Question

I have never worked on non ARC based project. I just came across a zombie on my ARC based project. I found it was because of retain cycle.I am just wondering what is a retain cycle.Can

Could you give me an example for retain cycle?

Answer 1

A retain cycle is a situation when object A retains object B, and object B retains object A at the same time^*. Here is an example:

@class Child;
@interface Parent : NSObject {
    Child *child; // Instance variables are implicitly __strong
}
@end
@interface Child : NSObject {
    Parent *parent;
}
@end

You can fix a retain cycle in ARC by using __weak variables or weak properties for your “back links”, i.e. links to direct or indirect parents in an object hierarchy:

@class Child;
@interface Parent : NSObject {
    Child *child;
}
@end
@interface Child : NSObject {
    __weak Parent *parent;
}
@end

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^* This is the most primitive form of a retain cycle; there may be a long chain of objects that retain each other in a circle.