Swift 3.0 中的铸造答案

【问题标题】：Casting in Swift 3.0Swift 3.0 中的铸造
【发布时间】：2016-09-21 08:16:08
【问题描述】：

我在 Swift 3.0 迁移指南中找不到有关类型转换更改的任何信息。但是，我偶然发现了一些发行版。

考虑一下这个游乐场：（顺便说一句，它不能在 Xcode 7.3.1 版本的 Swift 中编译）

var data1: AnyObject?
var data2: AnyObject?
var data3: AnyObject?

var tmpAny: Any?
var tmpString = "Hello!"

tmpAny = tmpString

data1 = tmpAny as AnyObject
data2 = tmpAny as AnyObject?
data3 = tmpAny as? AnyObject // Warning "Conditional cast from 'Any?' to 'AnyObject' always succeeds

print(type(of: data1))
print(type(of: data1!))

print()

print(type(of: data2))
print(type(of: data2!))

print()

print(type(of: data3))
print(type(of: data3!))

打印出来：

Optional<AnyObject>
_SwiftValue

Optional<AnyObject>
_NSContiguousString

Optional<AnyObject>
_SwiftValue

在 Swift 3.0 中。

主要是tmpAny as AnyObject和tmpAny as AnyObject?有什么区别？

【问题讨论】：

只需更改它：tmpAny as AnyObject 和警告消失

标签： ios swift3 optional

【解决方案1】：

选角：

switch data1 {
    case 0 as Int: 
    // use 'as' operator if you want to to discover the specific type of a constant or variable that is known only to be of type Any or AnyObject.
}

在 Swift 1.2 及更高版本中，as 只能用于向上转换（或消歧）和模式匹配。 Upcasting 表示保证施法，即会施法成功）。

data1 as AnyObject? 
// means that it's an Optional Value, it may either contain an AnyObject or it may be nil

data2 = tmpAny is AnyObject // when used 'is', data2 will be true if the instance is of that subclass type and false if it is not

向下转型：

由于向下转换可能会失败，类型转换可以用? 或! 标记。

data3 = tmpAny as? AnyObject 
// returns optional value of the type you are trying to downcast to
// do this if you're not sure if it succeeds
// if data3 couldn't be AnyObject, it would assign nil to the optional 
// means when you don't know what you're downcasting, you are assuming that it is Any, but it might be AnyObject, Integer or Float...
// that's why compiler warns - casting from 'Any?' to 'AnyObject' always succeeds

data4 = tmpAny as! AnyObject 
// attempts to downcast and force-unwraps the result after, as well
// do this if you're sure it succeeds, otherwise it will cause a runtime error if a wrong class/type is set

【讨论】：