// iOS 命令式：Target-Action
textField.addTarget(self, action: #selector(textDidChange), for: .editingChanged)
func textDidChange() {
    let text = textField.text ?? ""
    if text.count >= 3 { searchUsers(text) }
}

let (signal, observer) = Signal<String, Never>.pipe()
signal.observeValues { print($0) }
observer.send(value: "Hi")

let producer = SignalProducer<String, Never> { obs, _ in
    obs.send(value: "Hello")
    obs.sendCompleted()
}
producer.startWithValues { print($0) }

searchTextField.reactive.continuousTextValues
    .filter { ($0 ?? "").count >= 2 }
    .debounce(0.3, on: QueueScheduler.main)
    .flatMap(.latest) { keyword in
        searchAPI(keyword ?? "")
    }
    .observe(on: UIScheduler())
    .observeValues { [weak self] users in
        self?.updateSearchResults(users)
    }

// 命令式：监听文本框变化
textField.addTarget(self, action: #selector(textDidChange), for: .editingChanged)

func textDidChange() {
    let text = textField.text ?? ""
    if text.count >= 3 {
        searchUsers(text)
    }
}

// 响应式：声明数据流关系
textField.reactive.continuousTextValues
    .filter { ($0?.count ?? 0) >= 3 }
    .debounce(0.3, on: QueueScheduler.main)
    .observeValues { [weak self] text in
        self?.searchUsers(text ?? "")
    }

public enum Event<Value, Error: Swift.Error> {
    case value(Value)      // 携带一个值
    case failed(Error)     // 失败（携带错误）
    case completed         // 正常完成
    case interrupted       // 被中断
}

let (signal, observer) = Signal<String, Never>.pipe()

// observer 用于发送事件
observer.send(value: "Hello")
observer.send(value: "World")
observer.sendCompleted()

let disposable = signalProducer.start { value in
    print(value)
}

// 不再需要时取消订阅
disposable.dispose()

// 创建 Signal 的方式：pipe
let (signal, observer) = Signal<String, Never>.pipe()

signal.observeValues { value in
    print("收到: \(value)")
}

observer.send(value: "A")  // 输出: 收到: A
observer.send(value: "B")  // 输出: 收到: B
observer.sendCompleted()

let producer = SignalProducer<String, Never> { observer, _ in
    print("开始执行")  // 每次 start 都会打印
    observer.send(value: "Hello")
    observer.send(value: "World")
    observer.sendCompleted()
}

let d1 = producer.startWithValues { print("观察者1: \($0)") }
// 输出: 开始执行
// 输出: 观察者1: Hello
// 输出: 观察者1: World

let d2 = producer.startWithValues { print("观察者2: \($0)") }
// 再次输出: 开始执行
// 输出: 观察者2: Hello
// 输出: 观察者2: World

// MutableProperty：可读可写
let username = MutableProperty<String>("")

// 读取当前值
print(username.value)

// 监听变化
username.signal.observeValues { newValue in
    print("用户名变为: \(newValue)")
}

// 更新值
username.value = "张三"
username.modify { $0 = "李四" }

let searchAction = Action<String, [User], NSError> { keyword in
    return searchAPI(keyword)  // 返回 SignalProducer
}

// 绑定启用条件（例如：输入非空）
let enabledSearch = Action(state: viewModel.keyword, enabledIf: { $0.count > 0 }) { _, keyword in
    return searchAPI(keyword)
}

// 执行
searchAction.apply("Swift").startWithResult { result in
    switch result {
    case .success(let users):
        self.users = users
    case .failure(let error):
        self.showError(error)
    }
}

signal
    .map { $0.uppercased() }
    .observeValues { print($0) }

signal
    .filter { $0.count >= 3 }
    .observeValues { print($0) }

signal
    .reduce(0) { $0 + $1 }
    .observeValues { print("总和: \($0)") }

let combined = Signal.combineLatest(usernameSignal, passwordSignal)
combined.observeValues { (user, pwd) in
    loginButton.isEnabled = user.count > 0 && pwd.count >= 6
}

let zipped = Signal.zip(numbersSignal, lettersSignal)
// (1,A), (2,B), (3,C)...

searchKeywordSignal
    .flatMap(.latest) { keyword in
        searchAPI(keyword)  // 新关键词会取消上一次请求
    }
    .observeValues { users in
        self.updateUI(users)
    }

producer
    .flatMapError { _ in SignalProducer(value: []) }  // 失败时返回空数组
    .retry(upTo: 3)                                   // 失败重试 3 次
    .mapError { CustomError.wrapped($0) }             // 转换错误类型

// Signal 的核心：Generator 闭包
// 当有观察者订阅时，闭包被调用，传入 Observer
public init(_ generator: (Observer) -> Disposable?)

// pipe 创建方式
public static func pipe() -> (Signal, Signal.Observer) {
    var observer: Signal.Observer!
    let signal = Signal { innerObserver in
        observer = innerObserver
        return nil
    }
    return (signal, observer)
}

// 每次 start 时：
// 1. 创建一个新的 Signal（通过 pipe 或类似机制）
// 2. 执行 generator 闭包，将 observer 传入
// 3. 闭包内的逻辑开始执行，向 observer 发送事件
public func start(_ observer: Observer) -> Disposable {
    let (signal, pipeObserver) = Signal.pipe()
    let disposable = CompositeDisposable()
    disposable += signal.observe(observer)
    disposable += self.startWithSignal { signal, innerDisposable in
        pipeObserver.observe(signal)
        disposable += innerDisposable
    }
    return disposable
}

// map 的简化逻辑
extension Signal {
    public func map<U>(_ transform: @escaping (Value) -> U) -> Signal<U, Error> {
        return Signal { observer in
            return self.observe { event in
                switch event {
                case let .value(value):
                    observer.send(value: transform(value))
                case .completed:
                    observer.sendCompleted()
                case let .failed(error):
                    observer.send(error: error)
                case .interrupted:
                    observer.sendInterrupted()
                }
            }
        }
    }
}

class LoginViewModel {
    let username = MutableProperty("")
    let password = MutableProperty("")
    
    var canLogin: Property<Bool> {
        return Property(
            initial: false,
            then: Signal.combineLatest(username.signal, password.signal)
                .map { user, pwd in
                    user.count >= 3 && pwd.count >= 6
                }
        )
    }
    
    let loginAction: Action<(String, String), User, Error>
    
    init() {
        loginAction = Action(enabledIf: canLogin) { [weak self] _, input in
            return loginAPI(username: input.0, password: input.1)
        }
    }
}

// ViewController 中绑定
viewModel.canLogin.signal
    .observeValues { [weak loginButton] canLogin in
        loginButton?.isEnabled = canLogin
    }

loginButton.reactive.pressed = CocoaAction(viewModel.loginAction) { _ in
    (viewModel.username.value, viewModel.password.value)
}

searchTextField.reactive.continuousTextValues
    .filter { ($0 ?? "").count >= 2 }
    .debounce(0.3, on: QueueScheduler.main)
    .flatMap(.latest) { [weak self] keyword -> SignalProducer<[User], Error> in
        guard let keyword = keyword, !keyword.isEmpty else {
            return .init(value: [])
        }
        return self?.searchAPI(keyword) ?? .empty
    }
    .observe(on: UIScheduler())
    .startWithValues { [weak self] users in
        self?.updateSearchResults(users)
    }

let localUsers = loadLocalUsers()   // SignalProducer<[User], Never>
let remoteUsers = fetchRemoteUsers() // SignalProducer<[User], Error>

SignalProducer.combineLatest(
    localUsers.flatMapError { _ in .init(value: []) },
    remoteUsers.flatMapError { _ in .init(value: []) }
)
.map { local, remote in
    mergeAndDeduplicate(local: local, remote: remote)
}
.startWithValues { [weak self] users in
    self?.tableView.reload(with: users)
}

// UIControl 事件
button.reactive.controlEvents(.touchUpInside)
    .observeValues { _ in print("点击") }

// UITextField 文本
textField.reactive.continuousTextValues
    .observeValues { print($0 ?? "") }

// 双向绑定（需要导入 ReactiveCocoa）
label.reactive.text <~ viewModel.title  // 单向：ViewModel -> Label

// CocoaAction 绑定按钮
button.reactive.pressed = CocoaAction(viewModel.submitAction)