神奇的KVC ( key-value coding)

	[myObj setValue:@"peter" forKey:@"name"];
	(1) 呼叫 setName:
	(2) 如果setName:不存在，存取instance variable name。
	NSLog(@"name %@", [mutableArray valueForKey:@"name"]);
	回傳array，array裡為對mutableArray裡的每個物件呼叫valueForKey:的結果。
	ex:
	->
	name (
	Peter,
	Wendy
	)
	Artist *artist = [[Artist alloc] init];
	Album *album = [[Album alloc] init];
	[artist setValue:album forKey:@"newAlbum"];
	[artist setValue:@"怎麼開始忘了" forKeyPath:@"newAlbum.name"];
	NSLog(@"album name %@", [artist valueForKeyPath:@"newAlbum.name"]);
	利用keyPath，可以串聯

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