iCloud Document Storage with UIDocument - Beginning iOS Cloud and Database Development - page 80

Inside our implementation we need to first implement the writing method for UIDocument , which is
contentsForType:error: . To do this we will add a second method called encodeObject:toWrappers:
withFilename:
#pragma mark - Document Writing Methods
- (id)contentsForType:(NSString *)typeName error:(NSError *__autoreleasing *)outError {
if(self.metadata == nil || self.data == nil)
return nil;
NSMutableDictionary *wrappers = [NSMutableDictionary dictionary];
[self encodeObject:_data toWrappers:wrappers withKey:kDataKey];
[self encodeObject:_metadata toWrappers:wrappers withKey:kMetadataKey];
NSFileWrapper *fileWrapper = [[NSFileWrapper alloc] initDirectoryWithFileWrappers:wrappers];
return fileWrapper;
}
- (void)encodeObject:(id<NSCoding>)object toWrappers:(NSMutableDictionary *)wrappers
withKey:(NSString *)key {
@autoreleasepool {
NSMutableData *data = [NSMutableData data];
NSKeyedArchiver *archiver = [[NSKeyedArchiver alloc] initForWritingWithMutableData:data];
[archiver encodeObject:object forKey:@"DATA"];
[archiver finishEncoding];
NSFileWrapper *wrapper = [[NSFileWrapper alloc] initRegularFileWithContents:data];
[wrappers setObject:wrapper forKey:key];
}
}
Let's talk about encodeObject:toWrappers:withFilename: first. This method is passed three parameters.
The first is our object that conforms to the NSCoding protocol, which is going to be NSData or NSMetadata .
The second parameter is an NSMutableDictionary that we use to hold each NSFileWrapper that
contains the encoded data. The final parameter is the key used as the NSFileWrapper key for the
encoded object. We surround our actions in this method with an autoreleasepool so that the pool isn't
drained until we utilize the NSMutableDictionary in the calling method.
We start by creating an NSMutableData object called data. We then create an NSKeyedArchiver by
calling alloc and initForWritingWithMutableData : and passing our data object. We then call the
method encodeObject:forKey on the archiver. This takes our passed in object and encodes it. We
then wrap up the NSKeyedArchiver by calling finishEncoding .
The final step is to create an NSFileWrapper using alloc and initRegularFileWithContents : and
passing it our data object that has been encoded. Then we add our wrapper to the passed in
wrappers mutable dictionary using the key we passed into the object.
Looking at contentsForType:error: you can see that we first check to see if metadata or data are
empty. By calling self.metadata and self.data , we utilize an accessor method that we will create
later to be able to lazily load our data. If either of these is nil , then we return out of this method.