Why do UITableView actions only work on visible cells - ios

Question

I'm working with a UITableView which is a list of friends (I have 20 friends). I have, on top of those 20 friends cells, a unique cell in another section called "ALL FRIENDS" which allows me to select all my friends in one click.
When this "ALL FRIENDS" cell is selected, this code is called :
var index = 0
for friend in self.friendsList {
var indexPath = NSIndexPath(forRow: index, inSection: 1)
self.tableView.selectRowAtIndexPath(indexPath, animated: true, scrollPosition: UITableViewScrollPosition.None)
index += 1
}
Everything works fine for the visible cells. When a cell is selected the background color changes but this only works for visible cells.
Any ideas why ?

Answer 1

Because UITableView is dynamically displayed and invisible cells are not there.
UITableView basically dequeues a cell that's about to disappear and appends it where you're scrolling. To select all of the cells, you have to do it differently:
You'll have to set a global variable to true, let's say var allSelected = true.
Then call self.tableView.reloadData()
Which will be reflected in tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell by doing cell.selected = true.
That will ensure that all of the future displayed cells will be selected as well.

Answer 2

So Michal's answer is great, but I would like to give you an alternative: you can create that tableview without dequeuing and reusing cells, so the cells remain alive at all points.
You can do this by creating individual cells, and adding them to an array
var array = [UITableViewCell]
for friend in self.friendsList {
var cell = UITableViewCell()
// customize your cell however you want, or use custom nibs
array.append(cell)
}
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) {
return array[indexPath.row]
}