Make iOS blocks execute synchronously

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;
}