不能下标'[NSObject：AnyObject]类型的值吗？索引类型为'String' [英] Cannot subscript a value of type '[NSObject : AnyObject]?' with an index of type 'String'

问题描述

错误：

不能使用
类型索引下标类型'[NSObject：AnyObject]？'的值'String'

Cannot subscript a value of type '[NSObject : AnyObject]?' with an index of type 'String'

代码：

  func getApple(appleId: String) {
     var apples = userDefaults.dictionaryForKey("apples_array")
     println(apples[appleId])

推荐答案

应该是：

var apples = userDefaults.dictionaryForKey("apples_array")
println(apples?[appleId])

这里的问题是类型 [NSObject：AnyObject]？意味着一个可选类型，这意味着你试图调用一个基本上是enum的下标。当你尝试这样做时，没有声明下标，所以系统会窒息。

The issue here is that type [NSObject : AnyObject]? implies an optional type, which means you're attempting to call a subscript on what is essentially an enum. When you try to do this, there's no subscript declared, so the system chokes.

通过添加？我们如果可能的话，打开这个值，然后调用下标。通过这种方式，系统可以查看类型 [NSObject：AnyObject] 以获取下标声明，一切正常。

By adding the ? we're saying, unwrap this value if possible, and then call the subscript. This way the system infers to look on type [NSObject : AnyObject] for subscript declarations and everything is ok.

您也可以使用！强制解包，但如果 apples 为nil，则会崩溃。另一种可能的方法是：

You could also use ! to force an unwrap, but this will crash if apples is nil. Another possible way to write this would be:

let apples = userDefaults.dictionaryForKey("apples_array") ?? [:]
println(apples[appleId])

这样，苹果就不再了可选，它将始终具有下标语法。不需要打包。

This way, apples is no longer optional and it will always have the subscript syntax. No unwrapping necessary.

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