object-c ARC与block笔记

Avoiding Retain Cycles with __block

• To avoid retain cycles

  • use __weak , __block id x

    __block variables are retained under ARC;

    __block id x;//retains x under ARC

  • manually break cycle by setting __block value to nil;

//arc
dispatch_block_t getfoo(int i){
    return ^{
        printf("%d",i);
    };
}
//mrc
dispatch_block_t getfoo(int i){
    return [[^{
        printf("%d",i);
    }copy] autorelease];
}

Casting id <--> void*(Core Foundation and toll-free bridging)

• Bridged

  NSString *a = (__bridge NSString *)my_cfref;//Noop cast
  CFStringRef *b = (__bridge NSString *)my_id//Noop cast

• Move CF object to ARC
  NSString *c = CFBridgingRelease(my_cfref);//-1 on the CFRef
• Move ARC object to CF
  CFStringRef d = CFBridingRetain(my_id);// returned CFRef + 1