可以使用delay方法，生成一个新的Publisher。
// 只发送一次消息
let justPublisher = Just("一次消息")
// 延迟2秒发布消息
let delyPublisher = justPublisher.delay(for: 2, scheduler: DispatchQueue.main)
