Swift AnyObject 類型轉換

AnyObject

官方api中AnyObject的介紹分爲三方面內容，看完相信你會有所收穫

The protocol to which all classes implicitly conform. //隱含類型確認協議

You use AnyObject when you need the flexibility of an untyped object or when you use bridged Objective-C methods and properties that return an untyped result. AnyObject can be used as the concrete type for an instance of any class, class type, or class-only protocol.
當你需要靈活的類型或者當你調用返回未知類型結果的oc方法和屬性時，你需要使用AnyObject。
AnyObject可以作爲任何類，類的類型（type），或者是類協議的有型的類型來使用。
For example:
//例如

     class FloatRef {
         let value: Float
         init(_ value: Float) {
             self.value = value
         }
     }
     let x = FloatRef(2.3)
     let y: AnyObject = x
     let z: AnyObject = FloatRef.self

AnyObject can also be used as the concrete type for an instance of a type that bridges to an Objective-C class.
Many value types in Swift bridge to Objective-C counterparts, like String and Int.
AnyObject也可以被用作爲一個oc對象的有型的類型。swift中的許多值類型和oc裏面的是相通的，比如String 和 Int。

 let s: AnyObject = "This is a bridged string." as NSString
     print(s is NSString)
     // Prints "true"

     let v: AnyObject = 100 as NSNumber
     print(type(of: v))
     // Prints "__NSCFNumber"

The flexible behavior of the AnyObject protocol is similar to
Objective-C’s id type. For this reason, imported Objective-C types
frequently use AnyObject as the type for properties, method parameters,
and return values.
AnyObject協議的靈活性與oc中的id是類似的。由於這個原因，AnyObject被頻繁地作爲oc類的屬性、方法參數的類型來使用

Casting AnyObject Instances to a Known Type //拋出AnyObject對象給一個已知類型

Objects with a concrete type of AnyObject maintain a specific dynamic type and can be cast to that type using one of the type-cast operators
(as, as?, or as!).
This example uses the conditional downcast operator (as?) to conditionally cast the s constant declared above to an instance of Swift’s String type.
固定的類型的AnyObject對象具有一個具體的動態類型，並且可以使用(as, as?, or as!)被拋向那個具體的類型。下面的例子使用(as?)拋向上面聲明的Swift中String 類型的對象s

     if let message = s as? String {
         print("Successful cast to String: \(message)")
     }
     // Prints "Successful cast to String: This is a bridged string."

If you have prior knowledge that an AnyObject instance has a particular
type, you can use the unconditional downcast operator (as!). Performing
an invalid cast triggers a runtime error.
如果你事先知道AnyObject 對象的詳細類型，可以使用 (as!)。執行無效的拋向操作會引起運行時錯誤。

  let message = s as! String
     print("Successful cast to String: \(message)")
     // Prints "Successful cast to String: This is a bridged string."

     let badCase = v as! String
     // Runtime error

Casting is always safe in the context of a switch statement.
但在switch 語句中拋向操作是不會引起運行時錯誤的。

    let mixedArray: [AnyObject] = [s, v]
     for object in mixedArray {
         switch object {
         case let x as String:
             print("'\(x)' is a String")
         default:
             print("'\(object)' is not a String")
         }
     }
     // Prints "'This is a bridged string.' is a String"
     // Prints "'100' is not a String"

Accessing Objective-C Methods and Properties //訪問OC方法和屬性

When you use AnyObject as a concrete type, you have at your disposal every @objc method and property—that is, methods and properties imported from Objective-C or marked with the @objc attribute. Because Swift can’t guarantee at compile time that these methods and properties are actually available on an AnyObject instance’s underlying type, these @objc symbols are available as implicitly unwrapped optional methods and properties, respectively.

當你使用 AnyObject 作爲一個有型的類型，你必須處理每一個帶 @objc 標記或者來自於oc的方法和屬性。因爲Swift不能保證 潛在類型的AnyObject對象的這些方法和屬性是真正可用，帶有@objc 標記的方法和屬性會分別在暗中被解綁。

This example defines an IntegerRef type with an @objc method named
getIntegerValue().
這個例子定義了一個IntegerRef ，它有一個標記了@objc 方法 getIntegerValue()

     class IntegerRef {
         let value: Int
         init(_ value: Int) {
             self.value = value
         }

         @objc func getIntegerValue() -> Int {
             return value
         }
     }

     func getObject() -> AnyObject {
         return IntegerRef(100)
     }

    let obj: AnyObject = getObject()

In the example, obj has a static type of AnyObject and a dynamic type of IntegerRef. You can use optional chaining to call the @objc method getIntegerValue() on obj safely. If you’re sure of the dynamic type of obj, you can call getIntegerValue() directly.
這個例子中 ocj有一個AnyObject靜態類型和一個IntegerRef動態類型。 你可以使用可選鏈通過 obj調用帶有 @objc標記的方法getIntegerValue()。如果你確定obj的動態類型，你可以直接調用。

  let possibleValue = obj.getIntegerValue?()
     print(possibleValue)
     // Prints "Optional(100)"

     let certainValue = obj.getIntegerValue()
     print(certainValue)
     // Prints "100"

If the dynamic type of obj doesn’t implement a getIntegerValue()
method, the system returns a runtime error when you initialize
certainValue.
Alternatively, if you need to test whether obj.getIntegerValue() exists, use optional binding before calling the method.
如果動態類型obj沒有getIntegerValue()方法，當你初始化certainValue的時候，系統會出現運行時錯誤。除非，你調用這個方法之前，先使用可選的綁定檢驗下obj.getIntegerValue()是否存在

  if let f = obj.getIntegerValue {
         print("The value of 'obj' is \(f())")
     } else {
         print("'obj' does not have a 'getIntegerValue()' method")
     }
     // Prints "The value of 'obj' is 100"