How to achieve UITableView row animation similar to Mail - ios

Question

In Mail, if you manually refresh a mailbox via pull to refresh, if you don't have any new emails, the view slides back up to hide the spinner but the appearance of existing cells don't change - they don't animate at all. But if you did have new emails, those animate in from the top. If you had no emails listed before the refresh, all the cells animate in quite nicely. Also, if you refresh and an existing email had been deleted, its disappearance is animated but none of the other cells animate besides the entire table moving up to take the place of the removed cell. I would like to achieve this same behavior in my app.
Currently, when my data is fetched for the first time, it immediately appears in the table without any animation. If some data is deleted and the user manually refreshes, the entire table is instantly updated so there is no animation of the cell disappearing, it's just instantly replaced with the row underneath it. The same behavior occurs if cells were already displayed and refreshing results in new data being added to the table - it just instantly appears.
How can I implement animation for appearance and disappearance of cells when I reloadData? But not animate existing cells if they don't change.
My setup is to fetch the data, parse the JSON into a data structure (array or dictionary), then call reloadData, and then cellForRowAtIndexPath gets the data from the structure. When the user refreshes, it performs those same steps again.
My attempts so far haven't been successful. I tried instead of simply calling reloadData, I call [self.tableView reloadSections:[NSIndexSet indexsetWithIndex:0] withRowAnimation:UITableViewRowAnimationTop]; but this only animates the first section and it always animates it, even if none of the data has changed. My other attempt also always animates it:
[self.tableView beginUpdates];
[self.tableView deleteSections:[NSIndexSet indexsetWithIndex:0] withRowAnimation:YES];
[self.tableView insertSections:[NSIndexSet indexsetWithIndex:0] withRowAnimation:YES];
[self.tableView endUpdates];

Answer 1

When inserting the rows, keep the index paths being inserted in an array, then:
[self.tableView beginUpdates];
[self.tableView insertRowsAtIndexPaths:arrayOfInsertedIndexPaths withRowAnimation: UITableViewRowAnimationTop];
[self.tableView endUpdates];
I haven't confirmed directly, but I'm pretty sure this will behave as you wish when now insertions are made.
EDIT - Row-accurate animation is going to require row-accurate knowledge about the change in the model. How to get the index paths being inserted depends on how you're getting the data. In the toughest scenario, you're just getting a new collection and will need to work out for yourself what has changed. Your model objects probably need to implement equivalence** so you can conclude that different instances should be treated as the same.
** as in NSString isEqualToString: which is an equivalence test, not an equality test as it's name might suggest.
We're comparing the objects themselves, not row indexes, so knowing about deletions is hard, but no harder than knowing about insertions. As an example, here's a category I created for NSMutableArray that handles only insertions (it's ideal for an email or other timeline sort of app that's expecting new stuff, not deletions).
#import "NSMutableArray+Inserts.h"
#implementation NSMutableArray (NSMutableArray_Inserts)
// insert elements of array into the receiver, sorting on a key
// answer an array of index paths where the insertions happened
- (NSArray *)insertFromArray:(NSArray *)array sortedOn:(NSString *)key ascending:(BOOL)ascending {
// first insert everything from array (preserving uniqueness according to shallow equality)
for (id newElement in array) {
NSInteger position = [self indexOfObject:newElement];
if (position == NSNotFound) [self addObject:newElement];
}
// now sort using key
[self sortUsingDescriptors:[NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:key ascending:ascending]]];
// now find the insertions using deep equality
NSMutableArray *answer = [NSMutableArray array];
for (id newElement in array) {
NSInteger position = [self indexOfObject:newElement];
id element = [self objectAtIndex:position];
if (element == newElement) {
[answer addObject:[NSIndexPath indexPathForRow:position inSection:0]];
}
}
return [NSArray arrayWithArray:answer];
}
A couple of notes: 1) I lazily hardcoded the index path section to 0. If your model has multiple sections, you'll want to call this for each and add a section parameter so you get good index paths back, 2) the sorted insert feature of this is pertinent to my particular app, you might not need it, and can just remove those two params and delete the self sortUsing... line. 3) one way to account for deletions would be to add an analogous method that answers the index paths of elements in the receiver NOT in a passed array.