How can I change the properties of a cell that is not yet on the screen?

Question

I've an app that consists of a timetable (UITableView) and a daybar (UICollectionView). The user can change their selected day by one of two methods:

Select a day from the DayBar
Click the "JumpToTomorrow" Button at the bottom of the table view.

The problem with 2. right now is that when I click it in a scenario where the next day has not yet been scrolled on to the screen, the cell for that day is not selected (see below).

DayCollectionViewCell.isSelected (not being called for some reason)

    override var isSelected: Bool {
    didSet {
        self.backgroundColor        = self.getBackgroundColor(selected: isSelected)
        self.layer.borderColor      = self.getBorderColor(selected: isSelected)
        self.number.attributedText  = NSAttributedString(string: number.text!, attributes: self.getTextAttributes(selected: isSelected))
    }
}

DayBarController Functions

    override func viewDidLoad() {
    super.viewDidLoad()
    // To preserve selection between presentations
    self.clearsSelectionOnViewWillAppear = false
    setEdgeInsets()
    BroadcastManager.listen(to: PrepNotifications.JumpToTomorrowClicked, listenerId: "UpdateSelectedDay", react: updateSelectedDay)
}
    /// change the selected day and notify the app about the change
override func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    PrepGlobals.Calendar.SELECTED_DAY = indexPath.row
    BroadcastManager.broadcast(PrepNotifications.SelectedDayChanged)
    //scroll to center on selected day in case of clicked on today  
    if (PrepGlobals.Calendar.TODAY_INDEX == indexPath.row){
        self.collectionView?.scrollToItem(
            at: indexPath,
            at: .centeredHorizontally,
            animated: true
        )
    }
}

private func updateSelectedDay(_ userInfo: [AnyHashable: Any]? = nil) {
    PrepGlobals.Calendar.SELECTED_DAY += 1
    let selectedDayIndex = IndexPath(row: PrepGlobals.Calendar.SELECTED_DAY, section: 0)
    self.collectionView!.selectItem(at: selectedDayIndex, animated: true, scrollPosition: .right)
    BroadcastManager.broadcast(PrepNotifications.SelectedDayChanged)
}

score 1 · Answer 1 · answered Jun 01 '17 at 09:31

1

You should modify your collectionView(_:cellForItemAt:) to check the cell index should be selected or not and update the style there.

answered Jun 01 '17 at 09:31

Morty Choi

2,466
1
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This doesn't appear to be called for the cell I need. In the example pictured within my question `collectionView(_cellForItemAt)` is called for the cells 5 and 7. When we're looking to change the selection from 5 to 6. – Declan McKenna Jun 01 '17 at 11:57
Looks like prefetching causes this to be called prior to it appearing on the screen or being set to `selected`. – Declan McKenna Jun 01 '17 at 13:15

PMT · Accepted Answer · 2017-06-01T10:22:28.163

1

I edited my answer. Actually you have to set up isSelected manually.

override func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
      cell.isSelected = (indexPath.row == PrepGlobals.Calendar.SELECTED_DAY)
}

edited Jun 01 '17 at 10:22

answered Jun 01 '17 at 09:35

PMT

1,082
8
18

Declan McKenna · Answer 3 · 2017-06-01T13:12:50.143

1

Disabling Pre-fetching fixed this.

edited Jun 01 '17 at 13:12

answered Jun 01 '17 at 12:30

Declan McKenna

4,321
6
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72

How can I change the properties of a cell that is not yet on the screen?

3 Answers3