Wait For Asynchronous Operation To Complete in Swift

Question

I am not sure how to handle this situation as I am very new to iOS development and Swift. I am performing data fetching like so:

func application(application: UIApplication!, performFetchWithCompletionHandler completionHandler: ((UIBackgroundFetchResult) -> Void)!) {     loadShows()     completionHandler(UIBackgroundFetchResult.NewData)     println("Background Fetch Complete") } 

My loadShows() function parses a bunch of data it gets from a website loaded into a UIWebView. The problem is that I have a timer that waits for 10 seconds or so in the loadShows function. This allows for the javascript in the page to fully load before I start parsing the data. My problem is that the completion handler completes before my loadShows() does.

What I would like to do is add a bool for "isCompletedParsingShows" and make the completionHandler line wait to complete until that bool is true. What is the best way to handle this?

Answer 1

you have to pass your async function the handler to call later on:

func application(application: UIApplication!, performFetchWithCompletionHandler completionHandler: ((UIBackgroundFetchResult) -> Void)!) {     loadShows(completionHandler) }  func loadShows(completionHandler: ((UIBackgroundFetchResult) -> Void)!) {     //....     //DO IT     //....      completionHandler(UIBackgroundFetchResult.NewData)     println("Background Fetch Complete") } 

OR (cleaner way IMHO)

add an intermediate completionHandler

func application(application: UIApplication!, performFetchWithCompletionHandler completionHandler: ((UIBackgroundFetchResult) -> Void)!) {     loadShows() {         completionHandler(UIBackgroundFetchResult.NewData)         println("Background Fetch Complete")     } }  func loadShows(completionHandler: (() -> Void)!) {     //....     //DO IT     //....     completionHandler() } 

Answer 2

two ways to solve this, both use Grand Central Dispatch (which is similar in Swift and Objective C):

1. change loadShows method to make it synchronous and use the same dispatch queue as completionHandler, then wrap the entire body of the method in a dispatch_async ; this way the method call ends right away, but the completionHandler will be called after loadShows if finished, just like in a synchronous program

2. use a GCD semaphore - just like the BOOL you mention, but created with dispatch_semaphore_create ; you call dispatch_semaphore_wait before completionHandler to make it wait for the semaphore to be unlocked (unlock it with dispatch_semaphore_signal ) ; remember to place your method body inside a dispatch_async call in order not to have it block the rest of the app while waiting for loadShows to complete.