不能为'[NSObject: AnyObject]类型的值加上下标?'具有'String'类型的索引'

ERROR:

错误:

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

不能为'[NSObject: AnyObject]类型的值加上下标?'具有'String'类型的索引'

CODE:

代码:

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

2 个解决方案

#1

Should be:

应该是:

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

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.

这里的问题是类型[NSObject: AnyObject]?表示一个可选类型，这意味着您试图在实质上是枚举的内容上调用下标。当您尝试这样做时，没有声明下标，所以系统会阻塞。

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.

通过添加?我们说，如果可能的话打开这个值，然后调用下标。通过这种方式，系统推断要查找类型[NSObject: AnyObject]的下标声明，一切正常。

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

你也可以用!要打开包装，但如果苹果为零，就会崩溃。另一种可能的写法是:

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

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

这样，apple就不再是可选的，它将始终具有下标语法。没有必要展开。

#2

I think it is far more clear to use optional binding so that the println is only invoked when there is an actual value to print

我认为使用可选绑定要清楚得多，只有在有实际值要打印时才调用println

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

#1