删除 Swift 3 中的最后一个字符

我正在创建一个简单的计算器应用程序，目前正在努力在点击按钮时删除最后一个字符。我正在使用dropLast()方法，但我不断收到错误

调用中参数 #1 缺少参数

@IBAction func onDelPressed (button: UIButton!)  {
     runningNumber = runningNumber.characters.dropLast()
     currentLbl.text = runningNumber
}

Swift 4（附录）

在 Swift 中，您可以申请dropLast()直接在String实例，不再调用.characters访问一个CharacterView of the String:

var runningNumber = "12345"
runningNumber = String(runningNumber.dropLast())
print(runningNumber) // 1234

Swift 3（原始答案）

我假设runningNumber is a String实例。在这种情况下，runningNumber.characters.dropLast()不属于类型String, but a CharacterView:

let foo = runningNumber.characters.dropLast()
print(type(of: foo)) // CharacterView

您需要使用CharacterView实例化一个String在将实例分配回类型的属性之前String, e.g.

var runningNumber = "12345"
runningNumber = String(runningNumber.characters.dropLast())
print(runningNumber) // 1234

即，对于你的情况

@IBAction func onDelPressed (button: UIButton!)  {
  runningNumber = String(runningNumber.characters.dropLast())
  currentLbl.text = runningNumber
}