How to pass a block as an argument into another block in Objective C

Question

I'm trying to define a block that takes a block as an argument.

What's wrong with the following line of code?

id (^cacheResult)(NSString *, id(^)(void)) = ^(NSString *name, id(^)(void)block) { NSObject *item = nil; block(); return item; };

Why does the compiler keep giving errors like Parameter name omitted and Expected ")"?

Max · Accepted Answer · 2011-12-30 18:26:37Z

id (^cacheResult)(NSString *, id(^)(void)) = ^(NSString *name, id(^block)(void)) { NSObject *item = nil; block(); return item; };

Blocks have similar syntax to function pointers. You have to declare block name after the ^

Jonathan Grynspan · Accepted Answer · 2011-12-30 18:29:43Z

This is why typedef was invented. Embedding function pointers or block types like this is a pain. Try this instead:

typedef id (^ InnerBlock)(void); typedef id (^ OuterBlock)(NSString *, InnerBlock);

It'll make working with block types a lot easier to read. :)

ah, thanks for the pointer(no pun intended). I find blocks definition really hard to read sometimes and typedef is indeed the perfect solution! Though my particular error was misunderstanding where to place the block variable name.

huskerchad · Accepted Answer · 2011-12-30 18:21:56Z

3

Did you possibly mean id(^block)(void) on the RHS of the assignment?

answered Dec 30, 2011 at 18:21

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