如何拨打多个网络电话，只在成功后发送结果？

Question

我想要进行两个网络调用，并且只想在两个调用都成功完成时将结果发送回viewcontroller。在下面的代码中，我使用了布尔值GotCountrations&；gotocceans以了解我是否得到响应。我想移除这些布尔标志，并确保使用完成块发送结果。

func fetchCountries() {
    countries = Network call to get countries success
    gotCountries = true // Boolean
    if gotCountries && gotOcceans {
        onCompletion?(self.countries, self.occeans)
    }
}

func fetchOcceans() {
    occeans = Network call to get occeans success
    gotOcceans = true // Boolean
    if gotCountries && gotOcceans {
        onCompletion?(self.countries, self.occeans)
    }
}

Answer 1

对于这类情况，最好使用DispatchGroup在多个完成块完成后执行方法。下面是一个例子:

let dispatchGroup = DispatchGroup()

func fetchContriesAndOceans() {
    fetchCountries()
    fetchOcceans()
    dispatchGroup.notify(queue: .main) {
//      execute what needs to be executed after the completion of both methods here.
        onCompletion?(self.countries, self.occeans)
    }
}

func fetchCountries() {
    dispatchGroup.enter()
//    dispatchGroup.leave() // Put this line in the completion of network call
}

func fetchOcceans() {
    dispatchGroup.enter()
//    dispatchGroup.leave() // Put this line in the completion of network call
}