replace the NSMutableArray from mutableArrayValueForKey do extra set

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I'm learning objective-c recently. The behavior of replacement of the NSMutableArray from mutableArrayValueForKey confused me. The code is as follows.

#import <Foundation/Foundation.h>

@interface ItemType : NSObject
@property(nonatomic) NSString* tag;
@end

@implementation ItemType
- (instancetype) initWithTagName:(NSString*) tag{
    self = [super init];
    if (self != nil) {
        _tag = tag;
    }
    return self;

}

- (void) setTag:(NSString *)tag{
    NSLog(@"setTag to:%@ with: %@", tag,_tag);
    _tag = tag;
}

@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        ItemType *item0 = [[ItemType alloc] initWithTagName:@"Alpha"];
        ItemType *item1 = [[ItemType alloc] initWithTagName:@"Beta"];
        ItemType *item2 = [[ItemType alloc] initWithTagName:@"Gamma"];
        NSArray *items = @[item0, item1, item2];
        NSMutableArray* tags = [items mutableArrayValueForKey:@"tag"];
        NSLog(@"tags:%@",tags);
        [tags replaceObjectAtIndex:1 withObject:@"Rho"];
        NSLog(@"tags:%@",tags);
    }
    return 0;
}

and the result:

tags:(
    Alpha,
    Beta,
    Gamma
)
setTag to:(
    Alpha,
    Rho,
    Gamma
) with: Alpha
setTag to:(
    Alpha,
    Rho,
    Gamma
) with: Beta
setTag to:(
    Alpha,
    Rho,
    Gamma
) with: Gamma
tags:(
        (
        Alpha,
        Rho,
        Gamma
    ),
        (
        Alpha,
        Rho,
        Gamma
    ),
        (
        Alpha,
        Rho,
        Gamma
    )
)

I wonder why the extra setTag are called. Thank you very much.

asked 1 min ago

Lebin Jiang

1

replace the NSMutableArray from mutableArrayValueForKey do extra set