Question

In the box2d physics loop of my game (the step), I must call the following block:

if ([objectA conformsToProtocol:@protocol(FMCollisionProtocol)]) {
    [objectA performSelector:@selector(collideWith:) withObject:objectB];
}
if ([objectB conformsToProtocol:@protocol(FMCollisionProtocol)]) {
    [objectB performSelector:@selector(collideWith:) withObject:objectA];
}

The problem is that collideWith: may alter my physics object and this isn’t allowed during the step. Once the step is finished, the physics objects are free to be altered once more. I need some way to remember the target, selector and object paramater of this block for later use. One option would be to use a struct like this:

typedef struct {
    id target;
    SEL selector;
    id object;
} FMCallback;

And store these in an array. Once I’ve finished my step, I then analyse this array and call performSelector:. Is there a better way around this?

EDIT:

I tried both ways and found blocks to be simpler. I’m using a protocol, so I was unsure how to handle this (it would always return a nil NSMethodSignature):

NSMethodSignature *sig = [FMObject instanceMethodSignatureForSelector:@selector(collideWith:)];
NSInvocation *invoc = [NSInvocation invocationWithMethodSignature:sig];
invoc.target = objectB;
invoc.selector = @selector(collideWith:);
[invoc setArgument:objectA atIndex:2];
[Presenter.physics.callbacks addObject:invoc];

It was also longer than I would have hoped. The blocks worked fine after some reading:

// In the collision
[Presenter.physics.callbacks addObject: Block_copy(^{
    [(id)objectB collideWith:objectA];
})];

// After the step
for (id collision in callbacks_) {
    ((dispatch_block_t)collision)();
    Block_release(collision);
}
[callbacks_ removeAllObjects];

Answer 1

How about blocks?

NSMutableArray *collisions = [NSMutableArray array];
…
[collisions addObject:^{
    [objectA collideWith:objectB];
}];
…
for (id collision in collisions)
    ((dispatch_block_t)collision)();

This is with ARC; without ARC you should probably add a copy and autorelease when adding the blocks to the array.