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 ")"?

Answer 1

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 ^

Answer 2

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. :)

Answer 3

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