Question

I’m trying to combine images in my app into one file and write it to disk.

NSMutableArray *array = [NSMutableArray arrayWithObjects: 
                         [NSData dataWithContentsOfFile:@"0.png"], 
                         [NSData dataWithContentsOfFile:@"1.png"],
                         [NSData dataWithContentsOfFile:@"2.png"],
                         nil];

NSData *data = [NSKeyedArchiver archivedDataWithRootObject:array];
NSError *error = nil;
NSString *path=@"/Users/myusername/Desktop/_stuff.dat";
[data writeToFile:path options:NSDataWritingAtomic error:&error];

or

NSArray *array = [NSArray arrayWithObjects:
                  [NSImage imageNamed:@"0"], 
                  [NSImage imageNamed:@"1"], 
                  [NSImage imageNamed:@"2"], 
                  nil];

NSData *data = [NSKeyedArchiver archivedDataWithRootObject:array];
NSError *error = nil;
NSString *path=@"/Users/myusername/Desktop/_stuff.dat";
[data writeToFile:path options:NSDataWritingAtomic error:&error];

But both produce a file that is 4KB (empty). If I NSLog the error it is (null). Am I making the data the wrong way?

Edit: If I open the resulting file with a text editor, it looks like this:

Answer 1

I wrote a quick example:
Missing: memory management / error handling / proper file handling

// Archive

NSMutableArray *array = [[NSMutableArray alloc] init];

NSString * input = @"/Users/Anne/Desktop/1.png";

[array addObject:[NSData dataWithContentsOfFile:input]];
[array addObject:[NSData dataWithContentsOfFile:input]];
[array addObject:[NSData dataWithContentsOfFile:input]];

NSData *data = [NSKeyedArchiver archivedDataWithRootObject:array];
NSString *path = @"/Users/Anne/Desktop/archive.dat";
[data writeToFile:path options:NSDataWritingAtomic error:nil];


// Unarchive

NSMutableArray *archive = [NSKeyedUnarchiver unarchiveObjectWithFile:path];

NSData * firstObject = [archive objectAtIndex:0];
NSString * output = @"/Users/Anne/Desktop/2.png";
NSURL *fileURL = [[NSURL alloc] initFileURLWithPath:output];
[firstObject writeToURL:fileURL atomically:YES];

You can also add NSImages to the NSMutableArray:

NSString * input = @"/Users/Anne/Desktop/1.png";
NSImage *image = [[NSImage alloc] initWithContentsOfFile: input];
[array addObject:image];

But that will significantly increase the file size.