save and restore an array of custom objects

Question

I have an NSArray of custom objects that I want to save and restore. Can this be done with NSUserDefaults?

Answer 1

Custom objects, no. NSUserDefaults only knows about a few basic types (NSData, NSString, NSNumber, NSDate, NSArray, or NSDictionary).

Could you use JSON (http://code.google.com/p/json-framework/) to convert your custom object to a string representation, then save an array of those to Defaults? (Using the setObject:forKey: method).

Otherwise, you could look at using sqlite, NSCoder, or even resort to fopen.

Answer 2

NSUserDefaults cannot write custom objects to file, only ones it knows about (NSArray, NSDictionary, NSString, NSData, NSNumber, and NSDate). Instead, you should take a look at the Archives and Serializations Programming Guide, as well as the NSProtocol Class Reference, if you're looking to save and restore custom objects to disk. Implementing the protocol is not terribly difficult, and requires very little work.

Answer 3

You can still use NSUserDefaults if you archive your array into NSData.

For Archiving your array, you can use the following code:

[[NSUserDefaults standardUserDefaults] setObject:[NSKeyedArchiver archivedDataWithRootObject:myArray] forKey:@"mySavedArray"];

And then for loading the custom objects in the array you can use this code:

NSUserDefaults *currentDefaults = [NSUserDefaults standardUserDefaults];
NSData *savedArray = [currentDefaults objectForKey:@"mySavedArray"];
if (savedArray != nil)
{
        NSArray *oldArray = [NSKeyedUnarchiver unarchiveObjectWithData:savedArray];
        if (oldArray != nil) {
                customObjectArray = [[NSMutableArray alloc] initWithArray:oldSavedArray];
        } else {
                customObjectArray = [[NSMutableArray alloc] init];
        }
}

Make sure you check that the data returned from the user defaults is not nil, because that may crash your app.

The other thing you will need to do is to make your custom object comply to the NSCoder protocol. You could do this using the -(void)encodeWithCoder:(NSCoder *)coder and -(id)initWithCoder:(NSCoder *)coder methods.

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EDIT.

Here's an example of what you might but in the -(void)encodeWithCoder:(NSCoder *)coder and -(id)initWithCoder:(NSCoder *)coder methods.

- (void)encodeWithCoder:(NSCoder *)coder;
{
    [coder encodeObject:aLabel forKey:@"label"];
    [coder encodeInteger:aNumberID forKey:@"numberID"];
}

- (id)initWithCoder:(NSCoder *)coder;
{
    self = [[CustomObject alloc] init];
    if (self != nil)
    {
        aLabel = [coder decodeObjectForKey:@"label"];
        aNumberID = [coder decodeIntegerForKey:@"numberID"];
    }   
    return self;
}