实现NSCoding

NSCoding 是一个protocol，里头只有两个method 要实现

@protocol NSCoding
- (void)encodeWithCoder:(NSCoder *)aCoder;
- (id)initWithCoder:(NSCoder *)aDecoder;
@end

encodeWithCoder:的用途是将我们的实例透过NSCoder转成NSData，至于initWithCoder:刚好相反，就是透过NSCoder，再把NSData转回实例。

假如我们现在有一个叫做KKSongTrack 的class，用来代表KKBOX 里头的一首歌，里头只有songName、albumName 与artistName 三个property，这三个property 都是NSString。

@interface KKSongTrack : NSObject <NSCoding>
@property (strong, nonatomic) NSString *songName;
@property (strong, nonatomic) NSString *albumName;
@property (strong, nonatomic) NSString *artistName;
@end

在实现的时候，我们就可以用NSCoder的相关method对资料做encode与decode。由于我们的property都是NSString，都是Objective-C实例，所以我们选择encodeObject:forKey:以及decodeObjectForKey:。实现如下：

#import "KKSongTrack.h"

static NSString *const kSongNameKey = @"song_name";
static NSString *const kAlbumNameKey = @"album_name";
static NSString *const kArtistNameKey = @"artist_name";

@implementation KKSongTrack

- (NSString *)description
{
  return [NSString stringWithFormat:@"<%@ %p %@ - %@ - %@>",
    NSStringFromClass([self class]), self,
    self.songName, self.albumName, self.artistName];
}

- (instancetype)initWithCoder:(NSCoder *)coder
{
    self = [super init];
    if (self) {
        self.songName = [coder decodeObjectForKey:kSongNameKey];
        self.albumName = [coder decodeObjectForKey:kAlbumNameKey];
        self.artistName = [coder decodeObjectForKey:kArtistNameKey];
    }
    return self;
}

- (void)encodeWithCoder:(NSCoder *)aCoder
{
    [aCoder encodeObject:self.songName forKey:kSongNameKey];
    [aCoder encodeObject:self.albumName forKey:kAlbumNameKey];
    [aCoder encodeObject:self.artistName forKey:kArtistNameKey];
}

除了Objective-C 实例类型外，NSCoder 还有处理BOOL、整数以及浮点数相关的method 可以使用。

接下来如果我们想把歌曲转成NSData：

KKSongTrack *song = [[KKSongTrack alloc] init];
song.songName = @"orz 之歌";
song.albumName = @"orz 專輯";
song.artistName = @"orz";
NSData *data = [NSKeyedArchiver archivedDataWithRootObject:song];

把NSData 再转回KKSongTrack：

KKSongTrack *decodeSong = [NSKeyedUnarchiver unarchiveObjectWithData:data];