Objective-C Syntax for a Block that has a Block as a Parameter?

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Created Dec ’21

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I'm not sure this is even possible (and I'm sure there will be questions as to why) but I'm curious how one could define a block that takes a block as a parameter? For instance...

MyFunctionWithSuccess:(void(^)((void(^)(NSString* value))blockAsParameter)success

This of course doesn't work.

The only thing I could find is on StackOverflow but I think in this case it returns a block. Regardless the syntax throws a compiler error..

StackOverflow - Syntax to define a Block that takes a Block and returns a Block in Objective-C

There it says to define it as :

void (^(^complexBlock)(void (^)(void)))(void)

However this throws a "Type-id cannot have a name" which it complains about the ^complexBlock part.

Again it just may be that this is not possible and bad to do. However I'd like to confirm that it is or isn't.

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Answer 1

DTS Engineer OP

Apple

Dec ’21

Quinn’s secret sauce when dealing with C function pointers (and now blocks, replacing the * with ^) is to lean on typedefs. For example:

@import Foundation;

typedef void (^CompletionHandler)(void);
typedef void (^WorkerBlockThatTakesACompletionHandler)(CompletionHandler);

int main(int argc, char **argv) {
    #pragma unused(argc)
    #pragma unused(argv)
    WorkerBlockThatTakesACompletionHandler w = ^(CompletionHandler ch) {
        NSLog(@"work will start");
        sleep(1);
        NSLog(@"work did finish");
        ch();
    };
    w(^{
        NSLog(@"completion");
    });

    return EXIT_SUCCESS;
}

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