How to save objects indexes inside an array and then display if the position of index has changed when the array gets updated

Question

I am making an app where i get data from an XML file, i parse it to an Array, and then it populates a TableViewController.

Now i want to find a way to save this display order, so whenever my app reloads the data it detects if the object has moved indexes or not and display and UP/DOWN/EQUAL sign accordingly.

E.g:

Loads data

1 = B
2 = A
3 = C

Reloads data

1 ^ C
2 = A
3 - B

I'm not sure how i can accomplish this, can anyone help me out?

Thank you.

Answer 1

There are multiple ways that you could do this, but the indexOfObject: method is going to be your best bet for making it happen. With that method you could write something like:

    NSArray *ar1 = @[@"first", @"second", @"third"];

    NSLog(@"%d", (unsigned)[ar1 indexOfObject:@"third"]);

And it would log out 2 .

Based on that something like the following should work for you.

    NSArray *ar1 = @[@"first", @"second", @"third"];
    NSArray *ar2 = @[@"third", @"first", @"second"];

    [ar2 enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        if (idx != [ar1 indexOfObject:obj]) {
            NSLog(@"Obj: %@ Old idx: %ld new idx: %ld", obj, [ar1 indexOfObject:obj], idx);
        }
    }];

For me that outputs:

Obj: third Old idx: 2 new idx: 0 
Obj: first Old idx: 0 new idx: 1 
Obj: second Old idx: 1 new idx: 2

Answer 2

I think you need to keep a copy of the old data and then when the update arrives compare the data. Something like:

-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
        NSObject * obj =  self.newData objectAtIndex:indexPath.row];

        int oldIndex = [self.oldData indexOfObject:obj];
        if (oldIndex == NSNotFound) { //new object
            //do something with the new object
        }
        else
        {
            //do something with the oldIndex
        }
}

Assuming oldData contains the data fetched last time and newData contains the just recived data