快速警卫队以空值调用字典键 [英] Swift guard else called on dictionary key with NULL value

问题描述

如果我从NSNotification返回了包含以下内容的字典

If I have a Dictionary returned from a NSNotification containing the following

print(notificationObj.object)
Optional({
    age = "<null>";
    names =     (
        David
    );
})

然后在尝试将其分配给变量时调用警卫：

Then the guard else is called when trying to assign this to a variable:

guard let categories = notificationObj.object as? [String:[String]] else {
  // Gets to here
  return
}

如何处理字典关键字为null的情况。

How can I handle the case where a Dictionary key is null.

推荐答案

您的字典中确实包含。 ..

Your dictionary does contain ...

Optional({
    age = "<null>";
    names =     (
        David
    );
})

...和。 ..

• age = ... 是 String =字符串（值是单个 String ），


• names =（。 ..）是 String = [String] （值是 String s的数组） 。

• age = ... is String = String (value is single String),
• names = ( ... ) is String = [String] (value is array of Strings).

您不能将其强制转换为 [String：[String]] ，因为第一对不适合这种类型。这就是为什么您的后卫语句达到 else 的原因。

You can't cast it to [String:[String]] because the first pair doesn't fit this type. This is the reason why your guard statement hits else.

很难回答你的问题。字典包含名称，您想要类别，名称键包含 David ，它看起来并不像类别，...至少您知道为什么 guard 命中<$ c的原因$ c> else 。

Hard to answer your question. Dictionary contains names, you want categories, names key does contain David, which doesn't look like category, ... At least you know why guard hits else.

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