Recursive Blocks in Objective C (under ARC)

recursive-blocks.m

#include <stdio.h>

/**
 This only works if the block in question is called synchronously.
 */
void try0() {
  typedef void(^RecursiveBlock)();

  __block int i = 5;

  __weak __block RecursiveBlock weakBlockBlock;
  RecursiveBlock block = ^{
    printf("i: %d\n", i);
    i -= 1;
    if (i > 0) {
      weakBlockBlock();
    }
  };

  weakBlockBlock = block;

  block();
}

void try1() {
  typedef void (^RecursiveBlock)(id continuation);

  __block int i = 5;

  RecursiveBlock block = ^(RecursiveBlock continuation){
    printf("i: %d\n", i);
    i -= 1;
    if (i > 0) {
      continuation(continuation);
    }
  };

  block(block);
}

/**
 This is the probably the most natural of the given approaches.
 Unfortunately, this results in a spurious warning from the static analyzer.
 @see http://stackoverflow.com/a/13091475/141220
 */
void try2() {
  typedef void (^RecursiveBlock)();

  __block int i = 5;

  __block RecursiveBlock block = ^(){
    printf("i: %d\n", i);
    i -= 1;
    if (i > 0) {
      block();
    } else {
      block = nil;
    }
  };

  block();
}

int main(int argc, const char * argv[]){
  @autoreleasepool {
    try0();
    try1();
    try2();
  }
  return 0;
}