UITableViewCell being reused while still on-screen? - ios

Question

I have what probably seems like a really weird problem (it does to me!)
I am using a UITableView to display cells which each contain a UIWebView. I realise that this is a bad idea on the face of it, but I can't really do this any other way.
I am caching the heights of each cell when the UIWebView finishes loading, and then calling:
[self.tableView beginUpdates];
[self.tableView reloadRowsAtIndexPaths:#[cellIndexPath]
withRowAnimation:UITableViewRowAnimationNone];
[self.tableView endUpdates];
All of the germane code is in a Gist here.
I also have the UIWebViews cached in a dictionary on the data source, so it can be reused when the cell is reloaded.
This seems to sort of work, but I am encountering a lot of issues whereby the cells' contents will randomly disappear. I have added some logging into determine what's going on, and in what order, and it seems like some of the cells are being reused while they're still on-screen.
I see this in my logs while scrolling down:
2014-02-11 13:45:49.091 EApp[45936:70b] Generating cell for 1: Panning
2014-02-11 13:45:49.245 EApp[45936:70b] Generating cell for 2: Calibration
2014-02-11 13:45:50.063 EApp[45936:70b] Generating cell for 3: Aperture Priority
2014-02-11 13:45:50.063 EApp[45936:70b] Reusing cell: Stopping down
"Stopping down" in this case is a cell that is still on-screen. The "generating cell" items are logged inside the data source's cellForRowAtIndexPath and the "reusing" messages inside the cells' prepareForReuse.
Does anyone know what could be happening here? I know this seems complex.

Answer 1

The following line in your prepareForReuse is probably the culprit:
if ([self.contentWebView isDescendantOfView:self.contentWebView]) {
[self.contentWebView removeFromSuperview];
}
As the contentWebView is never a descendant of itself, it will not be removed from the cell, and the contentView will contain two webviews after the cellForRowAtIndexPath:
You probably meant to say:
if ([self.contentWebView isDescendantOfView:self.contentView]) {
[self.contentWebView removeFromSuperview];
}
Or simply:
[self.contentWebView removeFromSuperview];

Answer 2

One of the features/limitations of UITableView is that you don't know if, and can't depend on, a cell is being created or reused. You should always be able to handle both.
GENERALLY, when you call -reloadRowsAtIndexPaths:withRowAnimation:, you will get the cell from that indexPath to reuse. If that indexPath was on screen, it will be a cell that was on screen.

Answer 3

I don't know if it's the problem, but in the code you provided, you don't even initialize your cell...
I'm even surprise it works.
static NSString *CellIdentifier = #"FeedItemCell";
EFeedItemCell *cell = [self.tableViewController.tableView dequeueReusableCellWithIdentifier:CellIdentifier];
you should add to it :
if (!cell) {
cell = [EfeedItemCell alloc] initWithReus....];
}

Answer 4

From your code it seems that you are caching the webViews and then are adding them to cells programmatically. This can create random problems similar to what I had faced in the passed.
You must use EFeedItemCellWebView in your storyboard. Just add a UIWebView and change the class name to your custom class. And then when the data is loaded just simply change its contents in - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath;