不能下标'[NSObject：AnyObject]类型的值吗？索引类型为'String'

Question

ERROR:

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

CODE:

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

Answer 1

Should be:

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.

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.

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])

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

Answer 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

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