setNeedsDisplay doesn't always call drawRect

Question

I have a custom view in a custom table cell. Every time a specific property on the custom view is changed I call [self setNeedsDisplay] which redraws the view in - (void)drawRect:(CGRect)rect. That property is set in the table view delegate's tableView:cellForRowAtIndexPath:. But when the table view is larger than the screen and cells have to be reused, drawRect isn't called every time setNeedsDisplay is. Especially when I flick the table quickly . Slow scrolling works fine. This leads to the information in the first and last cells often being incorrect.

In the log I can see that normally, for every call to setNeedsDisplay there is a call to drawRect, but when I scroll the table view quickly there are more calls to setNeedsDisplay than drawRect. Surely there should be a one-to-one ratio here.

I use the same custom view in a normal UIView, and it redraws perfectly every time I call setNeedsDisplay. The problem seems isolated to table views and reusing cells.

Does anyone know what's going on here? Is there any other way to force the custom view to redraw itself?

Answer 1

You should be calling [_table_ reloadData] to redraw the table. Not [_table_cell_ setNeedsDsiplay]

Answer 2

It sounds like there are two circumstances under which the view is refreshed: if the internal value changes (and you call [self setNeedsDisplay] to force an update), and also if the tableview is requesting a cell be drawn (or reused). You're not explicitly redrawing that custom view in that situation.

According to the docs:

The table view'€™s delegate in tableView:cellForRowAtIndexPath: should always reset all content when reusing a cell.

If tableview is recycling cells, it'll hand back a cell with the old values still in place. It's up to you to make sure the content is updated. Depending on how you've set up your custom table cell, you might have to do more to make sure the view is uptodate but the simplest thing would be in the custom table cell handler to force a call to [myCustomView setNeedsDisplay].

Answer 3

setNeedsDisplay will only call drawRect: if the view is visible, otherwise it invalidates the view's layer and drawRect: will be called when it does become visible.

When scrolling through a table view, the table view will request cells in the hopes of adding them to the view. If scrolling fast enough, some cells will take too long to prepare and by the time they are ready the location they should be positioned is offscreen. In these cases, the table view discards them instead of adding them to the scroll view (and thus drawRect: will not be called)

Answer 4

I'm having the a very similar problem - I have a cell that draws all its contents inside a custom drawRect. Every time cellForRow... is called i call [cell setNeedsDisplay] and sometimes when I scroll quickly, the cell redrew seems to get called late - I will often see the cell for half a second or so before it refreshes the display.

Is there anyway to force redraw immediately rather than waiting for the run loop to realize it needs doing? Sounds like this would solve both our problems (which may indeed be the same).

Answer 5

I think I struggled with this issue for almost a week before I found the problem. Here it is, and I kid you not:

The pointer to this custom view was defined in the .m file instead of the .h file, making it a class variable instead of an instance variable.

And yes I am very, very embarrassed.