Swift AnyObject 类型转换

AnyObject

官方api中AnyObject的介绍分为三方面内容，看完相信你会有所收获

The protocol to which all classes implicitly conform. //隐含类型确认协议

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.
当你需要灵活的类型或者当你调用返回未知类型结果的oc方法和属性时，你需要使用AnyObject。
AnyObject可以作为任何类，类的类型（type），或者是类协议的有型的类型来使用。
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也可以被用作为一个oc对象的有型的类型。swift中的许多值类型和oc里面的是相通的，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协议的灵活性与oc中的id是类似的。由于这个原因，AnyObject被频繁地作为oc类的属性、方法参数的类型来使用

Casting AnyObject Instances to a Known Type //抛出AnyObject对象给一个已知类型

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.
固定的类型的AnyObject对象具有一个具体的动态类型，并且可以使用(as, as?, or as!)被抛向那个具体的类型。下面的例子使用(as?)抛向上面声明的Swift中String 类型的对象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 对象的详细类型，可以使用 (as!)。执行无效的抛向操作会引起运行时错误。

  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 语句中抛向操作是不会引起运行时错误的。

    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 //访问OC方法和属性

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.

当你使用 AnyObject 作为一个有型的类型，你必须处理每一个带 @objc 标记或者来自于oc的方法和属性。因为Swift不能保证 潜在类型的AnyObject对象的这些方法和属性是真正可用，带有@objc 标记的方法和属性会分别在暗中被解绑。

This example defines an IntegerRef type with an @objc method named
getIntegerValue().
这个例子定义了一个IntegerRef ，它有一个标记了@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.
这个例子中 ocj有一个AnyObject静态类型和一个IntegerRef动态类型。 你可以使用可选链通过 obj调用带有 @objc标记的方法getIntegerValue()。如果你确定obj的动态类型，你可以直接调用。

  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.
如果动态类型obj没有getIntegerValue()方法，当你初始化certainValue的时候，系统会出现运行时错误。除非，你调用这个方法之前，先使用可选的绑定检验下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"