Passing blocks for methods

I would like to pass in different blocks into a method. The method would subsequently use the passed in block as parameter to dispatch_async.

I have my block declared like this:

typedef int (^ComputationBlock)(int);

The class method that accepts the block is implemented as:

- (void)doSomething:(int)limit withBlock:(ComputationBlock)block;
{

 dispatch_queue_t queue = dispatch_get_global_queue开发者_开发百科(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

   // typical in-lined block for dispatch_async:  
 dispatch_async(queue, ^{
      // do some work here
 });

 // I want to pass in the block as the 2nd parameter to dispatch_async
 // but the compiler will warn me of a type mismatch unless I cast
 // the block like:
 dispatch_async(queue, (dispatch_block_t)block);    
}

@end

Is it okay to typecast the 'block' parameter as dispatch_block_t?

No, that's not cool to do -- the block passed to dispatch_async needs to take no parameters and return nothing. Casting your ComputationBlock to that would be a Bad Idea (it's not nice to fool mother nature).

Simply wrap your block that you want to call inside one of the right type:

 dispatch_async(queue, ^{ block(0); } );    

(note that you also need to supply a parameter to your ComputationBlock when you invoke it.)

It may compile, but it won't work. dispatch_block_t blocks must not take any arguments and must not have a return value.

Pass value using __block

__block int backValue;

it can modify in block