重构代码疑问之as? as as!

  1. as 使用场景
  2. as! 使用场景
  3. as?
  4. as? XXX 和 as! XXX?的区别

1.as使用场景:

1). 向上转型 

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class Animal {}
class Cat : Animal {}
let cat = Cat()
let animal = cat as Animal

2). 消除二义性, 数值类型转换

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let num1 = 42 as CGFloat
let num2 = 42 as Int
let num3 = 42.5 as Int
let num4 = (42 / 2) as Double

3). switch语句中进行模式匹配
如果不知道一个对象是什么类型, 可以通过Switch语句来检测类型并进行相关处理.

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class Animal {}
class Cat: Animal {}
class Dog: Animal {} 
let animal = Cat()
switch animal {
case let cat as Cat:
    print("cat class")
case let dog as Dog:
    print("dog class")
default:
    print("")
}

2. as!使用场景

向下转型(Downcasting), 转换失败报runtime error

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let ani2: Animal = Cat()
let cat = ani2 as! Cat

3.as?

同as!,但转换失败返回nil, 成功返回Optinal对象

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if let cat = animal as? Cat {
    // cat is not nil
} else {
    // cat is nil
}

4. as? XXX 和 as! XXX?

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let dict = NSDictionary()
guard let swiftDict = dict as? [String:Any] else { return }
    
if let tmp = swiftDict["key"] as? String {
    print("\(tmp) is a string")
}
    
if let tmp = swiftDict["key"] as! String? {
    print("\(tmp) is a string")
}
// 编译不能通过, 因为无法确定swiftDict["key"] 向 String转是否是向上转型
/*if let tmp = swiftDict["key"] as String? {
    print("\(tmp) is a string")
}*/

两者区别在于 as? String会更安全, 当发现tmp不是String时返回nil, 否则返回一个Optinal<String>
但是 as! String? 当 tmp不是一个<Optinal>String时会运行时崩溃, 因为它使用了as!将swiftDict["key"]强制解包为一个<Optional>String, 但实际对象不为Optional<String>, 示例如下: 

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let dict = NSMutableDictionary()
dict.setValue(NSNumber(value:1), forKey: "key")
guard let swiftDict = dict as? [String:Any] else { return }
    
if let tmp = swiftDict["key"] as! String? {
    print("\(tmp) is a string")
}
    
// Error: Could not cast value of type '__NSCFNumber' (0x1017f3590) to 'NSString' (0x100dffc60).
if let tmp = swiftDict["key"] as! String? {
    print("\(tmp) is a string")
}

参考网站:
Swift - as、as!、as?三种类型转换操作符使用详解（附样例）