UITableViewCell when are they deallocated?

Question

Level 1

19 points

UITableViewCell when are they deallocated?

I have the following pretty standard code which returns a UITableViewCell to display in a table view. I can see the code happily creating and reusing instances when the table is scrolled. But when are the table view cell instances deallocated? I know that they are autoreleased but the cells never appear have their dealloc method called.

Any illumination on the subject would be appreciated.

Thanks, Dave.


- (UITableViewCell *) createTableViewCell:(UITableView *)aTableView
{
 MYTableViewCell *aTableViewCell;
 
 aTableViewCell=(MYTableViewCell *)[aTableView dequeueReusableCellWithIdentifier:@"UITableViewCell"];
 
 if (aTableViewCell==nil)
 {
 
#ifdef MB_DEBUG
 
 NSLog(@"Created new MYTableViewCell ");
#endif
 aTableViewCell=[[[MYTableViewCell alloc] initWithFrame:CGRectZero reuseIdentifier:@"MYTableViewCell"] autorelease];
 }
 else
 {
#ifdef MB_DEBUG
 
 NSLog(@"Reused MYTableViewCell %@ ",aTableViewCell);
 
#endif
 }
 
 return aTableViewCell;
}

Posted on Apr 28, 2009 1:03 PM

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Answer 1

K T

Level 7

23,889 points

Apr 28, 2009 1:21 PM in response to Dave iMac

AFAIK, (UITableView)-cells are dequeued and required rather than dealloc'd...

From UITableView.h:

UITableView.h: - (UITableViewCell *)dequeueReusableCellWithIdentifier:(NSString *)identifier; // Used by the delegate to acquire an already allocated cell, in lieu of allocating a new one.

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Answer 2

Dave iMac Author

Level 1

19 points

Apr 28, 2009 3:17 PM in response to K T

Thanks for this. I put some debugging code into my dealloc method on MYTableViewCell but it never appears to be called which (at some point) I would have thought it should have been. It is almost as if the table view is retaining a set of this table view cells but not releasing them...?

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Answer 3

K T

Level 7

23,889 points

Apr 28, 2009 3:49 PM in response to K T

K T wrote:
AFAIK, (UITableView)-cells are dequeued and required rather than dealloc'd...

Sorry, should read '...are dequeued and re-acquired...'

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Answer 4

Dave iMac Author

Level 1

19 points

Apr 28, 2009 4:38 PM in response to K T

I'm probably missing something here but the table view cells are not being deallocated even when the table view they belong too is deallocated.... They must get deallocated at some point?

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Answer 5

K T

Level 7

23,889 points

Apr 28, 2009 5:08 PM in response to Dave iMac

A cell doesn't exist unless it's visible on the screen. Cells are allocated, deallocated, and reused as they appear on, or disappear off of, the screen.

They are not objects in the same fashion as your data model...they only exist when viewed on the screen. If your list has twelve cell needs and only ten cells are currently being viewed, only ten cells exist (on the screen). Twelve data sets exist (10+2), however, and that is all you need to concern yourself with...the UI kit handles the rest.

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Answer 6

Dave iMac Author

Level 1

19 points

Apr 28, 2009 5:22 PM in response to K T

...er but when they are deallocated shouldn't they call dealloc on the table view cell at some point? I am obviously being stupid here...

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Answer 7

K T

Level 7

23,889 points

Apr 28, 2009 6:18 PM in response to Dave iMac

Have you taken a peek at any of the UIKit code....?

Do you grasp the concept that your code isn't 100% of the code at work?

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Answer 8

Dave iMac Author

Level 1

19 points

Apr 29, 2009 12:16 PM in response to K T

I have.

Most of the example Apple UITableViewCell projects which implement a sub-class of UITableViewCell implement dealloc. If they are doing this then it is reasonable to assume that they expect it to be called. I understand that the cells are cached and that this cache only extends to the number of cells required to display the visible portion of the table.

The problem I have is that those cached cells should be deallocate when the table view that cached them is deallocated. Or perhaps there is a global cache that is only purged when the app is terminated.

It appears that you are don't know anymore about how this works than I do. Thanks, Dave.

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Answer 9

K T

Level 7

23,889 points

Apr 29, 2009 12:21 PM in response to Dave iMac

It happens automatically. Why can't you accept that?

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Answer 10

Apr 29, 2009 9:56 PM in response to Dave iMac

Hi,

cells are cached by the table when they are moved off the screen. There's no global cache.

As long as you return an autoreleased cell in "cellForRowAtIndexPath" the cell will be destroyed, when the tableView is destroyed.

If it's not the case in one of your apps, than you should check if the tableView is truly destroyed or if the cell's are "overretained".

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Answer 11

Dave iMac Author

Level 1

19 points

May 9, 2009 5:39 AM in response to just.do.it

Thanks for this - my table view cells were indeed over retained.

Thanks, Dave.

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