iOS / Swift：无法为UILabel文本属性分配可选字符串 [英] iOS/Swift: Can't assign optional String to UILabel text property

问题描述

UILabel有一个text属性，它是一个可选的String，但它看起来像是一个隐式解包的可选项。为什么我不能为它分配另一个可选的字符串？谢谢。

UILabel has a text property, which is an optional String, but it seems to be behaving like an implicitly unwrapped optional. Why can't I assign it another optional String? Thanks.

@IBOutlet weak var tweetContent: UILabel!

...

var unopt: String = "foo"
var opt: String? = "bar"
var opt2: String?
opt2 = opt                       //Works fine
cell.tweetContent.text? = unopt  //Works fine
cell.tweetContent.text? = opt    //Compile error: Value of optional type 'String?' not unwrapped

推荐答案

您不需要打开文本。

离开 text as String？（又名可选< String> ）

cell.tweetContent.text = unopt // Works: String implicitly wraps to Optional<String>
cell.tweetContent.text = opt   // Works: Optional<String>

展开文本转为字符串？到字符串。

cell.tweetContent.text? = unopt // Works: String
cell.tweetContent.text? = opt   // Fails: Optional<String> cannot become String

---

UPDATE

也许这里需要更多的解释。 text？比我原先想象的要差，不应该使用。

Maybe there needs to be a bit more of an explanation here. text? is worse than I originally thought and should not be used.

想想 text = value 和文字？ = value 作为函数 setText 。

text = 具有签名 func setText（value：String？）。请记住，字符串？是可选< String> 。此外，无论 text 的当前值如何，它总是被调用。

text = has the signature func setText(value: String?). Remember, String? is Optional<String>. In addition, it's always called no matter the current value of text.

text？ = 具有签名 func setText（value：String）。这是捕获，只有在 text 有值时才会调用它。

text? = has the signature func setText(value: String). Here is the catch, it's only called when text has a value.

cell.tweetContent.text = nil
cell.tweetContent.text? = "This value is not set"
assert(cell.tweetContent == nil)

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