why_we_need_copy_attribute.m

#import <Foundation/Foundation.h>

#pragma mark - MyDescriptor

__attribute__((objc_direct_members))
@interface MyDescriptor : NSObject {
    @package NSInteger _index;
}
- (BOOL)direct_isEqual:(MyDescriptor *)other;
@end

@implementation MyDescriptor

- (BOOL)direct_isEqual:(MyDescriptor *)other {
    if (self == other) {
        return YES;
    } else {
        return _index == other->_index;
    }
}

@end


#pragma mark - MyController

__attribute__((objc_direct_members))
@interface MyController : NSObject {
    @private MyDescriptor *_descriptor;
}
- (BOOL)updateLayoutWithDescriptor:(MyDescriptor *)descriptor;
@end

@implementation MyController

- (BOOL)updateLayoutWithDescriptor:(MyDescriptor *)descriptor {
    if ([_descriptor direct_isEqual:descriptor]) {
        return NO;
    }
    
    _descriptor = descriptor;
    
    [NSThread sleepForTimeInterval:0.1];
    
    return YES;
}

@end


#pragma mark - main

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        MyController *controller = [MyController new];
        MyDescriptor *descriptor = [MyDescriptor new];
        
        descriptor->_index = 3;
        assert([controller updateLayoutWithDescriptor:descriptor]);
        
        descriptor->_index = 4;
        assert([controller updateLayoutWithDescriptor:descriptor]); // hit program assert
    }
    
    return 0;
}