Using objective-C blocks to write a completion handler

Question

I've had a look at lots of different answers about blocks and I'm still struggling to figure out how to use it.

Here is what I have so far...

@interface myController ()
   typedef void (^CompletionBlock)();
@end

Then I have declared a method like this:

-(void)reloadDataWithCompletions:(CompletionBlock)completionBlock{
    [self.tableView reloadData];
    completionBlock();
}

What I don't know how to do is how to write the code in the completionBlock. Do I write another method called completionBlock ? Like this

-(void) completionBlock{
    // do something here once the first method is finished?
 }

rmaddy · Accepted Answer · 2015-05-29T00:31:32.767

2

You need to call the reloadDataWithCompletions: method passing in the block you want executed. Something like this:

[self reloadDataWithCompletions:^{
    // The code you want executed in the block
}];

BTW - there is no need for a block in this case. A completion block is only really needed when dealing with asynchronous calls made on other threads. Since your reloadDataWithCompletions: method only calls reloadData on a table view, and since that method is synchronous, the use of a block is pointless.

You would get the same result in this case by doing:

-(void)reloadData {
    [self.tableView reloadData];
}

and calling it like:

[self reloadData];
// code you want executed after the reload is done

edited May 29 '15 at 00:31

answered May 29 '15 at 00:28

rmaddy

314,917
42
532
579

I'm asking specifically about how to specify what happens in the completion block, not how to call the method. – Linda Keating May 29 '15 at 00:30
I showed you. Replace the comment with the actual code you want to happen in the completion block. That code is up to you and what you want to happen after the table is reloaded. – rmaddy May 29 '15 at 00:32
Ah thanks. I get it now. – Linda Keating May 29 '15 at 00:33

Using objective-C blocks to write a completion handler

1 Answers1