Swift 合并发布两次答案

【问题标题】：Swift Combine Publish TwiceSwift 合并发布两次
【发布时间】：2020-09-01 22:50:01
【问题描述】：

class ObservableFormViewModel: ObservableObject {
  @Published var isSubmitAllowed: Bool = true
  @Published var username: String = ""
  @Published var password: String = ""
  var somethingElse: Int = 10
}

var form = ObservableFormViewModel()

let formSubscription = form.$isSubmitAllowed.sink { _ in
    print("Form changed: \(form.isSubmitAllowed) \"\(form.username)\" \"\(form.password)\"")
}


form.isSubmitAllowed = false
form.isSubmitAllowed = false
form.isSubmitAllowed = false

输出是：

Form changed: true "" ""
Form changed: true "" ""
Form changed: false "" ""
Form changed: false "" ""

我的问题是：

为什么true 输出为 2 而false 只有 2？
有更好的方法来删除重复项吗？

【问题讨论】：

标签： swift swiftui combine

【解决方案1】：

为什么真正的输出是 2 而假的只有 2？

第一个输出在您创建 formSubscription 时运行。接下来的三个是由您连续的form.isSubmitAllowed = false 语句触发的。

请注意，您将form.isSubmitAllowed 三次更改为false，并且在输出中它只出现两次：

form.isSubmitAllowed = false
form.isSubmitAllowed = false
form.isSubmitAllowed = false

// Form changed: true "" ""
Form changed: true "" ""
Form changed: false "" ""
Form changed: false "" ""

这是因为您打印的不是更改后的值，而是旧值。

试试这个：

let formSubscription = form.$isSubmitAllowed.sink { isSubmitAllowed in
    print("Form changed: \(isSubmitAllowed) \"\(form.username)\" \"\(form.password)\"")
}

打印出来：

// Form changed: true "" ""
Form changed: false "" ""
Form changed: false "" ""
Form changed: false "" ""

如果您想删除重复项，只需使用 removeDuplicates:

let formSubscription = form.$isSubmitAllowed.removeDuplicates().sink { value in
    print("Form changed: \(value) \"\(form.username)\" \"\(form.password)\"")
}

form.isSubmitAllowed = false
form.isSubmitAllowed = false
form.isSubmitAllowed = false

打印出来：

Form changed: true "" ""
Form changed: false "" ""

【讨论】：