Wait for Firebase to load before returning from a function

Question

I have a simple function loading data from Firebase.

func loadFromFireBase() -> Array<Song>? {
    var songArray:Array<Song> = []

    ref.observe(.value, with: { snapshot in
        //Load songArray
    })

    if songArray.isEmpty {
        return nil
    }
    return songArray
}

Currently this function returns nil always, even though there is data to load. It does this because it doesn't ever get to the perform the completion block where it loads the array before the function returns. I'm looking for a way to make the function only return once the completion block has been called but I can't put return in the completion block.

Coldiron

Answer 1

(Variations on this question come up constantly on SO. I can never find a good, comprehensive answer, so below is an attempt to provide such an answer)

You can't do that. Firebase is asynchronous. Its functions take a completion handler and return immediately. You need to rewrite your loadFromFirebase function to take a completion handler.

I have a sample project on Github called Async_demo (link) that is a working (Swift 3) app illustrating this technique.

The key part of that is the function downloadFileAtURL, which takes a completion handler and does an async download:

typealias DataClosure = (Data?, Error?) -> Void

/**
 This class is a trivial example of a class that handles async processing. It offers a single function, `downloadFileAtURL()`
 */
class DownloadManager: NSObject {

  static var downloadManager = DownloadManager()

  private lazy var session: URLSession = {
    return URLSession.shared
  }()

    /**
     This function demonstrates handling an async task.
     - Parameter url The url to download
     - Parameter completion: A completion handler to execute once the download is finished
     */

      func downloadFileAtURL(_ url: URL, completion: @escaping DataClosure) {

        //We create a URLRequest that does not allow caching so you can see the download take place
        let request = URLRequest(url: url,
                                 cachePolicy: .reloadIgnoringLocalAndRemoteCacheData,
                                 timeoutInterval: 30.0)
        let dataTask = URLSession.shared.dataTask(with: request) {
          //------------------------------------------
          //This is the completion handler, which runs LATER,
          //after downloadFileAtURL has returned.
          data, response, error in

          //Perform the completion handler on the main thread
          DispatchQueue.main.async() {
            //Call the copmletion handler that was passed to us
            completion(data, error)
          }
          //------------------------------------------
        }
        dataTask.resume()

        //When we get here the data task will NOT have completed yet!
      }
    }

The code above uses Apple's URLSession class to download data from a remote server asynchronously. When you create a dataTask, you pass in a completion handler that gets invoked when the data task has completed (or failed.) Beware, though: Your completion handler gets invoked on a background thread.

That's good, because if you need to do time-consuming processing like parsing large JSON or XML structures, you can do it in the completion handler without causing your app's UI to freeze. However, as a result you can't do UI calls in the data task completion handler without sending those UI calls to the main thread. The code above invokes the entire completion handler on the main thread, using a call to DispatchQueue.main.async() {}.

Back to the OP's code:

I find that a function with a closure as a parameter is hard to read, so I usually define the closure as a typealias.

Reworking the code from @Raghav7890's answer to use a typealias:

typealias SongArrayClosure = (Array<Song>?) -> Void

func loadFromFireBase(completionHandler: @escaping SongArrayClosure) {
    ref.observe(.value, with: { snapshot in
        var songArray:Array<Song> = []
        //Put code here to load songArray from the FireBase returned data

        if songArray.isEmpty {
            completionHandler(nil)
        }else {
            completionHandler(songArray)
        }
    })
}

I haven't used Firebase in a long time (and then only modified somebody else's Firebase project), so I don't remember if it invokes it's completion handlers on the main thread or on a background thread. If it invokes completion handlers on a background thread then you may want to wrap the call to your completion handler in a GCD call to the main thread.

---

Edit:

Based on the answers to this SO question, it sounds like Firebase does it's networking calls on a background thread but invokes it's listeners on the main thread.

In that case you can ignore the code below for Firebase, but for those reading this thread for help with other sorts of async code, here's how you would rewrite the code to invoke the completion handler on the main thread:

typealias SongArrayClosure = (Array<Song>?) -> Void

func loadFromFireBase(completionHandler:@escaping SongArrayClosure) {
    ref.observe(.value, with: { snapshot in
        var songArray:Array<Song> = []
        //Put code here to load songArray from the FireBase returned data

        //Pass songArray to the completion handler on the main thread.
        DispatchQueue.main.async() {
          if songArray.isEmpty {
            completionHandler(nil)
          }else {
            completionHandler(songArray)
          }
        }
    })
}

Erlindaerline

could you please explain the reason why you need to call Dispatch.main.async in this scenario data, response, error in //Perform the completion handler on the main thread DispatchQueue.main.async() { ? The code that is run after in keyword, already runs on the main thread. Why do we have to call DispatchQueue.main.async() ? – Snowfall

The short answer is that you don't have to use DispatchQueue.main.async() for this Firebase call because Firebase calls it's completion handlers on the main thread. (See the comment in my edit.) I left the code that way, though, because for other types of async calls (Like Apples URLSession completion handlers) you do have to wrap UIKit calls in a call to DispatchQueue.main.async(). – Erlindaerline

Answer 2

Making Duncan answer more precise. You can make the function like this

func loadFromFireBase(completionHandler:@escaping (_ songArray: [Song]?)->()) {
    ref.observe(.value) { snapshot in
        var songArray: [Song] = []
        //Load songArray
        if songArray.isEmpty {
            completionHandler(nil)
        }else {
            completionHandler(songArray)
        }
    }
}

You can return the songArray in a completion handler block.

Deeplaid

Thanks Raghav. I wasn't at my Mac when I first wrote that answer, and was getting ready to provide a more complete answer but you beat me to it. – Erlindaerline

It looks to me like you have an extra level of parentheses around your completionHandler in your function definition though. That makes it harder to read, IMO. – Erlindaerline

@DuncanC since Firebase invokes its listeners on the Main thread, I assume there is no reason to call completionHandler on the main thread. – Snowfall