Block completion handler reference is nil after completing background fetch - ios

Question

I am trying to implement a background fetch of an RSS Feed using performFetchWithCompletionHandler, but when I want to call the completion handler it's nil.
Am I missing a way to retain my reference to self.completionHandler?
Am I declaring self.completionHandler correctly?
in app delegate:
//background fetch new RSS Feeds
-(void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
UIStoryboard *mainStoryboard = [UIStoryboard storyboardWithName:#"Main" bundle:nil];
MasterViewController *navigationController = [mainStoryboard instantiateViewControllerWithIdentifier:#"MasterView"];
MasterViewController *viewController = navigationController;
[viewController startParsingWithCompletionHandler2: ^ (UIBackgroundFetchResult completionHandler2){
completionHandler (UIBackgroundFetchResultNewData);
}];
}
in main view controller:
#property (nonatomic, strong) void (^completionHandler)(UIBackgroundFetchResult);
- (void) startParsingWithCompletionHandler2:(void (^)(UIBackgroundFetchResult))completionHandler2
{
self.completionHandler = completionHandler2;
if (self.completionHandler) {
NSLog(#"completionHandler");
}else{
NSLog(#"not completionHandler");
}
[self performSelector: #selector(stopParsing) withObject: nil afterDelay: PARSER_TIME_LIMIT];
[self.activityIndicator startAnimating];
numberOfCompletedStories = 0;
[self.parserArray removeAllObjects];
//check for RSS Site data updates
for (int lCounter = 0; lCounter < self.rssFeedAddresses.count; lCounter ++) {
RSSParser *parser = [[RSSParser alloc] init];
[parser setDelegate: self];
[self.parserArray addObject: parser];
[parser setSiteTitle: [self.rssFeedNames objectAtIndex: lCounter]];
[NSThread detachNewThreadSelector: #selector(begin:) toTarget: parser withObject: [self.rssFeedAddresses objectAtIndex: lCounter]];
}
if (self.completionHandler) {
NSLog(#"#2 completionHandler");
}else{
NSLog(#"#2 not completionHandler");
}
}
- (void) storyIsDone//called when parser completed one rss feed
{
numberOfCompletedStories ++;
if (self.completionHandler) {
NSLog(#"storyIsDone YES completion handler %i", numberOfCompletedStories);
}else{
NSLog(#"storyIsDone Not completion handler");
}
if (numberOfCompletedStories == self.rssFeedAddresses.count)
{
//if all the feeds are done cancel time-out timer
[NSObject cancelPreviousPerformRequestsWithTarget: self selector: #selector(stopParsing) object: nil];
[self.activityIndicator stopAnimating];
[self.refreshControl endRefreshing];
[[NSNotificationCenter defaultCenter] addObserver: self selector: #selector(reloadRSSfeeds) name: #"ReloadFeeds" object: nil];
canRefresh = YES;
NSLog(#"call back");
[self performSelectorOnMainThread: #selector(callCompletion) withObject: self waitUntilDone: YES];
}//else not yet complete
}
- (void) callCompletion
{
if (self.completionHandler) {
NSLog(#"callCompletion YES completion handler");
self.completionHandler (UIBackgroundFetchResultNewData);
}else{
NSLog(#"callCompletion Not completion handler");
}
}
The output is:
completionHandler
#2 completionHandler
storyIsDone Not completion handler
storyIsDone Not completion handler
storyIsDone Not completion handler
storyIsDone Not completion handler
storyIsDone Not completion handler
storyIsDone Not completion handler
storyIsDone Not completion handler
call back
callCompletion Not completion handler

Answer 1

I always use (nonatomic, copy) for block properties. You might try that and see if it helps.
In your code, though, I think you can just pass through the completion handler that's passed into application:performFetchWithCompletionHandler:.

Answer 2

In the .h file define a completion handler like this:
typedef void (^CompletionHandler)(UIBackgroundFetchResult BFR);
Also define your completion handler property like this:
#property (copy) CompletionHandler completionHandler;
Then in the .m file set it like this:
self.completionHandler = handler;
You need the (copy) keyword here so that the block which is passed through via the startParsingWithCompletionHandler2: method is copied and that copy is then retained by the main view controller.
From the docs:
Note: You should specify copy as the property attribute, because a block needs to be copied to keep track of its captured state outside of the original scope. This isn’t something you need to worry about when using Automatic Reference Counting, as it will happen automatically, but it’s best practice for the property attribute to show the resultant behavior.
See this link
EDIT:
I thought it would be good to add that you MUSTN'T call a completion handler (i.e. completionHandler(/*someArgs*/)) when the completionHandler is nil. If you do it will cause your App to crash.
To overcome this you can put in a simple check e.g.
!completionHandler ?: completionHandler(/*someArgs*/);
The above code is semantically equivalent to:
if (completionHandler != nil) {
completionHandler(/*someArgs*/)
}
it just condenses it onto one line using the ternary operator.

Answer 3

Class not properly initialized
When using performFetchWithCompletionHandler for background fetch - methods are called in a different order resulting in some objects not being properly initialized.
When the app is launched in the foreground these methods are called: (in order)
initWithCoder
awakeFromNib
viewDidLoad
dispatchLoadingOperation
viewDidAppear
When performing a background fetch methods are called in the following order:
initWithCoder
awakeFromNib
startParsingWithCompletionHandler2
viewDidLoad
dispatchLoadingOperation
viewDidAppear
Of particular note: vieDidLoad was called before dispatchLoadingOperation which kicked off the parsing when running in foreground.
When running in background, startParsingWithCompletionHandler2 (which also kicks off parsing when running in background) was called before viewDidLoad.
Since several objects were initialized in viewDidLoad, the parsing was begun before expected and my array was not initialized to store my parsing results. This appeared to me that the app was not launching.
While I was looking at the call back for the completion handler being nil, the real issue was the Class not being setup properly.