iOS Combine自定义EvenSubject无法发射值问题排查

核心问题 #

你的EvenSubject无法发射值的原因有两个：

1. 订阅时机错误：你先调用send发送值，之后才通过sink订阅。底层的PassthroughSubject是无缓存的“热”Subject，只会传递订阅之后发送的值，之前的所有值都会丢失。
2. 底层Subject类型不匹配需求：如果需要订阅者能获取订阅前的初始值，PassthroughSubject无法满足，它不会缓存任何值。

修复方案 #

方案1：调整订阅时机（仅接收订阅后的偶数）

如果只需要订阅之后的偶数被接收，把订阅代码移到所有send调用之前即可：

class EvenSubject<Failure: Error>: Subject {
    
    typealias Output = Int
    
    private let wrapped: PassthroughSubject<Int, Failure>
    
    init(initialValue: Int) {
        self.wrapped = PassthroughSubject()
        let evenInitialValue = EvenSubject.isEven(value: initialValue) ? initialValue : 0
        send(evenInitialValue)
    }
    
    private static func isEven(value: Int) -> Bool {
        value % 2 == 0
    }
    
    func send(_ value: Output) {
        print("send")
        if value % 2 == 0 {
            wrapped.send(value)
        }
    }
    
    func send(completion: Subscribers.Completion<Failure>) {
        print("send completion")
        wrapped.send(completion: completion)
    }
    
    func send(subscription: Subscription) {
        print("send subscription")
        wrapped.send(subscription: subscription)
    }
    
    func receive<S>(subscriber: S) where S : Subscriber, Failure == S.Failure, Output == S.Input {
        wrapped.receive(subscriber: subscriber)
    }
}

// 先订阅，再发送值
let subject = EvenSubject<Never>(initialValue: 4)

let cancellable = subject.sink { _ in
    
} receiveValue: { value in
    print(value) // 输出：4、10、14
}

subject.send(10)
subject.send(7)
subject.send(14)

方案2：改用CurrentValueSubject作为底层（支持订阅前的初始值）

如果需要订阅者能获取订阅前的初始偶数，将底层PassthroughSubject替换为CurrentValueSubject，确保初始值为偶数：

class EvenSubject<Failure: Error>: Subject {
    
    typealias Output = Int
    
    private let wrapped: CurrentValueSubject<Int, Failure>
    
    init(initialValue: Int) {
        let evenInitialValue = EvenSubject.isEven(value: initialValue) ? initialValue : 0
        self.wrapped = CurrentValueSubject(evenInitialValue)
    }
    
    private static func isEven(value: Int) -> Bool {
        value % 2 == 0
    }
    
    func send(_ value: Output) {
        print("send")
        if value % 2 == 0 {
            wrapped.send(value)
        }
    }
    
    func send(completion: Subscribers.Completion<Failure>) {
        print("send completion")
        wrapped.send(completion: completion)
    }
    
    func send(subscription: Subscription) {
        print("send subscription")
        wrapped.send(subscription: subscription)
    }
    
    func receive<S>(subscriber: S) where S : Subscriber, Failure == S.Failure, Output == S.Input {
        wrapped.receive(subscriber: subscriber)
    }
    
    // 可选：获取当前最新偶数
    var currentValue: Int {
        wrapped.value
    }
}

let subject = EvenSubject<Never>(initialValue: 4)

subject.send(10)
subject.send(7)
subject.send(14)

let cancellable = subject.sink { _ in
    
} receiveValue: { value in
    print(value) // 输出：14（订阅时先发送最新缓存的偶数，后续send偶数会继续输出）
}

方案3：缓存所有历史偶数（订阅时补发全部历史值）

如果需要订阅者能收到所有发送过的偶数，在EvenSubject中维护缓存数组，订阅时补发所有历史值：

class EvenSubject<Failure: Error>: Subject {
    
    typealias Output = Int
    
    private let wrapped: PassthroughSubject<Int, Failure>
    private var evenValuesCache: [Int] = []
    
    init(initialValue: Int) {
        self.wrapped = PassthroughSubject()
        let evenInitialValue = EvenSubject.isEven(value: initialValue) ? initialValue : 0
        send(evenInitialValue)
    }
    
    private static func isEven(value: Int) -> Bool {
        value % 2 == 0
    }
    
    func send(_ value: Output) {
        print("send")
        if value % 2 == 0 {
            evenValuesCache.append(value)
            wrapped.send(value)
        }
    }
    
    func send(completion: Subscribers.Completion<Failure>) {
        print("send completion")
        wrapped.send(completion: completion)
    }
    
    func send(subscription: Subscription) {
        print("send subscription")
        wrapped.send(subscription: subscription)
    }
    
    func receive<S>(subscriber: S) where S : Subscriber, Failure == S.Failure, Output == S.Input {
        // 补发所有缓存的偶数
        evenValuesCache.forEach { wrapped.send($0) }
        wrapped.receive(subscriber: subscriber)
    }
}

let subject = EvenSubject<Never>(initialValue: 4)

subject.send(10)
subject.send(7)
subject.send(14)

let cancellable = subject.sink { _ in
    
} receiveValue: { value in
    print(value) // 输出：4、10、14
}

内容的提问来源于stack exchange，提问作者user19037628