function return value from block

Question

What if I have a function that returns an int and the return value of the int is taken from the block?

For example:

- (int) queryForKey:(NSString *)aKey view:(UIButton *)aView countView:(UIView *)aCountView counter:(int) count {
    //some initialization
    [query countObjectsInBackgroundWithBlock:^(int number, NSError * error) {
        [aCountView addSubview:self.generateCountLabel];

        if (number > 0){
            [aView setUserInteractionEnabled:YES];
            [aView setEnabled:YES];
        }
        //return number; //doing this will generate an error
    }];
}

also another question is, what if I have an int passed in as an argument of the function above and I would like to assign a value to it. Is some thing like that even possible?

simonpie · Accepted Answer · 2011-11-04 14:53:20Z

1

Well your block does not have a return value, it returns void.

To return a value you could use the __block modifier on a variable outside your block and store then answer there which can then be used by the rest of your method (or code).

answered Nov 4, 2011 at 14:53

simonpie

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5 Comments

xonegirlz Over a year ago

I can have a __block int result outside the block and have it return result at the end of the method, but this won't guarantee that result will have the right value

simonpie Over a year ago

Would you care answering that. Is that an asynchronous thing ? If it is, then just use GCD at the end of your block to queue a block that notifies whatever object that should know the answer.

xonegirlz Over a year ago

Yes.. this is an asynchronous thing, can you give me an example of the GCD you just mentioned

simonpie Over a year ago

at the end of your block, use the function dispatch_async(dispatch_get_main_queue(),^(){[myobject_object_that_needs_to_know setNewCount:newcount] }) This will queue a block that only contains one method on the main queue. In this setNewCount method, you can then update your UI if needed.

xonegirlz Over a year ago

all I want to do is basically to set an int property that I have with the number that is got from the callback

bbum · Accepted Answer · 2011-11-04 15:15:17Z

1

The problem is that you have a synchronous method (one that wants to return the value immediately) that needs to return a value derived from an asynchronous method (one that goes about it's business in a different thread).

There are a couple of ways of fixing this:

wait for the countObjectsInBackgroundWithBlock: method to complete, use the __block pattern as @simonpie described.
replace the return number; with a call to something somewhere interested in the resulting number. This also means that queryForKey:view:countView: will likely return void.

The latter is the preferred solution as it will not block the thread calling the queryForKey:... method.

Note that you can't diddle UIKit classes on anything but the main thread. If that block is executed on a background thread, then doing what you are doing in the block is invalid.

answered Nov 4, 2011 at 15:15

bbum

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1 Comment

xonegirlz Over a year ago

Yea.. I am aware of this solution, it just seems to me that this is taking multiple steps to do, but if there's no other alternative solution then be it

Joel Fischer · Accepted Answer · 2012-11-29 15:37:40Z

I've found a better solution. Hopefully this will help someone else who stumbles across the question. I would implement a solution to your code like so:

- (int) queryForKey:(NSString *)aKey view:(UIButton *)aView countView:(UIView *)aCountView counter:(int) count {
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    __block int number;

    //some initialization
    [query countObjectsInBackgroundWithBlock:^(int number, NSError * error) {
        dispatch_async(queue, ^{
            [aCountView addSubview:self.generateCountLabel];

            if (number > 0){
                [aView setUserInteractionEnabled:YES];
                [aView setEnabled:YES];
            }
            dispatch_semaphore_signal(sema);
        });
    }];
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
    return number; //doing this will no longer generate an error
}

Then encapsulate your call with another dispatch_async so that your semaphore wait call doesn't block the main thread.

dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_async(queue, ^{
    [self queryForKey:@"AKey" view:myView countView:myCountView counter:aCounter];
});

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