SwiftUI学习(8)-Combine入门Part II-Subscriber

SwiftUI学习(8)-Combine入门Part II-Subscriber

2021, Oct 24    

コニクマル撰写

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

订阅者-Subscriber

Subscriber也是一个协议，协议包含了Input的数据类型，这个类型对应着Publisher的Output类型以及请求数据的数量。

实现Subscriber协议

先看看Subscriber协议

public protocol Subscriber : CustomCombineIdentifierConvertible {
    associatedtype Input
    associatedtype Failure : Error
  
    func receive(subscription: Subscription)

    func receive(_ input: Self.Input) -> Subscribers.Demand
    func receive(completion: Subscribers.Completion<Self.Failure>)
}
  • CustomCombineIdentifierConvertible 这个协议用来实现对Publisher流的唯一ID，系统用已经扩展实现了。
  • Input是输入类型，对应Publisher的Output类型
  • Failure是错误类型，对应Publisher的Failure类型
  • ` func receive(subscription: Subscription)`
    • 当订阅成功后调用,传递了订阅的订阅内容(Subscription)
    • 当Subscriber可以准备好开始接受数据了，可以调用subscription的request(_:)方法请求数据,request(_:)传递Subscribers.Demand作为参数。
      • Subscribers.Demand.max 表示请求最多几个
      • Subscribers.Demand.none 表示请求0个元素
      • Subscribers.Demand.unlimited表示请求无限个
  • func receive(_ input: Self.Input) -> Subscribers.Demand
    • 当订阅者接收消息的时候调用
    • input就是接受到Input类型数据
    • 需要返回一个Subscribers.Demand表示接下来还要接收几个参数
  • func receive(completion: Subscribers.Completion<Self.Failure>)
    • 当事件结束了调用

接下来自定义一个Subscriber：

class TestSubscriber: Subscriber{
    func receive(subscription: Subscription) {
        print("Received Subscription")
        subscription.request(.max(1))
    }
    
    func receive(_ input: Int) -> Subscribers.Demand {
        print("Received Value", input)
        return .unlimited
    }
    
    func receive(completion: Subscribers.Completion<Never>) {
        print("Received completion")
    }
}

简单运行下:

let sub = TestSubscriber()
(0 ..< 5).publisher.receive(subscriber: sub)

运行结果:

Received Subscription
Received Value 0
Received Value 1
Received Value 2
Received Value 3
Received Value 4
Received completion

然后把 func receive(subscription: Subscription)也就第一个函数修改下:

 subscription.request(.none)

再次运行:

Received Subscription

收不到数据也收不到完成。

在修改下代码:

...
 func receive(subscription: Subscription) {
        print("Received Subscription")
        subscription.request(.max(2))
    }
    
    func receive(_ input: Int) -> Subscribers.Demand {
        print("Received Value", input)
        return .none
    }
...

再次运行:

Received Subscription
Received Value 0
Received Value 1

再次修改代码

....
    func receive(_ input: Int) -> Subscribers.Demand {
        print("Received Value", input)
         return  input == 0 ? .max(2) : .none
    }
....

再次运行:

Received Subscription
Received Value 0
Received Value 1
Received Value 2
Received Value 3

通过两次修改可以发现接受的事件数量等于 subscription.request的次数加上func receive(_ input: Int) -> Subscribers.Demand返回的次数和。

sink

在Part I-Publisher中的很多例子都是用到sink(receiveCompletion:receiveValue:),这个方法是通过receiveCompletion和receiveValue闭包创建订阅者来完成订阅。

  • receiveCompletion当Publisher返回完成后调用
  • receiveValue当接受到事件后调用.

sink(receiveCompletion:receiveValue:)一订阅成功就马上请求的无限个数据, 返回的Cancellable对象必须持有，否则Cancellable被释放，数据流就会被取消。

如果不需要处理完成或者错误可以使用sink(receiveValue: )这个方法。

assign

assgin会将接受到的数据分发(assign)给标记成发布者的属性(Property)进行重新发布。

assgin可以通过keypath或者inout把接受到的数据分配给属性(Property)。举个例子:

class Test{
    @Published var i: Int = 0
    @Published var ii: Int = 0
}

变量i和ii需要使用@Published属性修饰, @Published就标记这些属性是Published, 通过$来获取属性对应publisher。@Published修饰的属性在SwiftUI开发中比较常用, 是连接View和ViewModel的很好方式。

测试下上面的代码:


class Test{
    @Published var i: Int = 0
    @Published var ii: Int = 0
}

let test = Test()

Just(1).assign(to: \.i, on: test)
Just(3).assign(to: &test.$ii)
test.$i.sink(receiveValue: { print("i", $0)})
test.$ii.sink(receiveValue: { print("ii", $0)})

输出如下:

i 1
ii 3

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