UITableViewCell selector setSelected:animated: gets called many times?

Question

I\'ve discovered a strange behavior with setSelected:animated: in my custom UITableViewCell class. I discovered that this function gets called multiple times if

Answer 1

What's going on is simply that the UITableView keeps track of which cells are selected in the table.

Since cells are reused when you scroll through a large table view, the table view has to keep the list of selected cells separate. Not only that, but whenever it reuses a cell it has to set its selected property, because it may be using an old, invalid selected state from a previous incarnation.

When you tap a cell, several things happen: the previously selected cell is deselected (using setSelected:). The new cell is highlighted. It's de-highlighted (at least if you tap, instead of holding your finger down), and the setSelected: method is called because the new cell was selected. That's one.

The second call is a delayed perform call, possibly from a point where the table view didn't yet know what the final state of the table would be. This call goes to _selectAllSelectedRows, which, as the name suggests, calls 'setSelected:animated:' on all selected rows. That's the second call. The reason for this is most likely to address potential issues due to the the table view being in a "transition", but who knows.

Whether it's a bug or not is up for interpretation. A fix for the duplicate calls is to simply do:

if (self.selected == selected) return;

right before the call to super (you do not have to call super if self.selected == selected).

Answer 2

It should definitely be called when table is scrolled. Cells are reused, that means, if you scroll cells in invisible areas will be reused and reinitialized, including the call to setSelected, which is basically a lightweight property setter.

If you really want to see what's happening, add a NSLog to tableView:cellForRowAtIndexPath: which will log indexPath and the returned cell.

The entire log should give you a good understanding what happens inside and why. I suppose it will be something like this (Clicked on IndexPath 1:1)

Give me cell on 1:0 (previously selected cell).
Deselect 1:0
Give me cell on 1:0 again (updated after deselection)
Deselect 1:0 (update selected flag on this cell and trigger animation)
Give me cell on 1:1
Select 1:1
Give me cell on 1:1 again (updated after selection)
Select 1:1 (update selected flag on this cell and trigger animation)

Clicking on a selected cell again is only slightly different - instead of triggering unselecting, it triggers another update.

Answer 3

Ideally you should not be calling setSelected from anywhere in your code. UIKit will take care of calling it.

If you want to show a cell/row as selected in cellForRowAtIndexPath method simply call

tableView.selectRowAtIndexPath(indexPath, animated: true, scrollPosition: .None)

for that specific indexPath.

Again never ever call setSelected explicitly unless you really mean to.

Answer 4

This is a normal behavior if you're using iPad. (it is only called once on iPhone).

In order to stop getting multiple "setSelected:YES" or multiple "setSelected:NO", all you have to do is this:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    [tableView deselectRowAtIndexPath:indexPath animated:YES];
}

Now, 1 click on any cell gives you:

• 1 entry of setSelected:YES animated:NO
• 1 entry of tableView: didSelectRowAtIndexPath:
• 1 entry of setSelected:NO animated:YES

So, calls are now stable regardless of what you do.