ios - 函数在完成处理程序完成之前完成它的执行

Question

我想返回从 HelperConnection.shared.getInfo() 计算的结果。但我总是在结果变量中得到 nil(也在 group.wait 之后)。如何在 HelperConnection.shared.getInfo() 完成之前停止当前函数的执行？

  func install(lists:[String]) {
    let group = DispatchGroup()
    var result : String!

    if lists.count != 0 {
        for i in lists {
            group.enter()
            HelperConnection.shared.getInfo(str: i, completion: { (str) in
                if str == "1" {
                    DispatchQueue.main.async {
                        result = str
                        group.leave()
                    }
                } else {
                    DispatchQueue.main.async {
                        result = "Hello"
                        group.leave()
                    }
                }
            })
        }
    }
    print(result)
    print(".........")
    group.wait(timeout: .now() + 5)
    print(result)
    return result
   }

Answer 1

使用notify 而不是wait 并添加完成处理程序。强制异步任务同步是不好的做法。

func install(lists:[String], completion: @escaping: (String)->Void) {
    let group = DispatchGroup()
    var result = ""

    if lists.isEmpty { (completion("")) }
    for i in lists {
        group.enter()
        HelperConnection.shared.getInfo(str: i, completion: { str in
            defer(group.leave())
            if str == "1" {
               result = str
            } else {
               result = "Hello"
            }
        })
    }       

    group.notify(queue: DispatchQueue.main) {
        print(".........")
        print(result)
        completion(result)
   }
}