how to redraw non-visible UICollectionViewCell's after rotation ready for when reuse occurs?

Question

How to redraw non-visible UICollectionViewCell's ready for when reuse occurs???

One approach I thought of was per the code in the Layout Cell prepareForReuse function, however whilst it works it non-optimal as it causes more re-drawing then required.

Background: Need to trigger drawRect for cells after an orientation change that are not current visible, but pop up to be used and haven't been redraw, so so far I can only see that prepareForReuse would be appropriate. Issue is I'm re-drawing all "reuse" cells, whereas I really only want to redraw those that initially pop up that were created during the previous orientation position of the device.

ADDITIONAL INFO: So currently I'm doing this:

In ViewController:

override func viewWillLayoutSubviews() {
    // Clear cached layout attributes (to ensure new positions are calculated)
    (self.cal.collectionViewLayout as! GCCalendarLayout).resetCache()
    self.cal.collectionViewLayout.invalidateLayout()

    // Trigger cells to redraw themselves (to get new widths etc)
    for cell in self.cal?.visibleCells() as! [GCCalendarCell] {
        cell.setNeedsDisplay()
    }

    // Not sure how to "setNeedsDisplay" on non visible cells here?
}

In Layout Cell class:

override func prepareForReuse() {
    super.prepareForReuse()
    // Ensure "drawRect" is called (only way I could see to handle change in orientation
    self.setNeedsDisplay() 
    // ISSUE: It does this also for subsequent "prepareForReuse" after all
    // non-visible cells have been re-used and re-drawn, so really
    // not optimal
}

Example of what happens without the code in prepareForReuse above. Snapshot taken after an orientation change, and just after scrolling up a little bit:

cells shouldn't need to know the orientation. if you need to re-draw them when orientation changes, put that logic in the collection view instead, then let the collection view tell the cells wether they need to re-draw or not. — Simon, Oct 21 '15 at 12:32
@Simon I'm not sure in this case how to identify (from the controller) the cells that are non-visible (and will pop back into the picture when re-use occurs) to call "setsNeedsDisplay" on them? I'll add some more detail in my question — Greg, Oct 21 '15 at 19:33
the cellForRow:AtIndexPath: is called just before they becomes visible :) — Simon, Oct 22 '15 at 13:08
cellForRow is a table view method no? Not a UICollectionView — Greg, Oct 22 '15 at 21:18
you're right, it's called -cellForItemAtIndexPath: in a collectionView — Simon, Oct 23 '15 at 07:52
but here you would still need to determine whether it's a cell that came from a previous orientation first, else if you trigger a redraw here you're doing it much more than necessary still...so still the same overall — Greg, Oct 23 '15 at 10:25
You need to define your sizes in `sizeForItemAtIndexPath` delegate method of `UICollectionViewDelegateFlowLayout`. That is where you should calculate the size of your cells. Then in the `viewWillTransitionToSize` you need to invalidate your collectionview layout like this : `[self.collectionView.collectionViewLayout invalidateLayout];` All of this must be done within your view controller that contains the collection view — Marc-Alexandre Bérubé, Oct 23 '15 at 13:17
Try calling `setNeedsDisplay ` inside `layoutSubviews` of your cell class, — deoKasuhal, Oct 23 '15 at 19:43
@Marc-AlexandreBérubé I'm not using flowlayout here, it's a custom UICollectionViewLayout. — Greg, Oct 23 '15 at 23:13
@deoKaushal This causes "drawRect" to be called unnessarily (from my testing) for the normal cases of cell reuse, where a forced redraw is not required. I'm just looking for a way to detect the cases of reuse whereby the cell was cached by not showing/visible, then a rotation occurred, and then it's pulled out to be reused and display. Just in this case I want to trigger the redraw — Greg, Oct 23 '15 at 23:28

score 5 · Accepted Answer · answered Oct 24 '15 at 00:49

I think I have it now here:

import UIKit

@IBDesignable class GCCalendarCell: UICollectionViewCell {
    var prevBounds : CGRect?

    override func layoutSubviews() {
        if let prevBounds = prevBounds {
            if !( (prevBounds.width == bounds.width) && (prevBounds.height == bounds.height) ) {
                self.setNeedsDisplay()
            }
        }
    }

    override func drawRect(rect: CGRect) {
        // Do Stuff
        self.prevBounds = self.bounds
    }

}

Noted this check didn't work in "prepareForReuse" as at this time the cell had not had the rotation applied. Seems to work in "layoutSubviews" however.

This will work if you have different cell sizes after rotation. — deoKasuhal, Oct 24 '15 at 02:02
yes, have different cell sizes, so the drawing needs to be redone to accomodate — Greg, Oct 24 '15 at 06:01

score 1 · Answer 2 · answered Oct 29 '15 at 14:51

You can implement some kind of communication between the cells and the view controller holding the collection view ( protocol and delegate or passed block or even direct reference to the VC ). Then You can ask the view controller for rotation changes.

Its a bit messy, but if You have some kind of rotation tracking in Your view controller You can filter the setNeedsDisplay with a simple if statement.

score 0 · Answer 3 · answered Mar 24 '17 at 16:32

I had similar challenged updating cells that were already displayed and off the screen. While cycling through ALLL cells may not be possible - refreshing / looping through non-visible ones is. IF this is your use case - then read on. Pre - Warning - if you're adding this sort of code - explain why you're doing it. It's kind of anti pattern - but can help fix that bug and help ship your app albeit adding needless complexity. Don't use this in multiple spots in app.

Any collectionviewcell that's de-initialized (off the screen and being recylced) should be unsubscribed automatically.

Notification Pattern

let kUpdateButtonBarCell = NSNotification.Name("kUpdateButtonBarCell")

class Notificator {
     static func fireNotification(notificationName: NSNotification.Name) {
          NotificationCenter.default.post(name: notificationName, object: nil)
     }
}

extension UICollectionViewCell{
    func listenForBackgroundChanges(){
         NotificationCenter.default.removeObserver(self, name: kUpdateButtonBarCell, object: nil)
        NotificationCenter.default.addObserver(forName:kUpdateButtonBarCell, object: nil, queue: OperationQueue.main, using: { (note) in

            print( " contentView: ",self.contentView)

        })
    }
}



override func collectionView(collectionView: UICollectionView!, cellForItemAtIndexPath indexPath: NSIndexPath!) -> UICollectionViewCell! {
    let cell =  collectionView.dequeueReusableCellWithReuseIdentifier("die", forIndexPath: indexPath) as UICollectionViewCell
    cell.listenForBackgroundChanges()
    return cell
}


 // Where appropriate broadcast notification to hook into all cells past and present
 Notificator.fireNotification(notificationName: kUpdateButtonBarCell)

Delegate Pattern

It's possible to simplify this.... an exercise for the reader. just do not retain the cells (use a weak link) - otherwise you'll have memory leaks.

how to redraw non-visible UICollectionViewCell's after rotation ready for when reuse occurs?

3 Answers3

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