如何正确使用完成处理程序 [英] How to use completion handler correctly

问题描述

据我所知完成处理程序是如何工作的，但即时通讯的语法有点迷茫。下面是一个函数，给定一个用户名，调用分析查询，找出相应的用户ID。查询结束返回功能之后（所以它返回nil），这就是为什么我们需要完成处理。如何实现呢？

I understand how completion handlers work, but im a bit confused on the syntax. Below is a function that, given a username, calls a parse query to find out the corresponding userId. The query ends after the function is returned (so it returns nil), which is why we need the completion handler. How do i implement it?

func getUserIdFromUsername(username: String) -> String {
    var returnValue = String()
    let query = PFQuery(className: "_User")
    query.whereKey("username", equalTo: username)
    query.findObjectsInBackgroundWithBlock { (objects, error) -> Void in
        if let objects = objects {
            for object in objects {
                returnValue = object.objectId!
            }
        }
    }
    return returnValue

}

请注意：我知道类似这样的例子存在，但他们要么不迅速，或极长。这是一个包含解析一个简短的版本。

NOTE: I know examples similar to this exist, but they are either not swift, or extremely lengthy. This is a short and concise version that contains Parse.

推荐答案

下面是如何实现它：

func getUserIdFromUsername(username: String, completionHandler: String -> Void) {

    let query = PFQuery(className: "_User")
    query.whereKey("username", equalTo: username)
    query.findObjectsInBackgroundWithBlock { (objects, error) -> Void in
        if let objects = objects {
            for object in objects {
                completionHandler(object.objectId!)
            }
        }
    }
}

和这里是如何使用它：

getUserIdFromUsername("myUser") { id in
    doSomethingWithId(id)
}

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