Removing and Adding same object to different classes - objective-c

Question

I'm reading a book to learn Objective-C. I'm stuck in one of the exercises. You have to make a program that has a class song, playlist, and music collection. If you create a song, it has to automatically add to the music collection, who has a NSMutableArray for collecting songs. And if You remove an object from music collection, the song has to remove from every playlist containing that song.
Song Interface
#import <Foundation/Foundation.h>
#interface Song : NSObject{
NSString *title;
NSString *artist;
NSString *album;
}
#property (copy) NSString *title;
#property (copy) NSString *artist;
#property (copy) NSString *album;
-(Song *) initWithNames:(NSString*) title1 and: (NSString*) artist1 and: (NSString*) album1;
#end
Playlist Interface
#import <Foundation/Foundation.h>
#interface Playlist : NSObject{
NSString * title;
NSMutableArray *collecsongs;
}
#property (strong) NSString *title;
#property (strong) NSMutableArray *collecsongs;
-(Playlist *) initWithName: (NSString *) name;
#end
Music Collection Interface
#import <Foundation/Foundation.h>
#import "Playlist.h"
#interface MusicCollection : NSObject{
NSMutableArray *collecplist;
Playlist *library;
}
#property (strong) NSMutableArray *collecplist;
#property (strong) Playlist *library;
#end
So if i create a song for example song1, is there a way if i add it to a playlist, automatically add it to the mastercoleection variable "library", instead of doing this
Song *song1 = [[Song alloc] initWithNames:#"Somebody That I Used To Know" and: #"Gotye" and: #"First Album"];
Playlist *Misrolas = [[Playlist alloc] initWithName: #"Misrolas"];
MusicCollection *music = [[MusicCollection alloc] init];
[Misrolas.collecsongs addObject: song1];//adds song1 to the playlist named "Misrolas"
[music.library.collecsongs addObject: song1];//adds song1 to the music collection
So i don't know what to do, i was thinking overriding maybe addObject:, but that doesn't seem right and easy, thanks for the help =)
I do it like this, is there more efficient or better ways to add it ???
-(void) addsong: (Song *)song addtocollection: (Playlist *) library{
NSAssert([song isKindOfClass: [Song class]], #"Not the same class");
[self.collecsongs addObject:song];
[library.collecsongs addObject: song];
}

Answer 1

-(Song *) initWithNames:(NSString*) title1 and: (NSString*) artist1 and: (NSString*) album1;
This is a pretty bad naming. Your selector shortens to initWithNames:and:and: (which is not really descriptive). Consider using
- (id)initWithTitle:(NSString *)title artist:(NSString *)artist album:(NSString *)album;
Notice how I use return type of (id) here. It allows easier subclassing, as any descendant class can use the init... "constructor" without any type mismatch warnings.
Speaking of your issue, I'd suggest you to expose only NSArray * property accessors (so that you cannot modify the array contents) and make a method on Playlist class:
- (void)addSong:(Song *)song
{
NSAssert([song isKindOfClass:[Song class]]);
[self.collecplist addObject:song];
}
That's OOP's encapsulation in action. You don't expose private interface (the array), you provide interface for adding exactly songs (you cannot add other kind of object), finally, you do a verification, that what you add is really a song. Here you can also add the song to your music collection.

Answer 2

You will probably find this easier if you call a method on the Playlist object to add the song, rather than accessing its collecsongs property. In that method, it can add the song to the array and then add it to the library. (And then you can make the collecsongs property return an NSArray, rather than an NSMutableArray, which seems much cleaner to me.