初始化具有多个依赖项的 RAC ReactiveSwift 属性？

Question

我对使用 ReactiveSwift 和 ReactiveCocoa 还很陌生，我似乎在初始化具有依赖关系的属性的正确方法方面遇到了障碍。

例如在下面的代码中，我尝试初始化一个属性，但我得到一个预期的编译器错误。我的问题是如何/什么是“正确”的方式来做到这一点。

class SomeViewModel {
// illustration purposes, in reality the property (dependency) we will observe will change over time
let dependency = Property(value: true)
let dependency2 = Property(value: "dependency2")
let dependency3 = Property(value: 12345)
let weightLabel: Property<String>

// private(set) var weightLabel: Property<String>!
// using private(set) var weightLabel: Property<String>! works, 
// however this changes the meaning behind using let, because we could
// reinitalize weightLabel again which is not similar to using a let so not a good alternative

// let weightLabel: Property<String> = Property(value: "")
// another solution that will work but will result in a wrong value
// upon initalization then, changed into the "correct value" thus, i
// am discrading this as well

init() {
    weightLabel = dependency.map {
        // compiler error, 'self' captured by closure before all members were initalized. 
        // My question is if there is a way to handle this scenario properly
        if $0 && self.dependency2.value == "dependency2" && self.dependency3.value == 12345 {
            return ""
        }
        return ""
    }
}
}

因此，正如您在上面的 cmets 中可能已经注意到的那样，我想知道是否有一种方法可以使用 ReactiveSwift 来处理这种情况，而不是我上面提到的那些并不是真正理想的解决方案。

Answer 1

适合该场景的工具是combineLatest，它提供所有这些属性（流）的组合版本，只要它们中的任何一个被更新。

weightLabel = Property.combineLatest(dependency, dependency2, dependency3)
    .map { d1, d2, d3 in
        return "Hello World! \(d1) \(d2) \(d3)"
    }

关于编译器错误，问题是您在初始化每个存储属性之前在闭包中捕获/引用self。根据意图，您可以使用 捕获列表 直接捕获您感兴趣的值和对象，而无需 self。

let title: String
let action: () -> Void

init() {
    title = "Hello World!"

    // ? `action` has not been initialised when `self` is
    // being captured.
    action = { print(self.title) } 

    // ✅ Capture `title` directly. Now the compiler is happy.
    action = { [title] in print(title) }
}