For years I've been following a great pattern called Target-Action which goes like this:
An object calls a specified selector on a specified target object when the time comes to call. This is very useful in lots of different cases where you need a simple callback to an arbitrary method.
Here's an example:
- (void)itemLoaded {
[specifiedReceiver performSelector:specifiedSelector];
}
Under ARC it now turns out that doing something like this all of a sudden became dangerous.
Xcode throws a warning that goes like this:
PerformSelector may cause a leak because its selector is unknown
Of course the selector is unknown since as part of the Target-Action design pattern you can specify whatever selector you want in order to get a call when something interesting happens.
What bugs me most about this warning is that it says there can be a potential memory leak. From my understanding ARC doesn't bend the memory management rules but instead simply automates the insertion of retain/release/autorelease messages at the right locations.
Another thing to note here: -performSelector: does have an id
return value. ARC analyzes method signatures to figure out through application of naming conventions if the method returns a +1 retain count object or not. In this case ARC doesn't know if the selector is a -newFooBar
factory or simply calling an unsuspicious worker method (which is almost always the case with Target-Action anyways). Actually ARC should have recognized that I don't expect a return value, and therefore forget about any potential +1 retain counted return value. Looking at it from that point of view I can see where ARC is coming from, but still there is too much uncertainty about what this really means in practice.
Does that now mean under ARC something can go wrong which would never happen without ARC? I don't see how this could produce a memory leak. Can someone give examples of situations in which this is dangerous to do, and how exactly a leak is created in that case?
I really googled the hell out of the internet but didn't find any site explaining why.