ReactiveCocoa：创建一个在观察者上应用地图的信号

Question

据我了解，RACSubject 等效于 ReactiveCocoa 4 是 Observer 类。 我希望将信号和观察者链接在一起，以便信号发送事件，将映射操作应用于发送给观察者的事件。 在 Obj-C 中它看起来像这样：

// ViewModel.h

@interface ViewModel

@property (readonly) RACSubject *someAction; //expects e.g. int values
@property (readonly) RACSignal *someSignal; //sends e.g. string values

@end

// ViewModel.m

//redeclaring the signal and action as readwrite

@implementation

- (id)init {
  _someAction = [RACSubject subject];
  _someSignal = [_someAction map:^id(NSNumber *index) {
     return "Some string based on index passed in";
  }];
}

@end

现在，当有人将值推送到 someAction 时，someSignal 将触发包含派生值的事件。 如何在 Swift 中实现同样的效果？

到目前为止，我能做的事情是这样的：

public class ViewModel: NSObject {
    public let (internalSignal, someAction) = Signal<Int, NoError>.pipe()
    public var someSignal: Signal<String, NoError> {
        get {
            return self.internalSignal.map({ [unowned self](index: Int) -> String in
                return "Some string value based on \(self.someArray[index])"
            })
        }
    }
    public let someArray = [1, 2, 3, 4, 5]
}

这看起来是个糟糕的解决方案，因为

1. internalSignal 应该是私有的，但需要声明为公共的，以便与 Signal 的管道匹配
2. 因此，someSignal 是在每次需要时计算的，即使相同的信号可以一遍又一遍地重复使用。也不能声明为let 常量。

Answer 1

您可以像 ObjC 一样初始化 init 中的成员...

public class ViewModel: NSObject {
    private let internalSignal: Signal<Int, NoError>
    public let someAction: Observer<Int, NoError>
    public let someSignal: Signal<String, NoError>

    override init() {
        (internalSignal, someAction) = Signal<Int, NoError>.pipe()
        someSignal = internalSignal.map { index in
            "Some string value based on \(index)"
        }
        super.init()
    }
}

---

对于someSignal，您也可以使用lazy initialization，它允许会员引用self：

public class ViewModel: NSObject {
    private let internalSignal: Signal<Int, NoError>
    public let someAction: Observer<Int, NoError>
    public private(set) lazy var someSignal: Signal<String, NoError> =
        self.internalSignal.map { [unowned self] index in
            "Some string value based on \(self.someArray[index])"
        }

    override init() {
        (internalSignal, someAction) = Signal<Int, NoError>.pipe()
        super.init()
    }
}

与第一段代码不同，lazy-var 仅在使用 someSignal 之前初始化，而不是在 ViewModel 初始化时。

另外，因为它是一个var，Swift 允许你改变它的值（没有lazy let 这样的东西）。我们可以使用private(set) 限制权限，但这不会阻止您在某处不小心写self.someSignal = ...。

---

或者，您可以将 someSignal 设为隐式展开的可选选项并手动初始化：

public class ViewModel: NSObject {
    private let internalSignal: Signal<Int, NoError>
    public let someAction: Observer<Int, NoError>
    public private(set) var someSignal: Signal<String, NoError>!

    override init() {
        (internalSignal, someAction) = Signal<Int, NoError>.pipe()
        super.init()
        someSignal = internalSignal.map { [unowned self] index in
            "Some string value based on \(self.someArray[index])"
        }
    }
}