RACObserve for value update in NSMutableArray?

Question

I'm having a problem reacting to a value change in NSMutableArray.

I have the following (somewhat simplified) code to detect a change:

[[RACObserve(self, postedImagesIds) filter:^BOOL(NSMutableArray * postedImagesIds) {
    return [postedImagesIds count] > 0;
}] subscribeNext:^(NSMutableArray * postedImagesIds) {
    [self uploadFields:fields];
}];

The idea here is to call uploadFields when there is a change in NSMutableArray postedImagesIds . But not only when a new element is added, but also when a value is updated like so:

[self.postedImagesIds replaceObjectAtIndex:i withObject:imagePosted.imagePostedId];

Then thing is that when the value is updated, RACObserve never knows!! Is there a way to recognize this change ?

Thanks in advance!

Answer 1

It can be done, sort of. The important part is that you must perform your NSMutableArray mutation via KVC on self , rather than on an independent reference to the NSMutableArray object. In other words, it won't work if you do this:

[self.postedImagesIds addObject:imagePosted.imagePostedId];
or
[self.postedImagesIds replaceObjectAtIndex:i withObject:imagePosted.imagePostedId];

instead, you must add or replace the object like this:

NSMutableArray *fromKVC = [self mutableArrayValueForKey:@"postedImagesIds"];
[fromKVC addObject:imagePosted.imagePostedId];
or
[fromKVC replaceObjectAtIndex:i withObject:imagePosted.imagePostedId];

This is because the KVO established by RACObserve() is relative to the object you pass in as its first parameter ( self , in this case), so only KVC-compliant mutations that pass through the observed object will trigger the observation notifications.