Make iOS blocks execute synchronously

Question

How can I make a block execute synchronously, or make the function wait for the handler before the return statement, so the data can be passed back from the block?

-(id)performRequest:(id)args
{
__block NSData *data = nil;   

    [xyzclass requestAccessToAccountsWithType:accountType withCompletionHandler:^(BOOL granted, NSError *error) {
        data = [NSData dataWithData:responseData];
    }];

    return data;
}

Answer 1

You can use semaphores in this case.

-(id)performRequest:(id)args
{
    __block NSData *data = nil;   
     dispatch_semaphore_t sem = dispatch_semaphore_create(0);
     [xyzclass requestAccessToAccountsWithType:accountType withCompletionHandler:^(BOOL granted, NSError *error) {
       data = [NSData dataWithData:responseData];
       dispatch_semaphore_signal(sem);
     }];
    dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
    return data;
}

semaphore will block execution of further statements until signal is received, this will make sure that your function does not return prematurely.

Answer 2

async is almost always better. but if you want synchronous:

-(id)performRequest:(id)args
{
__block NSData *data = nil;   

    [xyzclass requestAccessToAccountsWithType:accountType withCompletionHandler:^(BOOL granted, NSError *error) {
        data = [NSData dataWithData:responseData];
    }];

    while(!data) {
        [[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.1]];
    }
    return data;
}

---

Disclaimer: CocoaCouchDeveloper says that of course this will only work if the completion block and the runloop are on the same thread. I assumed that because many(most) COMPLETION handler I know work that way but of it is valid in principle.

The above is not thread safe
use a semaphore or something maybe.
also I said I don't promote this

Answer 3

You could just add a method which processes the returned data and call that in your block:

-(void)performRequest:(id)args{
    __block NSData *data = nil;   

    [xyzclass requestAccessToAccountsWithType:accountType withCompletionHandler:^(BOOL granted, NSError *error) {
        data = [NSData dataWithData:responseData];
        [self processData:data]; //method does something with the data 
    }];
}