iOS中Combine Framework如何为Publisher发射的值添加间隔延迟？

问题分析 #

你的代码存在几个关键问题导致delay操作符未生效：

1. 操作符链断裂：delay之后没有通过链式调用连接sink，而是直接订阅原subject，等于完全绕过了延迟处理逻辑。
2. 未保留订阅者：sink返回的AnyCancellable如果不存储，会被立即释放，订阅直接失效。
3. Playground执行限制：默认情况下Playground会在代码执行完后立即终止，无法等待延迟后的事件输出。
4. delay的作用逻辑：delay是给每个事件单独延迟指定时长，如果两个事件同时发送，延迟后依然会同时被接收，无法实现两个值之间的间隔延迟。

修正方案 #

要实现两个值之间的延迟接收，我们可以通过两种方式处理，以下是完整的修正代码：

方案一：用Combine操作符实现顺序延迟

import Combine
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

example(of: "delay between values"){
    
    struct Human {
        let fName: String
        let lName: String
    }

    let subject = PassthroughSubject<Human,Error>()
    let delayInSeconds = 4
    var isLoading = false
    var cancellables = Set<AnyCancellable>()
    
    subject
        .flatMap { human in
            // 每个值延迟指定时长后发送，实现顺序间隔
            Just(human)
                .delay(for: .seconds(delayInSeconds), scheduler: DispatchQueue.main)
        }
        .sink(
            receiveCompletion:{
                print("Received Completion", $0)
                isLoading = true
                print("isLoading =", isLoading)
                PlaygroundPage.current.finishExecution()
            },
            receiveValue: {
                print("Value Received:" ,$0.fName, $0.lName)
            })
        .store(in: &cancellables)

    subject.send(Human(fName: "John", lName: "Abraham"))
    subject.send(Human(fName: "Jane", lName: "Anne"))
    subject.send(completion: .finished)
}

方案二：手动延迟发送第二个值（更直接）

import Combine
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

example(of: "delay between values"){
    
    struct Human {
        let fName: String
        let lName: String
    }

    let subject = PassthroughSubject<Human,Error>()
    let delayInSeconds = 4
    var isLoading = false
    var cancellables = Set<AnyCancellable>()
    
    subject
        .sink(
            receiveCompletion:{
                print("Received Completion", $0)
                isLoading = true
                print("isLoading =", isLoading)
                PlaygroundPage.current.finishExecution()
            },
            receiveValue: {
                print("Value Received:" ,$0.fName, $0.lName)
            })
        .store(in: &cancellables)

    // 第一个值立即发送
    subject.send(Human(fName: "John", lName: "Abraham"))
    // 延迟指定时长后发送第二个值
    DispatchQueue.main.asyncAfter(deadline: .now() + delayInSeconds) {
        subject.send(Human(fName: "Jane", lName: "Anne"))
        subject.send(completion: .finished)
    }
}

关键注意点 #

• 必须将sink返回的AnyCancellable存储到集合中，否则订阅会被系统立即释放，无法接收事件。
• Playground必须开启needsIndefiniteExecution = true，否则会在初始代码执行完后直接终止，看不到延迟后的输出。

内容的提问来源于stack exchange，提问作者D.Yasiru Jayatissa