Storing images locally on an iOS device

Question

Some iOS photo-related apps store images created with the app somewhere other than the photo library. For example Fat Booth shows a scrolling list of photos created with the app at app start-up. Note these photos are persisted w/o the user explicitly saving them to the photo library. What is the simplest way to save and persist images inside an iOS application?

The only persistent stores I'm familiar with are NSUserDefaults and the key chain. However I've never heard of these used to store larger pieces of data such as an image. Now I'm wondering if Core Data is the easiest way.

Answer 1

The simplest way is to save it in the app's Documents directory and save the path with NSUserDefaults like so:

NSData *imageData = UIImagePNGRepresentation(newImage);

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];

NSString *imagePath =[documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.png",@"cached"]];

NSLog(@"pre writing to file");
if (![imageData writeToFile:imagePath atomically:NO]) 
{
    NSLog(@"Failed to cache image data to disk");
}
else
{
    NSLog(@"the cachedImagedPath is %@",imagePath); 
}

Then save the imagePath in some dictionary in NSUserDefaults or however you'd like, and then to retrieve it just do:

 NSString *theImagePath = [yourDictionary objectForKey:@"cachedImagePath"];
 UIImage *customImage = [UIImage imageWithContentsOfFile:theImagePath];

Answer 2

For Swift:

let imageData = UIImagePNGRepresentation(selectedImage)
let paths = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0] as String
let imagePath = paths.stringByAppendingPathComponent("cached.png")

if !imageData.writeToFile(imagePath, atomically: false)
{
   println("not saved")
} else {
   println("saved")
   NSUserDefaults.standardUserDefaults().setObject(imagePath, forKey: "imagePath")
}

For Swift 2.1:

let imageData = UIImagePNGRepresentation(selectedImage)
let documentsURL = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)[0]
let imageURL = documentsURL.URLByAppendingPathComponent("cached.png")

if !imageData.writeToURL(imageURL, atomically: false)
{
    print("not saved")
} else {
    print("saved")
    NSUserDefaults.standardUserDefaults().setObject(imageData, forKey: "imagePath")
}

stringByAppendingPathComponent is unavailable in Swift 2.1 so you can use URLByAppendingPathComponent. Get more info here.

Answer 3

You can do it with core data by storing binary data, but its not recommended. There is a much better way - especially for photos. Your application has a documents/file directory that only your app can access. I recommend starting here for the concepts and how to access it. Its relatively straightforward. You may want to combine this with core data to store file paths, metadata, etc. http://developer.apple.com/library/mac/#documentation/FileManagement/Conceptual/FileSystemProgrammingGUide/FileSystemOverview/FileSystemOverview.html