表达式类型 '@lvalue String?'当 += 到 UILabel.text 时，没有更多上下文是不明确的 [英] Expression type '@lvalue String?' is ambiguous without more context when += to UILabel.text

问题描述

当有人点击标记为 7 的按钮时，我试图将字符串7"添加到标签中.

I am trying to add the string "7" to a label when someone tabs the button labeled 7.

但是当使用 += "7" 时，它给了我错误表达式类型 '@lvalue String?'在没有更多上下文的情况下是模棱两可的"，当使用 operator = "7" 时它工作正常.为什么 += "7" 不起作用?

But when using += "7", it gives me the error "Expression type '@lvalue String?' is ambiguous without more context", when using the operator = "7" it works fine. Why doesn't += "7" work?

class NumberPadController: UIViewController {
    @IBOutlet weak var valueLabel: UILabel!

    /// set value in main vc and return to that.
    @IBAction func doneEntering(_ sender: Any) {
        guard let valueString = valueLabel.text, let valueDouble = Double(valueString), let presentingVC = self.presentingViewController as? ViewController else {
            // FIXME: Show error
            dismiss(animated: true, completion: nil)
            return
        }

        presentingVC.valuePassedFromNumPad = valueDouble
        dismiss(animated: true, completion: nil)
    }

    @IBAction func seven(_ sender: Any) {
        valueLabel.text += "7" // Expression type '@lvalue String?' is ambiguous without more context
        valueLabel.text = "a" // works fine
    }
}

推荐答案

text 属性是可选.一种安全的方法是使用 append 和 optional chaining:

The text property is optional. One way to do this safely would be to use append along with optional chaining:

valueLabel.text?.append("7")

或使用 += 和 可选链:

valueLabel.text? += "7"

如果标签是 nil，这些将安全地什么都不做.如果您希望标签为 "7" 如果它是 nil，则使用 @RickyMo 的解决方案.

If the label is nil, these would safely do nothing. If you'd like the label to be "7" if it was nil, then use @RickyMo's solution.

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