We have a scenario where we reuse the same UITableView to display different sets of records from the database as a user selects an item in a different UITableView (similar in concept to a splitviewcontroller, but all in the same view).
When the user selects a different area to view, we fetch the appropriate records from the database, update the Source with the new set of records, and call ReloadData.
The problem is that some of the cells in the UITableView still exist and are reused when switching between lists, resulting in invalid or overlapped data for various rows.
The question I have is: is there a method whereby we can tell the UITableView to discard all existing cells including those queued for reuse? I would have expected ReloadData to perform this task, but it does not.
Update with additional clarification:
Each cell in our table is composed of a variable number of subviews (we are trying to mimic a grid control). When the user selects a different list in the left-hand navigation, there is no guarantee that the subview positions (columns) in the newly selected list will overlap with those in the previous list. This is why we are looking for a method to remove the queued cells.
Since this is a situation that others may find themselves in, I will post the solution that we used, but am still interested in more "native" ways of doing this.
Here is how we solved the issue:
When the set of records being displayed is changed, we update the UITableView with a unique identifier for the current cell configuration (i.e. which list of data is the user currently viewing).
We then created a custom version of UITableViewCell and record the unique list identifier in the cell when it is created.
When we dequeue a reusable cell, we compare the view's current unique identifier with the unique identifier stored in the cell and, if they don't match, we discard the cell and create a new one.
Hopefully this will help someone down the road.
ReloadData does not clear the re-use queue. It simply triggers the callbacks to re-read the data which reloads the rows. As each cell get's loaded, if you're getting it out the re-use queue it will still be used. That's good even if the table view is re-used because the cell types are the same type especially since you use the CellIdentifier to ensure that.
But, it's kind of odd that you're overlapping data. When you get a cell out of the re-use queue, how are you putting the data in it? Are you painting the data directly on the cell? Are you adding subViews with the data (causing multiple sets of overlapped data)? Typically, a cell contains subviews like text labels and the data is set on them so there's no scenario where data gets overlapped - the subviews of cell just has their data updated ...
Your GetCell method needs to ensure that a dequeued cell is fully configured with the new data that it is going to display.
A common mistake is assuming that just dequeuing the cell for display will give you a fresh cell that you can use.
If you have done any customization with your cell, like adding views, modifies its properties (images, disclosure indicators, background colors) you must make sure in the GetCell method that is reusing a cell that every single one of those properties is properly set before returning the cell.
Related
I have a macOS application that contains a tab bar design (ie: Tweetbot). There are 4 tabs that are linked to 4 different view controllers. The initial view controller (view one) contains a NSCollectionView which displays 3 cells horizontally.
This all works fine, however when I switch to another view controller and then come back to the initial view controller, the order of the collection view changes for no reason. I am not making ANY changes to the data source (which is a NSMutableArray) and I am not adding/deleting any cells, nor am I calling reloadData. I don't understand why the order of the collection view cells keep changing.
I have done some testing and can confirm that the order of the data in my data source, does NOT change at all. So it makes no sense for the collection view, to just change the order of the cells.
Another weird issue I have noticed, is that if I limit the collection view to 2 cells, this issue does not occur. This makes me wonder whether or not, the issue is down to some sort of NSCollectionView caching method that runs in the background? Perhaps when the collection view recycles a cell, it uses the incorrect data??
I have tried lots of different solutions, nothing seems to work. Does anyone have any ideas of what I can test/try out, in order to find out what's wrong?
Alternative idea
I could use NSTableView instead, because it supports multiple columns (unlike UITableView). So I could just make an NSTableView with 3 columns. Would this approach be any worse performance wise, than a NSCollectionView (especially if I wanted to add lots of cells?).
I couldn't find out what was wrong with the collection view. So I ended up using a NSTableView with multiple columns (instead of collection view rows). I set each column to have one row (index 0), which makes the table view look exactly like the collection view I was working with.
Performance wise the table view seems to be a lot better too. Even with hundreds of columns, it's still nice and smooth when scrolling/interacting.
Is there a way to make table view not recycle UITableViewCell so after all cells are loaded, it won't ask for cells(ie tableView:cellForRow...)?
I know I can put the table view in a scroll view and make the table view to have the same size as the scroll view, so all cells will be loaded, but some of my cells have variable sizes, then I need to update the scroll view's content size after the cells' sizes are changed.
Is there any other way to do this?
update:
Sorry for the misleading, actually I mean how to make it not REMOVE cells once created, so if there are ten cells, and there won't be any cell returned by dequeueReusableCell and after ten tableView:cellForRow: it won't call it again.
Simply not use reusable identifier will make it create new cell every time as needed, even the cell at position 1 has been created but then reused for position 2.
Set the reuseIdentifier to nil.
This is much better than using a different identifier for each cell, as the table can now dispose unneeded cells quickly. If they have some identifier set, there's no way to tell if you will ever reuse them.
So you don't have to miss the reuse part completely, just for the cells you don't want to. If the new cell contains the same subviews (maybe just layout and configured differently) - it might still be better to reuse.
Just don't use a cellIdentifier, or use a different identifier for each cell.
just dont use - (id)dequeueReusableCellWithIdentifier:(NSString *)identifier
in the cellForRowAtIndexPath method and create a new cell everytime.
Other way is to give a different cell Identifier for each cell you create.
Now, If you can elaborate on why you dont want the tableview to recycle its cells (which is one of the powerful feature of tableview), may be you can find out other ways to achieve the same thing with reusing the cells..!!
Hope it helps..:)
I've got a custom UITableViewCell that allows a user to input text when in edit mode. I've noticed that on stock UITableViewCells, when I swipe left, then hit the Delete button, it's table view receives the commitEditingStyle:forRowAtIndexPath message. I'm curious where this message is coming from. As far as I can tell, individual cells don't have a reference to the table view they belong to. If I want my custom cell to send this message, how would I go about doing that? I've thought about using the superview of the cell, but it seems like there's got to be an easier way.
This method gets called automatically (if you implement the data source protocol) before adding any row or column.
If you want to know to which table view a cell is belonging, use the superview method.
I have a UITableView Controller and a UITableView. I have everything set up using the delegates etc, and it populates fine. I notice a small bug however with the following method:
:cellForRowAtIndexPath
I am noticing that this method is continually called every time I scroll the table. Even after the table is populated, it continues to call. Basically, a cell moves out of view, when it comes back in view, it is calling it again. I had NSLog print out the cell contents within that method, which is how I know it continues to call.
Should that function not just call once per cell, to populate it, then be done?
Nope. That's called every time a cell is needed, and a cell is needed every time one is rendered.
UITableView is a very clever little critter. It literally keeps only the cells it's displaying, plus a cache of literally two or three more. When the time comes to scroll a cell on that wasn't on, it can be asked for a cell from the cache, and if there is one, it gets reused.
This way HUGE data sets can be displayed in a UITableView, because there really aren't a large number of cells in memory, only the ones on the screen and a small cache.
This is correct behavior. It has to do with the dequeueing of UITableViewCells. That is why cell reuse is important. This way there are ideally a finite, small amount built in memory that continually get reused.
Check out the docs for more info and code samples.
I have an NSTableView with some cells that where the value of enabled is set through bindings. I want to be able to change the colour of the cells depending on whether the cell is enabled or not. Previously I have done this through the NSTAbleView delegate method tableView: willDisplayCell: forTableColumn: row: which up until now worked fine. I've had to switch off the table delegate though and so I need to find another way to do it. I suspect it's a problem with an obvious answer, but how do I access each cell in the table? I can get the number of rows and columns in the table, and I can cycle trough them, I'm just not sure what method to call to get the cell in column i, row j.
Generally speaking, you shouldn't be cycling through the rows and columns. Your code should be able to do the right thing, given any row or column.
One way to do it is to write your own cell class and implement the appropriate drawing and highlighting methods that you'll find in the NSCell documentation. You'd implement
– drawWithFrame:inView: and probably -highlight:withFrame:inView:
Yet another way is to subclass NSTableView, and override -preparedCellAtColumn:row: to do whatever manipulation of the cell you want just before it's drawn.
The easiest way to do what you want though, is to just use the delegate method as you were before. Why don't you want to do it that way?
Assuming you don't want to color an entire column, the easiest way is tableView:willDisplayCell:forTableColumn:row:, since there is only one cell per column (the view uses the same cell for that column in every row).
Why did you get rid of your delegate?