Can I reference an instance method when I declare a block in objective-c

Question

I have an instance method that I'd like to invoke directly using the callback param from a block in objective-c. I prefer this approach when I need to do more than a simple 1 liner in the callback.

Here is the basic setup...

I init a class w/ some type of call back so I can parse json after the http request lets say

- (void)initFooAndDoStuff {
    Foo *foo = [[Foo alloc] initWithCallback:^(NSData * response){
        // do stuff
    }];
}

//this is the instance method I'd like to invoke instead of an inline function
- (void)callBackWithHttpResponse:(NSData *)response {
    // do stuff ... assuming it's more complex than a 1 liner that is
}

I can setup a call back that does something inline (but again more complex stuff would be better in a stand alone instance method perhaps)

Foo *foo = [[Foo alloc] initWithCallback:^(NSData * response){ 
    NSLog(@"foo"); 
}];

I can use the instance method like below but it feels a little long winded. Any way I can cut this down (the syntax that is).

Foo *foo = [[Foo alloc] initWithCallback:^(NSData * response){ 
    [self callBackWithHttpResponse:response]; 
}];

I don't understand what you're trying to do. Do you want to declare one block and copy or reuse it again and again instead of having to re-declare it inline each time? — Arlen Anderson
– Arlen Anderson, Commented Jun 16, 2012 at 22:36
Blocks are generally less cumbersome than the old target-and-selector technique that predated them. Your last code snippet is pretty concise and simple, I think. If you want to do the target-and-selector approach, then design Foo to work that way. — Ken Thomases
– Ken Thomases, Commented Jun 16, 2012 at 23:29
@Ken can u show and example of this? Evan would the delegate syntax reduce the noise I'm talking about? — Toran Billups
– Toran Billups, Commented Jun 16, 2012 at 23:32
Foo might define - (id) initWithTarget:(id)target callbackSelector:(SEL)selector. It would save the target and the selector in ivars. It would be called like [[Foo alloc] initWithTarget:self callbackSelector:@selector(callBackWithHttpResponse:)]. To call the callback, Foo would do [_target performSelector:_selector withObject:response]. — Ken Thomases
– Ken Thomases, Commented Jun 16, 2012 at 23:37

Christopher Pickslay · Accepted Answer · 2012-06-18 17:02:48Z

2

If you're going to put the implementation in a method, you might as well just have the method return the entire block:

-(void (^)(NSData *))httpResponseHandler {
    return ^(NSData *responseData){
        // do something with responseData
    };
}

Then the places you use it would be a bit more succinct:

Foo *foo = [[Foo alloc] initWithCallback:[self httpResponseHandler]];

answered Jun 18, 2012 at 17:02

Christopher Pickslay

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4 Comments

Toran Billups Over a year ago

awesome! this is exactly what I was looking for! (funny I ended up on your blog this morning after some issue with ocmock. Keep up the great objective-c / iOS mentoring **we need more developers like yourself in the community)

bbum Over a year ago

I'm not really sure how that is a simplification -- more of a lateral move in terms of complexity -- but it certainly is a perfectly valid (and useful -- I do similar things all the time) pattern. Upvote!

Christopher Pickslay Over a year ago

@bbum I agree, not a simplification per se, depends a lot on how you might re-use the implementation--the callback method embedded in inline block is probably more flexible overall. But I do like to return blocks from methods as a design practice where it's relevant.

Just a coder Over a year ago

I dont know if its valid/accepted way to use this pattern, but i use it to free up delegate methods for cleaner code. Example: If there is much communication between VCa and VCb, i send a dictionary from VCa to VCb containing all the methods VCb needs to call on VCa. So instead of cluttering up the VC-chain with delegate call backs, VCb can just call the methods stored in the dictionary. This keeps "encapsulation" i think? and it reduces multiple delegate methods to only one. Maybe you guys can comment if this is a bad idea?

bbum · Accepted Answer · 2012-06-17 00:09:43Z

1

Foo *foo = [[Foo alloc] initWithCallback:^(NSData * response){ 
    [self callBackWithHttpResponse:response]; 
}];

OK -- pretty darned readable as it is.

Now, go in the other room and hit yourself in the head 3 times with a brick to simulate not having looked at this project in 6 months to a year.

Now, read the code again.

Still readable, isn't it?

There is nothing wrong with a bit of verbosity when that verbosity says exactly what the code is doing. You could go with a target/action pattern as Ken Thomases suggested, but that would require additional complexity elsewhere.

Instead of callback, I would change it to be even more specific. I.e. if the callback was really a completion handler:

Foo *foo = [[Foo alloc] initWithHTTPResponseCompletionHandler:^(NSData * response){ 
    [self httpResponseCompletionHandler:response]; 
}];

answered Jun 17, 2012 at 0:09

bbum

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1 Comment

Toran Billups Over a year ago

I'll agree -the complexity on the other end appears to be > what I'm already doing when I alloc/init the web service class. I was simply curious to see what other options I had as I'm fairly new to objective-c block syntax (thanks for the replies)

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