calling asynchronous method inside for-loop [duplicate]

问题

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Closed last year.

Trying to make my for loop behave synchronously when I am making an asynchronous call in each iteration of the loop. I have a feeling I will need to use Grand Central Dispatch in some way but not sure.

func test(strings: [String], completion: @escaping ((_ value: [String]) -> Void)) {
    var results: [String] = []
    for string in strings {
        Service.shared.fetch(with: string, completion: { (result) in
            results.append(result)
        })
    }
    // this will run before asynchronous method in for-loop runs n times.
    completion(results)
}

回答1:

You don't need to make this loop synchronous. What you really want is to call completion when you will get all results. You can use this solution (for free):

func test(strings: [String], completion: @escaping ((_ value: [String]) -> Void)) {
    var results: [String] = []  

    for string in strings {
        Service.shared.fetch(with: string, completion: { (result) in
            DispatchQueue.main.async { 
                results.append(result)
                if results.count >= strings.count {
                    completion(results)
                }
            }
        )
    }
}

来源：https://stackoverflow.com/questions/48254073/calling-asynchronous-method-inside-for-loop