Objective C - Firebase - How to add completion handler to FDataSnapshot

Question

I'm experimenting with Firebase's FDataSnapshot to pull in data and I would like it to write its data to my core data using MagicalRecord.

According to Firebases "best practice" blog I need to keep a reference to the "handle" so it can be cleaned up later on. Further, they mention to put the FDSnapshot code in viewWillAppear.

I am wanting a callback so that when its finished doing its thing to update core data.

But I'm really note sure how to do that; its doing two things and giving a return at the same time.

// In viewWillAppear:

__block NSManagedObjectContext *context = [NSManagedObjectContext MR_context];

    self.handle = [self.ref observeEventType:FEventTypeValue withBlock:^(FDataSnapshot *snapshot) {
        if (snapshot.value == [NSNull null])
        {
            NSLog(@"Cannot find any data");
        }
        else
        {
            NSArray *snapshotArray = [snapshot value];

// cleanup to prevent duplicates
               [FCFighter MR_truncateAllInContext:context];

            for (NSDictionary *dict in snapshotArray)
            {

                FCFighter *fighter = [FCFighter insertInManagedObjectContext:context];
                fighter.name = dict[@"name"];

                [context MR_saveToPersistentStoreWithCompletion:^(BOOL contextDidSave, NSError *error){
                    if (error)
                    {
                        NSLog(@"Error - %@", error.localizedDescription);
                    }
                }];
            }
        }
    }];

    NSFetchRequest *fr = [[NSFetchRequest alloc] initWithEntityName:[FCFighter entityName]];
    fr.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES]];
    self.fighterList = (NSArray *) [context executeFetchRequest:fr error:nil];
    [self.tableView reloadData];

In the above code, the core data reading does not wait for the firebase to complete.

Thus, my query -- how would I best combine a completion handler so that when it is complete to update core data, and reload the tableview.

Many thanks

Answer 1

This is a common issue when working with Asynchronous data.

The bottom line is that all processing of data returned from an async call (in this case, the snapshot) needs to be done inside the block.

Anything done outside the block may happen before the data is returned.

So some sudo code

observeEvent  withBlock { snapshot
     //it is here where snapshot is valid. Process it.
     NSLog(@"%@", snapshot.value)
}

Oh, and a side note. You really only need to track the handle reference when you are going to do something else with it later. Other than that, you can ignore the handles.

So this is perfectly valid:

[self.ref observeEventType:FEventTypeValue withBlock:^(FDataSnapshot *snapshot) {
   //load your array of tableView data from snapshot
   //  and/or store it in CoreData
   //reload your tableview
}