ios - Collection View (创建可拖动的 UICollectionViewCell) fatal error

Question

我正在尝试在 Collection View 中获取单元格的拖放功能。与下面的类似，只不过这是一个简单的 UIView: https://www.cocoacontrols.com/controls/lgdragdrop

为了将其扩展到 Collection View ，我依靠这个巧妙的小技巧 https://adoptioncurve.net/archives/2014/07/creating-a-draggable-uicollectionviewcell/

这是黑客“创建单元格内容的副本作为图像，然后将其拖动到手指下方的屏幕上。”

我已将代码转换为 Swift:

func handlePan(panRecognizer: UIPanGestureRecognizer){
    var locationPoint: CGPoint! = panRecognizer.locationInView(self.collectionView)

    //var locationPoint: CGPoint = panRecognizer.locationInView(self.collectionView)

    if panRecognizer.state == .Began {
        print("Pan Gesture BEGAN")

        var indexPathOfMovingCell: NSIndexPath = self.collectionView.indexPathForItemAtPoint(locationPoint)!
        var cell: UICollectionViewCell = self.collectionView.cellForItemAtIndexPath(indexPathOfMovingCell)!
        UIGraphicsBeginImageContext(cell.bounds.size)
        cell.layer.renderInContext(UIGraphicsGetCurrentContext()!)
        var cellImage: UIImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        self.movingCell = UIImageView(image: cellImage)
        //self.movingCell.userInteractionEnabled = true
        self.movingCell.center = locationPoint
        self.movingCell.alpha = 0.75
        self.collectionView.addSubview(self.movingCell)
    }
    if panRecognizer.state == .Changed {
        print("Pan Gesture CHANGED")

        self.movingCell.center = locationPoint
    }
    if panRecognizer.state == .Ended {
        print("Pan Gesture ENDED")

        self.movingCell.removeFromSuperview()
    }

}`

但是，我收到错误

Pan Gesture BEGAN
fatal error: unexpectedly found nil while unwrapping an Optional value
(lldb)

在下面一行

self.movingCell.center = locationPoint

还有其他与“展开可选值”相关的帖子，但对我的具体情况没有帮助

请帮忙...

此外，我不确定是否可以将单元格拖动到 Collection View 之外。如果没有，需要添加什么代码。

Answer 1

看起来您正在强制解开这两个:

var indexPathOfMovingCell: NSIndexPath = self.collectionView.indexPathForItemAtPoint(locationPoint)!
var cell: UICollectionViewCell = self.collectionView.cellForItemAtIndexPath(indexPathOfMovingCell)!

您不应该这样做，因为您的手指当前位置可能不在任何单元格上，这将返回零。相反，你应该做什么，将它们保留为可选。并像这样安全地打开它们:

var indexPathOfMovingCell: NSIndexPath? = self.collectionView.indexPathForItemAtPoint(locationPoint)

if let indexPath = indexPathOfMovingCell {

    if let cell = self.collectionView.cellForItemAtIndexPath(indexPath) {
        UIGraphicsBeginImageContext(cell.bounds.size)
        cell.layer.renderInContext(UIGraphicsGetCurrentContext()!)
        var cellImage: UIImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        // Rest of your code
        ...
    }
}

希望有帮助:)

我根据 Swift 教程写了一篇文章，它还包含一些改进，例如将单元格拖到 Collection View 之外。你可以去看看here .