Remove `mutable container` functionality.

[nlutsenko]

Mutable containers right now power the ability to mutate NSArray, NSDictionary, PFGeoPoint after setting them on PFObject.
Here is an example:

NSMutableArray *array = [NSMutableArray array];

PFObject *object = [PFObject objectWithClassName:@"Example"];
object[@"array"] = array;
[object save];
// This will save an object with empty `array` field.

[array addObject:@"test1"];
[object save];
// This will save an object with `array: [ test1 ]` field

This issue is for tracking proposal to remove this functionality from our SDKs. Here are just a few reasons:

• Inconsistency with Cocoa(iOS/OSX) conventions, where you always pass an object by copy, specifically where related to closest thing to Parse - Core Data.
• Inconsistency when fetching an object, where it will remove the mutable containers tracking, as the data will be overwritten by new data from the server.
• Slower Local Datastore performance - a lot of time spent on pinning objects to LDS is spent in checkpointing all mutable containers.

cc @grantland, @hallucinogen, @richardjrossiii

[richardjrossiii]

+1

[nlutsenko]

This is potentially breaking, if someone relies on the flow discussed above.
Please raise questions/issues here, so we are more aware of the public impact.

[gonzalonunez]

+1

[gavinbunney]

👍 would be great to have it removed (as per #102) - would be more consistent with how you imagine it would work since they are NSArray objects, not NSMutableArray.

Typically we've handed this through helper methods on the parent objects such as (add a new Note object):

- (Note *)addNote:(Note*)note {
    [self willChangeValueForKey:@"notes"];

    NSMutableArray *mutableArray = self.notes.mutableCopy;
    if (!mutableArray) {
        mutableArray = [NSMutableArray array];
    }

    [mutableArray addObject:note];
    self.notes = mutableArray;
    [self didChangeValueForKey:@"notes"];

    return note;
}

[FreudGit]

+1

[alex-vasenin]

Guys, I agree you've had a good reasons to remove mutability. But the bad thing the following code which worked well in 1.9.1 silently fails under 1.10.1 without any compile or runtime errors/warnings: [myPfObjectSubclass[@"someArrayKey"] addObject:someObject]. If it doesn't allowed anymore SDK must warn the developers (preferably at compile time).