问题
I understand the ARC Release Notes, however I have been wondering what does this mean exactly and what are the system classes:
You may implement a dealloc method if you need to manage resources other than releasing instance variables. You do not have to (indeed you cannot) release instance variables, but you may need to invoke [systemClassInstance setDelegate:nil] on system classes and other code that isn’t compiled using ARC.
This is right here ARC Release Notes under the new rules enforced by ARC
What are the so called system classes here?.
回答1:
I take this to mean any class starting with 'NS' or 'UI'. Apple have not rebuilt all the frameworks from the ground up to use ARC. Instead, your new ARC code should happily interoperate with the existing frameworks if you follow the rules.
In particular, delegate properties of system classes (such as UIApplication) are still declared as (nonatomic, assign) instead of (nonatomic, weak). This means that these properties do not automatically zero themselves when the delegate is deallocated. In fact, assign is a synonym for unsafe_unretained under ARC. Hence the advice to manually nil the delegate property in your dealloc method. This is so that there is no chance that the system class will attempt to access its delegate after it has disappeared.
来源：https://stackoverflow.com/questions/7934615/iphone-arc-release-notes-dealloc-on-system-classes-delegates