Swift CollectionView可重用单元注册

Question

I have a Problem with a CollectionView inside a UIViewcontroller . I get my data from a JSON and in special conditions the cells don't seem to be reused properly, and I think there is something wrong with the way I've setup my Collectionview.

A overview:

My MyCollectionViewCell.swift :

import UIKit

class MyCollectionViewCell: UICollectionViewCell {

    @IBOutlet weak var bottomLabel: UILabel!
    @IBOutlet weak var topLabel: UILabel!

}

As you can see I have two labels in my Cell. And Parts of my Viewcontroller:

import UIKit

private let reuseIdentifier = "colcel"

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate {

    override func viewDidLoad() {

        super.viewDidLoad()

        // Register cell classes
        //collectionView.register(MyCollectionViewCell.self, forCellWithReuseIdentifier: reuseIdentifier)

        //SOME JSON PARSING HAPPENS HERE

        collectionView.delegate = self
        collectionView.dataSource = self
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let colcel = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! MyCollectionViewCell

        colcel.contentView.layer.cornerRadius = 5.0
        colcel.layer.shadowColor = UIColor.darkGray.cgColor
        colcel.layer.shadowOffset = CGSize(width: 12, height: 4.0)
        colcel.layer.shadowRadius = 12.0
        colcel.layer.masksToBounds = false

        return colcel
    }
}

Now when I try to use for Example:

colcel.topLabel.text = "someText"

I get:

fatal error: unexpectedly found nil while unwrapping an Optional value

When I remove the collectionview.register in the viewDidLoad() it works. Now is the question is my register action not neccesary or is it just wrong?

//edit.

My Cell REordering:

In the viewDidLoad() is this:

//Long Pressure for reordering
let longPressGesture = UILongPressGestureRecognizer(target: self, action: #selector(ViewController.handleLongGesture))
self.collectionView.addGestureRecognizer(longPressGesture)

This is the corresponding function:

func handleLongGesture(gesture: UILongPressGestureRecognizer) {

    switch(gesture.state) {

    case UIGestureRecognizerState.began:
        guard let selectedIndexPath = self.collectionView.indexPathForItem(at: gesture.location(in: self.collectionView)) else {
            break
        }
        collectionView.beginInteractiveMovementForItem(at: selectedIndexPath)
    case UIGestureRecognizerState.changed:
        collectionView.updateInteractiveMovementTargetPosition(gesture.location(in: gesture.view!))
    case UIGestureRecognizerState.ended:
        collectionView.endInteractiveMovement()
    default:
        collectionView.cancelInteractiveMovement()
    }
}

and I have this function:

func collectionView(_ collectionView: UICollectionView,
                    moveItemAt sourceIndexPath: IndexPath,
                    to destinationIndexPath: IndexPath)
{
    // Update data model to reflect the change
    let temp = devArray[sourceIndexPath.row]
    devArray[sourceIndexPath.row] = devArray[destinationIndexPath.row]
    devArray[destinationIndexPath.row] = temp
}

when I run

self.collectionView.reloadData()

sometimes the positions jump

Answer 1

If you have designed your storyboard with a prototype cell with the appropriate base class, you should not also register the class. The storyboard prototype cell does all of that for you.

The only time you manually register a class is when you are creating the cell programmatically. In that case, you should remove the prototype cell, but your custom class has to create the subviews itself and hook up the outlets (no longer @IBOutlet references) programmatically.

In terms of why you'd use prototype cells vs. programmatically creating the cells, it's just much easier to use prototype cells. You have to write less code. If you use cell prototypes, storyboards do all of this quite gracefully, so I see no benefit in going old-school and registering the class and programmatically creating the cells manually. It's just how we used to do it before we had prototype cells. But using storyboard vs programmatically created cells shouldn't affect the cell reuse logic.

You said:

... in special conditions the cells don't seem to be reused properly.

You should show us in what case the cells aren't reused. It's worth noting that when scrolling extremely quickly, cells aren't reused immediately after they scroll out of view. There's a little bit of lag before they're reused. If that's the scenario you're seeing that behavior, then, yes, I'm not surprised by that behavior. If that's not where you're seeing it not successfully dequeue/reuse cells, then share details regarding when and where you're seeing this behavior.

---

Regarding your separate reordering problem, the collectionView(_:moveItemAt:to:) is swapping the source and the destination. But what you want is to remove and insert :

override func collectionView(_ collectionView: UICollectionView, moveItemAt sourceIndexPath: IndexPath, to destinationIndexPath: IndexPath) {
    let value = devArray.remove(at: sourceIndexPath.item)
    devArray.insert(value, at: destinationIndexPath.item)
}

Because you're swapping, the one cell in question looks fine (so everything looks OK when you finish your drag), but all of the other cells in-between will be wrong. And you won't see that until you reload the collection view and/or scroll away and then back. By doing remove and insert , instead, the model should stay in sync.

Answer 2

请检查UICollectionViewCell属性上的Custom Class，确保值为MyCollectionViewCell - > ScreenShot