What is the best way to back up an NSMutableArray on NSUserDefaults?

I have a custom object called Occasion defined as follows:


@interface Occasion : NSObject {

NSString *_title;
NSDate *_date;
NSString *_imagePath;    


@property (nonatomic, retain) NSString *title;
@property (nonatomic, retain) NSDate *date;
@property (nonatomic, retain) NSString *imagePath;

Now I have an NSMutableArray of Occasions which I want to save to NSUserDefaults. I know it’s not possible in a straight forward fashion so I’m wondering which is the easiest way to do that? If serialization is the answer, then how? Because I read the docs but couldn’t understand the way it works fully.

You should use something like NSKeyedArchiver
to serialize the array to an NSData
, save it to the NSUserDefaults
and then use NSKeyedUnarchiver
to deserialize it later:

NSData *serialized = [NSKeyedArchiver archivedDataWithRootObject:myArray];
[[NSUserDefaults standardUserDefaults] setObject:serialized forKey:@"myKey"];


NSData *serialized = [[NSUserDefaults standardUserDefaults] objectForKey:@"myKey"];
NSArray *myArray = [NSKeyedUnarchiver unarchiveObjectWithData:serialized];

You will need to implement the NSCoding
protocol in your Occasion
class and correctly save the various properties to make this work correctly. For more information see the Archives and Serializations Programming Guide
. It shouldn’t be more than a few lines of code to do this. Something like:

- (void)encodeWithCoder:(NSCoder *)coder {
    [super encodeWithCoder:coder];

    [coder encodeObject:_title forKey:@"_title"];
    [coder encodeObject:_date forKey:@"_date"];
    [coder encodeObject:_imagePath forKey:@"_imagePath"];

- (id)initWithCoder:(NSCoder *)coder {
    self = [super initWithCoder:coder];

    _title = [[coder decodeObjectForKey:@"_title"] retain];
    _date = [[coder decodeObjectForKey:@"_date"] retain];
    _imagePath = [[coder decodeObjectForKey:@"_imagePath"] retain];

    return self;
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