Is there a way to update the height of a single UITableViewCell, without recalculating the height for every cell?

Question

I have a UITableView with a few different sections. One section contains cells that will resize as a user types text into a UITextView. Another section contains cells that render HTML content, for which calculating the height is relatively expensive.

Right now when the user types into the UITextView, in order to get the table view to update the height of the cell, I call

[self.tableView beginUpdates];
[self.tableView endUpdates];

However, this causes the table to recalculate the height of every cell in the table, when I really only need to update the single cell that was typed into. Not only that, but instead of recalculating the estimated height using tableView:estimatedHeightForRowAtIndexPath:, it calls tableView:heightForRowAtIndexPath: for every cell, even those not being displayed.

Is there any way to ask the table view to update just the height of a single cell, without doing all of this unnecessary work?

Update

I'm still looking for a solution to this. As suggested, I've tried using reloadRowsAtIndexPaths:, but it doesn't look like this will work. Calling reloadRowsAtIndexPaths: with even a single row will still cause heightForRowAtIndexPath: to be called for every row, even though cellForRowAtIndexPath: will only be called for the row you requested. In fact, it looks like any time a row is inserted, deleted, or reloaded, heightForRowAtIndexPath: is called for every row in the table cell.

I've also tried putting code in willDisplayCell:forRowAtIndexPath: to calculate the height just before a cell is going to appear. In order for this to work, I would need to force the table view to re-request the height for the row after I do the calculation. Unfortunately, calling [self.tableView beginUpdates]; [self.tableView endUpdates]; from willDisplayCell:forRowAtIndexPath: causes an index out of bounds exception deep in UITableView's internal code. I guess they don't expect us to do this.

I can't help but feel like it's a bug in the SDK that in response to [self.tableView endUpdates] it doesn't call estimatedHeightForRowAtIndexPath: for cells that aren't visible, but I'm still trying to find some kind of workaround. Any help is appreciated.

Answer 1

As noted, reloadRowsAtIndexPaths:withRowAnimation: will only cause the table view to ask its UITableViewDataSource for a new cell view but won't ask the UITableViewDelegate for an updated cell height.

Unfortunately the height will only be refreshed by calling:

[tableView beginUpdates];
[tableView endUpdates];

Even without any change between the two calls.

If your algorithm to calculate heights is too time consuming maybe you should cache those values. Something like:

- (CGFloat)tableView:(UITableView *)tableView
heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
    CGFloat height = [self cachedHeightForIndexPath:indexPath];

    // Not cached ?
    if (height < 0)
    {
        height = [self heightForIndexPath:indexPath];
        [self setCachedHeight:height
                 forIndexPath:indexPath];
    }

    return height;
}

And making sure to reset those heights to -1 when the contents change or at init time.

Edit:

Also if you want to delay height calculation as much as possible (until they are scrolled to) you should try implementing this (iOS 7+ only):

@property (nonatomic) CGFloat estimatedRowHeight

Providing a nonnegative estimate of the height of rows can improve the performance of loading the table view. If the table contains variable height rows, it might be expensive to calculate all their heights when the table loads. Using estimation allows you to defer some of the cost of geometry calculation from load time to scrolling time.

The default value is 0, which means there is no estimate.

Answer 2

This bug has been fixed in iOS 7.1.

In iOS 7.0, there doesn't seem to be any way around this problem. Calling [self.tableView endUpdates] causes heightForRowAtIndexPath: to be called for every cell in the table.

However, in iOS 7.1, calling [self.tableView endUpdates] causes heightForRowAtIndexPath: to be called for visible cells, and estimatedHeightForRowAtIndexPath: to be called for non-visible cells.

Answer 3

Variable row heights have a very negative impact on your table view performance. You are talking about web content that is displayed in some of the cells. If we are not talking about thousands of rows, thinking about implementing your solution with a UIWebView instead of a UITableView might be worth considering. We had a similar situation and went with a UIWebView with custom generated HTML markup and it worked beautifully. As you probably know, you have a nasty asynchronous problem when you have a dynamic cell with web content:

• After setting the content of the cell you have to
• wait until the web view in the cell is done rendering the web content,
• then you have to go into the UIWebView and - using JavaScript - ask the HTML document how high it is
• and THEN update the height of the UITableViewCell.

No fun at all and lots of jumping and jittering for the user.

If you do have to go with a UITableView, definitely cache the calculated row heights. That way it will be cheap to return them in heightForRowAtIndexPath:. Instead of telling the UITableView what to do, just make your data source fast.

Answer 4

Is there a way? The answer is no.

You can only use heightForRowAtIndexPath for this. So all you can do is make this as inexpensive as possible by for example keeping an NSmutableArray of your cell heights in your data model.

Answer 5

I had a similar issue(jumping scroll of the tableview on any change) because I had

• (CGFloat)tableView:(UITableView *)tableView estimatedHeightForRowAtIndexPath:(NSIndexPath *)indexPath { return 500; }

commenting the entire function helped.

Answer 6

Use the following UITableView method:

- (void)reloadRowsAtIndexPaths:(NSArray *)indexPaths withRowAnimation:(UITableViewRowAnimation)animation

You have to specify an NSArray of NSIndexPath which you want to reload. If you want to reload only one cell, then you can supply an NSArray that holds only one NSIndexPath.

NSIndexPath* rowTobeReloaded = [NSIndexPath indexPathForRow:1 inSection:0];
NSArray* rowsTobeReloaded = [NSArray arrayWithObjects:rowTobeReloaded, nil];
[UITableView reloadRowsAtIndexPaths:rowsTobeReloaded withRowAnimation:UITableViewRowAnimationNone];

Answer 7

The method heightForRowAtIndexPath: will always be called but here's a workaround that I would suggest.

Whenever the user is typing in the UITextView, save in a local variable the indexPath of the cell. Then, when heightForRowAtIndexPath: is called, verify the value of the saved indexPath. If the saved indexPath isn't nil, retrieve the cell that should be resized and do so. As for the other cells, use your cached values. If the saved indexPath is nil, execute your regular lines of code which in your case are demanding.

Here's how I would recommend doing it:

Use the property tag of UITextView to keep track of which row needs to be resized.

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    ...

    [textView setDelegate:self];
    [textView setTag:indexPath.row];

    ...
}

Then, in your UITextView delegate's method textViewDidChange:, retrieve the indexPath and store it. savedIndexPath is a local variable.

- (void)textViewDidChange:(UITextView *)textView
{
    savedIndexPath = [NSIndexPath indexPathForRow:textView.tag inSection:0];
}

Finally, check the value of savedIndexPath and execute what it's needed.

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (savedIndexPath != nil) {
        if (savedIndexPath == indexPath.row) {
            savedIndexPath = nil;

            // return the new height
        }
        else {

            // return cached value
        }
    }
    else {
        // your normal calculating methods...
    }
}

I hope this helps! Good luck.

Answer 8

I ended up figuring out a way to work around the problem. I was able to pre-calculate the height of the HTML content I need to render, and include the height along with the content in the database. That way, although I'm still forced to provide the height for all cells when I update the height of any cell, I don't have to do any expensive HTML rendering so it's pretty snappy.

Unfortunately, this solution only works if you've got all your HTML content up-front.