Declare a block method parameter without using a typedef

Question

Is it possible to specify a method block parameter in Objective-C without using a typedef? It must be, like function pointers, but I can't hit on the winning syntax without using an intermediate typedef:

typedef BOOL (^PredicateBlock_t)(int);
- (void) myMethodTakingPredicate:(PredicateBlock_t)predicate

only the above compiles, all these fail:

-  (void) myMethodTakingPredicate:( BOOL(^block)(int) ) predicate
-  (void) myMethodTakingPredicate:BOOL (^predicate)(int)

and I can't remember what other combinations I've tried.

Solution 1

- ( void )myMethodTakingPredicate: ( BOOL ( ^ )( int ) )predicate

Solution 2

This is how it goes, for example...

[self smartBlocks:@"Pen" youSmart:^(NSString *response) {
        NSLog(@"Response:%@", response);
    }];


- (void)smartBlocks:(NSString *)yo youSmart:(void (^) (NSString *response))handler {
    if ([yo compare:@"Pen"] == NSOrderedSame) {
        handler(@"Ink");
    }
    if ([yo compare:@"Pencil"] == NSOrderedSame) {
        handler(@"led");
    }
}

Solution 3

http://fuckingblocksyntax.com

As a method parameter:

- (void)someMethodThatTakesABlock:(returnType (^)(parameterTypes))blockName;

Solution 4

Another example (this issue benefits from multiple):

@implementation CallbackAsyncClass {
void (^_loginCallback) (NSDictionary *response);
}
// …


- (void)loginWithCallback:(void (^) (NSDictionary *response))handler {
    // Do something async / call URL
    _loginCallback = Block_copy(handler);
    // response will come to the following method (how is left to the reader) …
}

- (void)parseLoginResponse {
    // Receive and parse response, then make callback

   _loginCallback(response);
   Block_release(_loginCallback);
   _loginCallback = nil;
}


// this is how we make the call:
[instanceOfCallbackAsyncClass loginWithCallback:^(NSDictionary *response) {
   // respond to result
}];

Solution 5

Even more clear !

[self sumOfX:5 withY:6 willGiveYou:^(NSInteger sum) {
    NSLog(@"Sum would be %d", sum);
}];

- (void) sumOfX:(NSInteger)x withY:(NSInteger)y willGiveYou:(void (^) (NSInteger sum)) handler {
    handler((x + y));
}

View more solutions

Comments

[Kyle Clegg]

goshdarnblocksyntax.com

[Fred Foo]

+1, though a typedef should really be preferred for more complicated cases.

[Mohammad Abdurraafay]

- ( void )myMethodTakingPredicate: ( BOOL ( ^ )( NSString *name, NSString *age ) )predicate { //How Should I Access name & age here...? }

[Macmade]

Those are just parameter names. Just use them.

[mtmurdock]

@larsmans I agree, unless this particular predicate/block is used in a lot of places where it would be more clear to have it typedef'd. Apple has defined a number of blocks that were quite simple, but did so such that it was easy to find what they wanted in documentation.

[user4951]

and how to pass a block to that function.

[orkoden]

Is there a reason you don't use the [NSString isEqualToString:] method?

[Mohammad Abdurraafay]

Nothing specific. I'm just use to use 'compare:' a lot. '[NSString isEqualToString:]' is a better way though.

[devios1]

Ugh. There is nothing intuitive about this syntax at all. :(

[Macmade]

@chaiguy It is if you know how to deal with function pointers. It's the same syntax, with ^ instead of *

[devios1]

@Macmade That's the syntax I'm complaining about. Not criticizing your answer mind you; already +1'd.

[funroll]

Strong recommendation! Name your variables. They will autocomplete into usable code. So replace BOOL ( ^ )( int ) with BOOL ( ^ )( int count ).

[Ash]

Do you need the word response in the smartBlocks method definition? Couldn't you just say (NSString*))handler {?

[Mohammad Abdurraafay]

You may have (NSString *)) handler. That's valid as well.