ios - cellForRow(在 : indexPath) returns nil Swift3

Question

我有一个 UITableView 和一个自定义单元格 IconsTableViewCell，其中包含一个 UIImageView 和一个 UILable。
如果之前选择了一行，当用户点击新行时，将取消选择前一行并且新行的标签文本颜色应该更改。
但是，当我尝试使用 indexPath 获取对当前单元格的引用时，应用程序崩溃了。在过去的几个小时里，我一直坚持这一点。

class EighthViewController: UIViewController, UITableViewDelegate,UITableViewDataSource {

let checkedImage = UIImage(named: "checked")!
let uncheckedImage = UIImage(named: "unchecked")!

struct Item {
    var name:String // name of the rows
    var selected:Bool // whether is selected or not
    var amount: Int // value of the items
}
var frequency = [
        Item(name:"Every week",selected: false, amount: 0),
        Item(name:"Every 2 weeks",selected: false, amount: 0),
        Item(name:"Every 4 weeks",selected: false, amount: 0),
        Item(name:"Once",selected: false, amount: 0),
        Item(name:"End of tenancy cleaning", selected: false, amount: 0)
    ]

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)

    // retrieve indexPathForCellSelected from UserDefaults
   if let retrievedIndexPath = UserDefaults.standard.data(forKey: indexKey) {
    if let data1 = NSKeyedUnarchiver.unarchiveObject(with: retrievedIndexPath) as? IndexPath {
    indexPathForCellSelected = data1

 /* Inform the delegate that the row has already been selected.

      When calling 'tableView:didSelectRowAtIndexPath:', it will calculate the total amount depending on the type of cleaning:
           Weekly, End of Tenancy..etc
             Call calculateTotal() function which is using `indexPathForCellSelected`to calculate the total */
           self.tableView(self.tableView, didSelectRowAt: indexPathForCellSelected!)

  // assign the indexPath retrieved to StructS.indexPath
       StructS.indexPath = indexPathForCellSelected

      // assign StructS.price to  self.frequencyTotalPrice
                self.frequencyTotalPrice = StructS.price

            // assign self.frequencyTotalPrice to FullData.finalFrequecyAmount
                FullData.finalFrequecyAmount = self.frequencyTotalPrice

       // assign a Checkmark to the row with the corresponding indexPathForCellSelected retrieved
         tableView.cellForRow(at: indexPathForCellSelected!)?.accessoryType = .checkmark

         tableView.cellForRow(at: indexPathForCellSelected!)?.imageView?.image = checkedImage

    let cell = tableView.cellForRow(at: indexPathForCellSelected!) as! IconsTableViewCell
    cell.frequencyLabel.textColor = .black

        // assign frequency[indexPath.row].name to FullData structure
            FullData.finalFrequencyName = frequency[indexPathForCellSelected!.row].name

    //assign the row as Int value to a global var so as to determine which ViewController to unwind segue in 10th ViewController
    StructS.frequencyRowSelectedEighthVC = indexPathForCellSelected!.row
  }
 }

    // handle the selection of the row so as to update the values of labels in section header. 
   // if indexPathForCellSelected == nil, select a default type of cleaning for the first time
    if indexPathForCellSelected == nil {
        // construct an indexPath for the row we want to select when no previous row was selected ( not already saved in UserDefaults)
    let rowToSelect:IndexPath = IndexPath(row: 1, section: 0)

        // select the row at `rowToSelect` indexPath. This will just register the selectd row, However,the code that you have in tableView:didSelectRowAtIndexPath: is not yet executed because the delegate for the tablewView object in the ViewController has not been called yet. 
      self.tableView.selectRow(at: rowToSelect, animated: true, scrollPosition: UITableViewScrollPosition.none)

        // inform the delegate that the row was selected
        // stackoverflow.com/questions/24787098/programmatically-emulate-the-selection-in-uitableviewcontroller-in-swift
        self.tableView(self.tableView, didSelectRowAt: rowToSelect)

        //assign the row as Int value to a global var so as to determine which ViewController to unwind segue in 10th ViewController
        StructS.frequencyRowSelectedEighthVC = rowToSelect.row
        print("the row that was selected is\(StructS.frequencyRowSelectedEighthVC) ")
    }
}


func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    return frequency.count
}


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

   // configure the cell
   func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath)
    -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "Cell") as! IconsTableViewCell
        cell.frequencyLabel.text = frequency[indexPath.row].name
        cell.frequencyLabel.textColor = .gray
        cell.iconImageView.image = uncheckedImage
         return cell
    }


    func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        if !frequency[indexPath.row].selected {
            // this avoid set initial value for the first time
            if let index = indexPathForCellSelected {
                // clear the previous cell
                frequency[index.row].selected = false
                tableView.cellForRow(at: index)?.accessoryType = .none
                tableView.cellForRow(at: index)?.imageView?.image = nil
            }
            //mark the new row
            frequency[indexPath.row].selected = true
            tableView.cellForRow(at: indexPath)?.accessoryType = .checkmark

            //assign checked image to row
            tableView.cellForRow(at: indexPath)?.imageView?.image = checkedImage

            //evaluates to nil when trying to get a reference to the cell at the selected indexPath
            let cell = tableView.cellForRow(at: indexPath) as! IconsTableViewCell   
            cell.frequencyLabel.textColor = .black

            //save indexPathForCellSelected in UserDefaults
            if indexPathForCellSelected != nil { 
                // used to check if there is a selected row in the table
                let data = NSKeyedArchiver.archivedData(withRootObject: indexPathForCellSelected!)
                UserDefaults.standard.set(data, forKey: indexKey)
                self.tableView.reloadData()
            } // end of if indexPathForCellSelected
        }
    }
} //end of class

Answer 1

• 创建属性 selectedRow。默认情况下，选择第一行。

  var selectedRow = 0
  

• 在 viewWillAppear 中从 UserDefaults 中读取选定的行并重新加载 TableView

  override func viewWillAppear(_ animated: Bool) {
      super.viewWillAppear(animated)
      selectedRow = UserDefaults.standard.integer(forKey: indexKey)
      frequency[selectedRow].selected = true
      tableView.reloadData()
  }
  

• 在 cellForRow 中根据 selected 属性设置颜色、图像和附件 View

  func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
      let cell = tableView.dequeueReusableCell(withIdentifier: "Cell" for: indexPath) as! IconsTableViewCell
      let freq = frequency[indexPath.row]
      if freq.selected {
          cell.accessoryType = .checkmark
          cell.imageView?.image = checkedImage
          cell.frequencyLabel.textColor = .gray
      } else {
          cell.accessoryType = .none
          cell.imageView?.image = uncheckedImage
          cell.frequencyLabel.textColor = .black
      }
  
      cell.frequencyLabel.text = freq.name
      return cell
  }
  

• 在 didSelectRowAt 中将实际索引路径与 selectedRow 进行比较。如果它们不相等，则将前一个选定单元格的 selected 属性设置为 false，将新选定单元格的属性设置为 true。然后将 selectedRow 设置为索引路径的行，将该行保存到 UserDefaults 并重新加载 TableView 。

  func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
  
      if indexPath.row != selectedRow {
          let previousIndexPath = IndexPath(row:selectedRow, section:0)
          frequency[selectedRow].selected = false
          frequency[indexPath.row].selected = true
          selectedRow = indexPath.row   
  
          UserDefaults.standard.set(selectedRow, forKey: indexKey)
          tableView.reloadRows(at: [indexPath, previousIndexPath], with: .none)
      }
  }