UICollection视图重新排序单元格使iOS Swift崩溃

Question

I want to reorder my cells of uicollection view. When i try, it works sometimes(with lags) but sometimes, my app got crashed. I searched everywhere on internet but unable to find the answer till now.

func handleLongGesture(panRecognizer: UIPanGestureRecognizer){

    let locationPoint: CGPoint! = panRecognizer.locationInView(collectionView)
    guard let selectedIndexPath : NSIndexPath = collectionView.indexPathForItemAtPoint(locationPoint) else {
        return
    }
    if panRecognizer.state == .Began{

        collectionView.beginInteractiveMovementForItemAtIndexPath(selectedIndexPath)
        indexPathSelectedItem = selectedIndexPath
    }
    else if panRecognizer.state == .Changed{

        collectionView.updateInteractiveMovementTargetPosition(locationPoint)

    }
    else if panRecognizer.state == .Ended{

        collectionView.endInteractiveMovement()
    }
}

This is the above code which i am trying. i am not able to find the bugs in the entire code. I want to let you know that i also tried to use breakpoints to find out where my app crashing and i found that sometimes control not able to go under the state "panRecognizer.state == .Ended" and i think that is the reason my app get crashing.

Answer 1

Without the crash log it's difficult to say exactly what's happening but here are a few suggestions in the meanwhile:

First:

You have a beautiful guard statement at the top of your method, I suggest you add the let locationPoint: CGPoint! = panRecognizer.locationInView(collectionView) let locationPoint: CGPoint! = panRecognizer.locationInView(collectionView) to it. This way you don't have to force unwrap and the code will be protected against this particular crash.

Second:

When you call the endInteractiveMovement() method of your collection view, it will in turn call your delegate method collectionView:moveItemAtIndexPath:toIndexPath: to let you now that you need to update your datasource and move the item there as well.

Make sure you implemented it and moved the object in question to the right place! If not, your app will crash because the datasource is not in sync with the collectionview anymore.

Third

I suggest you use a switch statement instead of a if-else to catch all other possible states, this will give you the possibility to cancel the move action (which you're not doing right now):

switch(panRecognizer.state) {

    case UIGestureRecognizerState.Began:
        // Begin movement
        collectionView.beginInteractiveMovementForItemAtIndexPath(selectedIndexPath)
    indexPathSelectedItem = selectedIndexPath

    case UIGestureRecognizerState.Changed:
        // Update movement
        collectionView.updateInteractiveMovementTargetPosition(locationPoint)

    case UIGestureRecognizerState.Ended:
        // End movement
        collectionView.endInteractiveMovement()

    default:
        collectionView.cancelInteractiveMovement()
    }

}