Your archiving in Homepwner saves and loads the itemKey for each BNRItem, but what about the images themselves? Let’s extend the image store to save images as they are added and fetch them as they are needed.
The images for BNRItem instances should also be stored in the Documents directory. You can use the image key generated when the user takes a picture to name the image in the filesystem.
Open BNRImageStore.m and add a new method declaration to the class extension.
- (NSString *)imagePathForKey:(NSString *)key;
Implement imagePathForKey: in BNRImageStore.m to create a path in the documents directory using a given key.
- (NSString *)imagePathForKey:(NSString *)key
{
NSArray *documentDirectories =
NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,
NSUserDomainMask,
YES);
NSString *documentDirectory = [documentDirectories firstObject];
return [documentDirectory stringByAppendingPathComponent:key];
}
To save and load an image, you are going to copy the JPEG representation of the image into a buffer in memory. Instead of just malloc’ing a buffer, Objective-C programmers have a handy class to create, maintain, and destroy these sorts of buffers – NSData. An NSData instance holds some number of bytes of binary data, and you will use NSData to store image data.
In BNRImageStore.m, modify setImage:forKey: to get a path and save the image.
- (void)setImage:(UIImage *)image forKey:(NSString *)key
{
self.dictionary[key] = image;
// Create full path for image
NSString *imagePath = [self imagePathForKey:key];
// Turn image into JPEG data
NSData *data = UIImageJPEGRepresentation(image, 0.5);
// Write it to full path
[data writeToFile:imagePath atomically:YES];
}
Let’s examine this code more closely. The function UIImageJPEGRepresentation takes two parameters: a UIImage and a compression quality. The compression quality is a float from 0 to 1, where 1 is the highest quality (least compression). The function returns an instance of NSData.
This NSData instance can be written to the filesystem by sending it the message writeToFile:atomically:. The bytes held in this NSData are then written to the path specified by the first parameter. The second parameter, atomically, is a Boolean value. If it is YES, the file is written to a temporary place on the filesystem, and, once the writing operation is complete, that file is renamed to the path of the first parameter, replacing any previously existing file. Writing atomically prevents data corruption should your application crash during the write procedure.
It is worth noting that this way of writing data to the filesystem is not archiving. While NSData instances can be archived, using the method writeToFile:atomically: copies the bytes in the NSData directly to the filesystem.
In BNRImageStore.m, make sure that when an image is deleted from the store, it is also deleted from the filesystem:
- (void)deleteImageForKey:(NSString *)key
{
if (!key) {
return;
}
[self.dictionary removeObjectForKey:key];
NSString *imagePath = [self imagePathForKey:key];
[[NSFileManager defaultManager] removeItemAtPath:imagePath
error:nil];
}
Now that the image is stored in the filesystem, the BNRImageStore will need to load that image when it is requested. The class method imageWithContentsOfFile: of UIImage will read in an image from a file, given a path.
In BNRImageStore.m, replace the method imageForKey: so that the BNRImageStore will load the image from the filesystem if it does not already have it.
- (UIImage *)imageForKey:(NSString *)key
{
return self.dictionary[key];
// If possible, get it from the dictionary
UIImage *result = self.dictionary[key];
if (!result) {
NSString *imagePath = [self imagePathForKey:key];
// Create UIImage object from file
result = [UIImage imageWithContentsOfFile:imagePath];
// If we found an image on the file system, place it into the cache
if (result) {
self.dictionary[key] = result;
}
else {
NSLog(@"Error: unable to find %@", [self imagePathForKey:key]);
}
}
return result;
}
Build and run the application again. Take a photo for an item, exit the application, and then press the Home button. Launch the application again. Selecting that same item will show all its saved details – including the photo you just took.
Also, notice that the images were saved immediately after being taken, while the instances of BNRItem were saved only when the application entered the background. You save the images right away because they are just too big to keep in memory for long.