滚动时的UITableView改组

Question

I have a UITableView set up that works almost correctly, but if you scroll all the way up or down rather quickly, it shuffles the cells out of order. I set the label to reflect the index path and even when it starts out

[0,0] [0,1] [0,2] [0,3] [0,4] [0,5] [0,6]

After a quick few swipes up and down, the cells look like this

[0,1] [0,6] [0,2] [0,3] [0,4] [0,5] [0,0] [0,1]

My code is similar to the following

import UIKit

class TableViewController : UITableViewController {

    var dates: [Date]?

    override func numberOfSections(in tableView: UITableView) -> Int {
        return 1
    }

    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return dates?.count ?? 0
    }

    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        guard let cell = tableView.dequeueReusableCell(withIdentifier: tableViewCell.storyboardID, for: indexPath) as?  tableViewCell else { fatalError() }

        let date = dates?[indexPath.row]
        cell.date = date
        cell.delegate = self
        cell.cellIndex = indexPath

        return cell
    }
}

Answer 1

From Apple docs on UITableView

When the table view asks the data source to configure a cell object for display, the data source can access the queued object by sending a dequeueReusableCellWithIdentifier: message to the table view, passing in a reuse identifier. The data source sets the content of the cell and any special properties before returning it. This reuse of cell objects is a performance enhancement because it eliminates the overhead of cell creation.

So in this case, you are storing the value of indexPath in the cell propeties, "cellIndex", which is reusable by other cells. Therefore, it will randomly show the value based on previously assigned in queue.

In order to encounter this, you can either store the value to array variable in viewController, so every time the datasource want to re-create the cells, it will get from the array consistently.

Secondly, you can directly assign the indexPath to the cell ui element such as UILabel.

cell.titleLabel.text = "\(indexPath)"