SwiftUI学习(9)-Combine入门Part III-Subject&AnyPublisher&Cancellable

SwiftUI学习(9)-Combine入门Part III-Subject&AnyPublisher&Cancellable

2021, Oct 25    

コニクマル撰写

本文使用目前最新版本的 xcode 13 + macOS Big sur + iOS15

Subject(主题)

应该可以这样翻译吧?

Subject是一个集成Publisher的协议,比起Publisher, Subject提供了一些方法来发布事件。

Subject提供了几个 send 方法：

  • send(_ :Output): 用于发送一个事件给Subscriber
  • send(subscription: Subscription): 发布订阅内容给发布者

Swift 提供了PassthroughSubject和CurrentValueSubject两个Subject使用。

  • PassthroughSubject: 直接发布事件给Subscriber

    let passThrough = PassthroughSubject<Int, Never>()
passThrough.sink(receiveCompletion: {print("PassThrough Completion ", $0)}, receiveValue: {print("PassThrough Broadcast ", $0)})
passThrough.send(1)
passThrough.send(2)
passThrough.send(completion: .finished)

    输出如下:

    PassThrough Broadcast  1
PassThrough Broadcast  2
PassThrough Completion  finished

  • CurrentValueSubject: 初始化一个Output类型的值并发布给Subscriber，每当发布一个事件给Subscriber，这个值就会随之更新。或者更新该值也会发布事件给Subscriber。

    let currentValue = CurrentValueSubject<Int, Never>(0)
currentValue.sink(receiveCompletion: {print("PassThrough Completion ", $0)}, receiveValue: {print("PassThrough Broadcast ", $0)})
currentValue.value = 1
currentValue.send(2)
print(currentValue.value)

    输出如下:

    PassThrough Broadcast  0
PassThrough Broadcast  1
PassThrough Broadcast  2
2

AnyPublisher

有些时候并不希望然后Subscriber了解发布者者的具体内容, 比如使用PassthroughSubject或者自定义的Publisher时候，并不想然后调用的地方看这个Publisher具体实现。这个时候可以使用 eraseToAnyPublisher 将 Publisher 转换为AnyPublisher来隐藏Publihser细节。

例如:

let passThrough = PassthroughSubject<Int, Never>()
let publisher = passThrough.eraseToAnyPublisher()

按 Optional 键并点击 publisher，可以看到 publisher 是一个 AnyPublisher<Int, Never>类型的对象。

Cancellable

Cancellable 是一个协议而订阅内容(Subscription)继承了这个协议，在SwiftUI学习(7)-Combine入门Part II-Subscriber一文中提到，Subscriber订阅成功后, 其func receive(subscription: Subscription)会被调用，这个 subscription 继承了Cancellable协议，调用subscription.cancel()就能订阅。

例如：

class TestSubscriber: Subscriber{
    private var subscription: Subscription? = nil

    func receive(subscription: Subscription) {
        print("Received Subscription")
        subscription.request(.max(1))
        self.subscription = subscription
    }

    func receive(_ input: Int) -> Subscribers.Demand {
        print("Received Value", input)
        return .unlimited
    }

    func receive(completion: Subscribers.Completion<Never>) {
        print("Received completion")
    }

    func cancel(){
        self.subscription?.cancel()
    }
}
let passThrough = PassthroughSubject<Int, Never>()
let subscriber = TestSubscriber()
passThrough.subscribe(subscriber)
passThrough.send(1)
passThrough.send(2)
subscriber.cancel()
passThrough.send(3)
passThrough.send(4)

输出:

Received Subscription
Received Value 1
Received Value 2

当 cancel 后Subscriber就无法收到数据了。

AnyCancellable

AnyCancellable是一个 final class, 当 class 被释放了会调用改类的cancel()取消操作。

给上面自定义的 Subscriber 添加一个 AnyCancellable 的只读对象:

...
var cancellable: AnyCancellable{
  return AnyCancellable {
      print("Cancel")
      self.cancel()
  }
}
...

编写测试代码：

let passThrough = PassthroughSubject<Int, Never>()
func doIt(){
    let subscriber = TestSubscriber()
    passThrough.subscribe(subscriber)
    passThrough.send(1)
    passThrough.send(2)
    // 获取AnyCancellable
    let c = subscriber.cancellable
}
doIt()
passThrough.send(3)
passThrough.send(4)

运行结果:

Received Subscription
Received Value 1
Received Value 2
Cancel

在 doIt()函数运行完后, AnyCancellable类型的 c 变量自动释放了。

其他相关