如何在swift3中以其他方法更新UITableviewcell标签？

Question

我正在尝试用其他方法更新 UITableviewcell 的标签。我还看到了很多关于 stackoverflow 的帖子，对我来说不是工作。我正在使用 Swift 3，我尝试了这个：

    let indexPath = IndexPath.init(row: 0, section: 0)
    let cell : CategoryRow = tableView.cellForRow(at: indexPath) as! CategoryRow

我收到了这个错误：

fatal error: unexpectedly found nil while unwrapping an Optional value

我在其他方法中使用它，我在 24 小时后更新 tableviewcell 标签文本。 代码在这里

var myQuoteArray = ["“If you have time to breathe you have time to meditate. You breathe when you walk. You breathe when you stand. You breathe when you lie down. – Ajahn Amaro”","We are what we repeatedly do. Excellence, therefore, is not an act but a habit. – Aristotle","The best way out is always through. – Robert Frost","“If you have time to breathe you have time to meditate. You breathe when you walk. You breathe when you stand. You breathe when you lie down. – Ajahn Amaro”","We are what we repeatedly do. Excellence, therefore, is not an act but a habit. – Aristotle","The best way out is always through. – Robert Frost"

    ]
func checkLastRetrieval(){

   //        let indexPath = IndexPath.init(row: 0, section: 0)
   //        let cell : CategoryRow = tableView.cellForRow(at: indexPath) as? CategoryRow


    let indexPath = IndexPath.init(row: 0, section: 0)
   guard let cell = tableView.cellForRow(at: indexPath) as? CategoryRow else { return }

    print("Getting Error line 320")// This is not printing on console

let userDefaults = UserDefaults.standard

if let lastRetrieval = userDefaults.dictionary(forKey: "lastRetrieval") {
if let lastDate = lastRetrieval["date"] as? NSDate {
if let index = lastRetrieval["index"] as? Int {
if abs(lastDate.timeIntervalSinceNow) > 86400 { // seconds in 24 hours
// Time to change the label
var nextIndex = index + 1

// Check to see if next incremented index is out of bounds
if self.myQuoteArray.count <= nextIndex {
// Move index back to zero? Behavior up to you...
nextIndex = 0
}



cell?.quotationLabel.text = self.myQuoteArray[nextIndex]

let lastRetrieval : [NSObject : AnyObject] = [
"date" as NSObject : NSDate(),
"index" as NSObject : nextIndex as AnyObject
]

userDefaults.set(lastRetrieval, forKey: "lastRetrieval")
userDefaults.synchronize()
}
// Do nothing, not enough time has elapsed to change labels
}
}
} else {

// No dictionary found, show first quote
cell?.quotationLabel.text = self.myQuoteArray.first!

     print("line 357")
// Make new dictionary and save to NSUserDefaults
let lastRetrieval : [NSObject : AnyObject] = [
"date" as NSObject : NSDate(),
"index" as NSObject : 0 as AnyObject
]

userDefaults.set(lastRetrieval, forKey: "lastRetrieval")
userDefaults.synchronize()
}

}

并在 ViewDidLoad() 中调用此方法

 override func viewDidLoad() {
    super.viewDidLoad()

     checkLastRetrieval()
    }

我该怎么办？ 提前致谢

Answer 1

如果请求的单元格当前不在屏幕上，则函数cellForRow(at:) 返回nil。由于您在viewDidLoad 中调用您的函数，因此屏幕上将没有单元格。这会导致崩溃，因为您强制向下转换 cellForRow(at:) 的结果。

你不应该使用强制展开或强制向下转换，除非你绝对 确定结果不能是nil 和/或如果是nil，唯一可能的操作是让您的应用崩溃。

通常您不应直接更新单元格。最好更新表格的数据模型，然后调用reloadRows(at:,with:) 并让您的数据源cellForRow(at:) 函数处理更新单元格。

如果您要直接更新单元格，请确保使用带有if let 或guard let 的条件向下转换，以避免在单元格不可见时发生崩溃。在这种情况下，您仍然需要更新数据模型，以便当单元格出现在屏幕上时单元格数据是正确的。

Answer 2

你应该使用guard let声明：

let indexPath = IndexPath(row: 0, section: 0)
guard let cell = tableView.cellForRow(at: indexPath) as? CategoryRow else { return }

如果您的cell 类型不是CategoryRow，您将从该方法返回。尝试找出为什么您的cell 类型不是CategoryRow。