how to redraw non-visible UICollectionViewCell'

问题:

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:

回答1:

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.

回答2:

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.

回答3:

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.