How to execute two asynchronous functions sequentially

问题

func getTopicIdFromMYSQL(){
    let myUrl = NSURL(string: "xxxx")
    let request = NSMutableURLRequest(URL: myUrl!)
    request.HTTPMethod = "POST"
    let email:String = "xxx@gmail.com"
    let postString = "email=\(email)"
    request.HTTPBody = postString.dataUsingEncoding(NSUTF8StringEncoding)
    let task = NSURLSession.sharedSession().dataTaskWithRequest(request){
        data, response, error in
        if(error != nil){

            print("Get all topic")
            print("error=\(error)")
            return
        }
        do {
            let json = try NSJSONSerialization.JSONObjectWithData(data!, options: .MutableContainers) as? NSDictionary

            if let parseJSON = json
            {
                let resultValue = parseJSON["status"] as? String

                print("Get all topic")
                favouriteTopic = parseJSON["getResult"]! as! [AnyObject]
                print("return topic:\(favouriteTopic)")

                dispatch_async(dispatch_get_main_queue(), {

                    if(resultValue == "Success"){

                    }
                    else{
                        let error = UIAlertController(title: "Error", message: "Please check your network configuration!:-(", preferredStyle: .Alert)
                        let cancel = UIAlertAction(title: "Cancel", style: .Cancel, handler: nil)
                        let ok = UIAlertAction(title: "OK", style: .Default, handler: nil)
                        error.addAction(cancel)
                        error.addAction(ok)
                    }
                })

            }

        }catch
        {
            print(error)
        }

    }
    task.resume()

} I have already got this code, how ever, I want to run another function after it. What should I do. like:

getTopicIdFromMYSQL()
getCommentFromMYSQL()
print("Finish")

I find the problem is my code does not excute in order, the function getCommentFromMYSQL is almost the same as getTopicIdFromMYSQL, I want to run these three in order what should I do?

回答1:

Add a completion handler to your asynchronous functions as a parameter:

func getTopicIdFromMYSQL(completion: (AnyObject?, ErrorType?)->())

func getCommentFromMYSQL(completion: (AnyObject?, ErrorType?)->())

Note: The completion handler must be eventually called when the asynchronous function completes - either with an error or the computed value.

You then call these functions as shown below:

getTopicIdFromMYSQL() { (result1, error) in
    if let result1 = result1 {
        // process result1
        // ...
        getCommentFromMYSQL() { (result2, error) in
            if let result2 = result2 {
                // process result2
                // ...
            } else {
                // handle error
            }
        }
    } else {
        // handle error
    }
}

You may implement these functions as follows:

func getTopicIdFromMYSQL(completion: (AnyObject?, ErrorType?) {
    let myUrl = NSURL(string: "xxxx")
    let request = NSMutableURLRequest(URL: myUrl!)
    request.HTTPMethod = "POST"
    let email:String = "xxx@gmail.com"
    let postString = "email=\(email)"
    request.HTTPBody = postString.dataUsingEncoding(NSUTF8StringEncoding)
    let task = NSURLSession.sharedSession().dataTaskWithRequest(request){
        data, response, error in
        if error != nil {
            completion(nil, error)
        }
        do {
            let json = try NSJSONSerialization.JSONObjectWithData(data!, options: .MutableContainers) as? NSDictionary
            if let parseJSON = json {
                let resultValue = parseJSON["status"] as? String
                print("Get all topic")
                favouriteTopic = parseJSON["getResult"]! as!  [AnyObject]
                completion(favouriteTopic, nil)
            } else {
                throw MyError.Error(message: "bogus JSON")
            }    
        } catch let error { 
            completion(nil, error)
        }
    }
    task.resume()
}

回答2:

The code is always executed in order. I think your problem is that you are making an asynchronous call in this line:

NSURLSession.sharedSession().dataTaskWithRequest(request)

But, as the code is executed sequentially, the method getCommentFromMYSQL() is called before the asynchronous call finishes. You should call getCommentFromMYSQL() inside this conditional:

if (resultValue == "Success") {
}

Or whenever you want. That will ensure the method getCommentFromMYSQL() to be called after the first method finishes its execution.

回答3:

This is because, you are making async calls, which run in background thread.

If you want them to be executed in order, call the next method after dispatching to main queue,

dispatch_async(dispatch_get_main_queue(), {
                //call here
                getCommentFromMYSQL()
                if(resultValue == "Success"){

                }
                else{
                    let error = UIAlertController(title: "Error", message: "Please check your network configuration!:-(", preferredStyle: .Alert)
                    let cancel = UIAlertAction(title: "Cancel", style: .Cancel, handler: nil)
                    let ok = UIAlertAction(title: "OK", style: .Default, handler: nil)
                    error.addAction(cancel)
                    error.addAction(ok)
                }
            })

来源：https://stackoverflow.com/questions/35551771/how-to-execute-two-asynchronous-functions-sequentially