您无需使该循环同步.您真正想要的是在获得所有results时调用completion.您可以免费使用此解决方案:

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)
                }
            }
        )
    }
}