class.m

@interface List : NSObject

@property (copy) NSArray *items;
@property (readonly) NSMutableArray *mutableItems;

@end


@implementation List

- (NSMutableArray *)mutableItems {
    // KVO trick: returns a proxy array that uses "items" key to get/set the value.
    return [self mutableArrayValueForKey:@"items"];
}

@end

usage.m

// Usage:
[list.mutableItems addObject:item];

// Equivalent of:
NSMutableArray *mutableItems = [list.items mutableCopy];
[mutableItems addObject:item];
list.items = mutableItems;

// Pro: Easiest way to trigger KVO events for collection mutations.
// Con: Too much copying when doing a lot of mutating operations.