Parse.com saveEventually with callback and use NSCache to store data - ios

Question

Parse.com has a saveEventually option that stores data to disc and waits for a network connection to proceed and save it to server.
However, sometimes when a network connection is available and the user quickly switches screens, the new data is not yet saved to the server, causing views to show old data.
For parse.com users, this happens when I run a new query and the saveEventually call is not yet complete.
I would like to implement my own cache system.
I would like to call saveEventually with callback upon completion, so then I would delete the cached data.
This way, I could check first if there's cached data before making network connection.
I think I can do it using NSCache and NSDiscardableContent, I already read the docs, but it would really help me if I could see some simple sample code for creating these objects and storing them on user's device.
Any suggestions?
UPDATE:
OK, this an idea of workaround solution.
Every object that I call saveEventually on I will add to a NSArray and store it in documentsDirectory, then, upon saveEventually completion I will delete the array in documentsDirectory.
If the app closes before completion, I wouldn't get a callback but it wouldn't be a problem because I will perform the query on the server and get the stored NSArray in device as well, I would then create a unique array giving preference to the objects in the documentsDirectory.
I think this could work. My concern is if it will make my app slower.
Any thoughts?

Answer 1

I haven't tried this, but how about setting your query's cache policy, like:
query.cachePolicy = kPFCachePolicyCacheElseNetwork;
If saveEventually is using the regular parse cache, then it should do just what you want.
EDIT - Since the above is apparently not going to work, I can think of several schemes to employ your own cache, but they're all stuck with the fundamental race condition problem: say you save eventually, then query again. Say you've fixed your query code to grab the local object that's being eventually saved.
What if you modify that cached object and save it eventually (again)? Do we know that parse will correctly serialize the two saves?
If you're not worried about this (maybe the second query never generates a save, or you're willing to roll the dice on the race condition), then you have a lot of choices. Let's take the straight-forward one:
Create a singleton with an NSCache property. Give these methods which are in it's public interface...
// call this instead of saveEventually
- (void)cacheThenSaveEventually:(NSArray *)pfObjects {
for (PFObject *object in pfObjects) {
[self.cache setObject:object forKey:[object objectId]];
[object saveEventually:^(BOOL success) {
if (success) [self.cache removeObjectForKey:[object objectId]];
}];
}
}
// call this with the result of a query that might have been in a race with saveEventually
- (NSArray *)freshenUpResults:(NSArray *)pfObjects {
NSMutableArray *fresherObjects = [NSMutableArray array];
for (PFObject *object in pfObjects) {
PFObject *fresher = [self.cache objectForKey:[object objectId]];
[fresherObjects addObject:(fresher)? fresher : object];
}
return [NSArray arrayWithArray:fresherObjects];
}