CODEWITHSUNDEEP
iosswiftuicollectionviewuicollectionviewcell
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Aggressor

Reputation: 13551

UICollectionView Cells Are Not Retaining Data --- How Can I Make Their Data Persistent?

I have a UICollectionView full of custom cell objects.

When the user selects one I draw a circle and put a number in the middle.

However as the user scrolls down and then back up, selected cells have their number data all messed up.

I dove deeper into what the problem is, and it turns out that cells are not cached, and when you scroll up with a UICollectionView, its creating new cell assets, as such data inside them is destroyed (and hence I can't store a number in them). So my thought is to put the number back each time that cell is back on the screen, but I can't tell when a cell is being displayed or not.

I've made a map which records the NSIndexPath of selected cells. However, I don't know how to tell if that path (i.e. a cell at that path) is currently being displayed.

Do you know how I can tell if a cell is being displayed from an NSIndexPath at any point along a scroll?

Here is my cell code for selection (my UICollectionView calls this).

 override public var selected: Bool
    {
        didSet
        {
            _checkMark?.hidden = !selected
            _number?.hidden = !selected
            println("did set")
        }
    }

    func setNumberText(number:UInt8)
    {
        if(_number == nil)
        {
            _number = UILabel(frame: CGRectMake(0,0,1,1))
            addSubview(_number)
        }
        _number.text = String(number)
        _number.sizeToFit()
        _number.frame = CGRectMake(_checkMark.frame.width/2 - _number.frame.width/2, _checkMark.frame.height/2 - _number.frame.height, _checkMark.frame.width, _checkMark.frame.height)
        _numberInt = number
    }

Is there some "cellAtPathIsNowOnScreen" event I can track?

Upvotes: 1

Views: 650

Answers (1)

i_am_jorf
i_am_jorf

Reputation: 54610

You're doing it wrong. Cells get reused as you scroll, and the method by which this happens is an implementation detail that you don't get to know.

Instead you should store the data about what the cell should draw, in this case the number, in the data source for your collection view. Then in - collectionView:cellForItemAtIndexPath: you can tell the cell what number to draw after you dequeue it, or clear the value if the number shouldn't be drawn for this particular cell.

Additionally, UICollectionView does have a - visibleCells method which will return an array of all the cells that are currently visible, but that's really not what you want to do.

Upvotes: 1

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