swift - 如何在 Swift 中等待函数调用结束

Question

我的问题很简单，但经过多次研究和测试，我在继续流程之前未能成功等待函数结束。

示例:

print("Before stuff")
do_stuff {
    print("After stuff")
}


func do_stuff(onCompleted: () -> ()) {
    let postEndpoint: String = "http://localhost:8080/users/1234567890987654"
    guard let url = NSURL(string: postEndpoint) else {
        print("Error: cannot create URL")
        return
    }
    let urlRequest = NSURLRequest(URL: url)
    let config = NSURLSessionConfiguration.defaultSessionConfiguration()
    let session = NSURLSession(configuration: config)
    let task = session.dataTaskWithRequest(urlRequest, completionHandler: {
    (data, response, error) in
    guard let responseData = data else {
        print("Error: did not receive data")
        return
    }
    guard error == nil else {
        print("error calling GET on /users/1234567890987654")
        print(error)
        return
    }
    // parse the result as JSON
    let user: NSDictionary
    do {
        user = try NSJSONSerialization.JSONObjectWithData(responseData,
            options: []) as! NSDictionary
    } catch  {
        print("error trying to convert data to JSON")
        // Means that user does not exist
        return
    }
    print("The user is: " + user.description)
    })
    task.resume()
    onCompleted()
}

如何在第二次打印之前等待 do_stuff() 结束？

感谢您的帮助，我想我错过了一些东西。

托马斯

Answer 1

有一些基本的东西你还没有理解。它实际上并不是正在执行的函数。它是函数内部定义的闭包。您需要等待的是请求完成后调用的闭包。事实上，您不应该等待，而是在函数外部分配另一个闭包，以便从函数内部的闭包调用。

print("Before stuff")
do_stuff {
    // Now the "function" has completed.
    print("After stuff")
}

func do_stuff(onCompleted: () -> ()) {
    let task = session.dataTaskWithRequest(urlRequest) { data, response, error in
        ...
        onCompleted()
    }
}