Keep uitableview static when inserting rows at the top

Question

I have a tableView that I'm inserting rows into at the top.

Whilst I'm doing this I want the current view to stay completely still, so the rows only appear if you scroll back up.

I've tried saving the current position of the underlying UIScrollview and resetting the position after the rows have been inserted but this results in a judder, up and down, although it does end up back in the same place.

Is there a good way of achieving this ?

Update: I am using beginUpdate, then insertRowsAtIndexPath, endUpdates. There is no reloadData call.

scrollToRowAtIndexPath jumps to the top of the current cell (saved before adding rows).

The other approach I tried, which ends up in exactly the right pace, but with a judder is.

save tableView currentOffset. (Underlying scrollView method)
Add rows (beginUpdates,insert...,endUpdates) 
reloadData ( to force a recalulation of the scrollview size )
Recalculate the correct new offset from the bottom of the scrollview
setContentOffset (Underlying scrollview method)

Trouble is the reloadData causes the scrollview/tableview to start scrolling briefly, then the setContentOffset returns it to the correct place.

Is there a way of getting a tableView to work out it's new size without starting display ?

Wrapping the whole thing in a beginAnimation commitAnimation doesn't help much either.

Update 2: This can clearly be done - see the offical twitter app for one when you pull down for updates.

Answer 1

There's really no need to sum up all rows height, the new contentSize after reloading the table is already representing that. So all you have to do is calculate the delta of contentSize height and add it to the current offset.

    ...
    CGSize beforeContentSize = self.tableView.contentSize;
    [self.tableView reloadData];
    CGSize afterContentSize = self.tableView.contentSize;

    CGPoint afterContentOffset = self.tableView.contentOffset;
    CGPoint newContentOffset = CGPointMake(afterContentOffset.x, afterContentOffset.y + afterContentSize.height - beforeContentSize.height);
    self.tableView.contentOffset = newContentOffset;
    ...

Answer 2

-(void) updateTableWithNewRowCount : (int) rowCount
{    
    //Save the tableview content offset 
    CGPoint tableViewOffset = [self.tableView contentOffset];                                                                                                            

    //Turn of animations for the update block 
    //to get the effect of adding rows on top of TableView
    [UIView setAnimationsEnabled:NO];

    [self.tableView beginUpdates];                    

    NSMutableArray *rowsInsertIndexPath = [[NSMutableArray alloc] init];        

    int heightForNewRows = 0;

    for (NSInteger i = 0; i < rowCount; i++) {

        NSIndexPath *tempIndexPath = [NSIndexPath indexPathForRow:i inSection:SECTION_TO_INSERT];
        [rowsInsertIndexPath addObject:tempIndexPath];

        heightForNewRows = heightForNewRows + [self heightForCellAtIndexPath:tempIndexPath];                        
    }

    [self.tableView insertRowsAtIndexPaths:rowsInsertIndexPath withRowAnimation:UITableViewRowAnimationNone];                                                                            

    tableViewOffset.y += heightForNewRows;                                                                                 

    [self.tableView endUpdates]; 

    [UIView setAnimationsEnabled:YES];

    [self.tableView setContentOffset:tableViewOffset animated:NO];           
}


-(int) heightForCellAtIndexPath: (NSIndexPath *) indexPath
{

    UITableViewCell *cell =  [self.tableView cellForRowAtIndexPath:indexPath];

    int cellHeight   =  cell.frame.size.height;

    return cellHeight;
}

Simply pass in the row count of the new rows to insert at the top.

Answer 3

@Dean's way of using an image cache is too hacky and I think it destroys the responsiveness of the UI.

One proper way: Use a UITableView subclass and override -setContentSize: in which you can by some means calculate how much the table view is pushed down and offset that by setting contentOffset.

This is a simplest sample code to handle the simplest situation where all insertions happen at the top of table view:

@implementation MyTableView

- (void)setContentSize:(CGSize)contentSize {
        // I don't want move the table view during its initial loading of content.
    if (!CGSizeEqualToSize(self.contentSize, CGSizeZero)) {
        if (contentSize.height > self.contentSize.height) {
            CGPoint offset = self.contentOffset;
            offset.y += (contentSize.height - self.contentSize.height);
            self.contentOffset = offset;
        }
    }
    [super setContentSize:contentSize];
}

@end