Question

I have a table that I’m doing some special loading for. The user starts scrolled to the bottom. When the user scrolls near the top, I detect this through scroll view delegate methods, and I quickly load some additional content, and populate more of the top part of the list. I want this to look seamless, like an “infinite scroll” upward. To do this, I have to set the content offset, so that the user doesn’t see the table “jump” upward. When I scroll slowly, this works perfectly. When I scroll quickly, so that the table is decelerating, the content offset I set is ignored. Here is the code I’m using:

CGFloat oldHeight = self.tableView.contentSize.height;
CGFloat oldOffset = self.tableView.contentOffset.y;
self.tableContentsArray = newTableContentsArray;
[self.tableView reloadData];
CGFloat newHeight = self.tableView.contentSize.height;
CGFloat newOffset = oldOffset + (newHeight - oldHeight);
self.tableView.contentOffset = CGPointMake(0, newOffset);

So if I scroll up quickly with a table 100px high and hit the top while decelerating, I load more data, get a new table height of, say, 250px. I set the offset to 150. However, since it’s still decelerating, the Apple code leaves the offset set to 150 for .1 seconds or something, then goes to the next calculated offset for deceleration, and sets the offset to 0, which makes it look to the user like they just skipped 150px of content, and are now at the new top of the list.

Now I’d LOVE to keep the acceleration from the list, so that it keeps going up for a while, slows down, and ends up somewhere around 120px offset, just like you would expect. Question is, how?

If I use [self.tableView setContentOffset: CGPointMake(0, newOffset) animated: NO]; it stops the content offset from being ignored, but stops the list dead.

Answer 1

We had an interesting situation like this at work a few months back. We wanted to use the UITableViewController because of it’s caching, loading, and animations, but we wanted it to scroll horizontally (in your case it would be scroll upward). The solution? Rotate the table, then rotate the cells the other direction.

In your case, the code would look like this:

#define degreesToRadian(x) (M_PI * (x) / 180.0) in the header

- (void) viewDidLoad {
   [super viewDidLoad];
   self.table.transform = CGAffineTransformMakeRotation(degreesToRadian(-180));
   ...
}

Then rotate the cell, so it appears in the right orientation for the user

- (UITableViewCell *) tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    ...
    cell.transform = CGAffineTransformMakeRotation(degreesToRadian(180));
}

Now you can postpend your cells like any normal table, and they’ll be added on top instead of the bottom.