Swift As

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1.从派生类转化为基类，向上转型

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class Animal {
    var a = "ani"
}
class Cat: Animal {
    var c = "cat"
}
let cat = Cat()
let animal = cat as Animal

2.消除二意性，数值类型转化

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let num1 = 25 as Int
let num2 = 25 as Float
let num3 = 25 as Double
print(num1,num2,num3)

3.switch语句进行模式匹配

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

4.0 as! 向下转型使用，强制转换类型，转换失败，会曝错误

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

5.0 as? 转化成功返回可选值类型，需要拆包使用，转化失败，返回nil

由于as?在转换失败也不会报错，如不能百分百确保转换成功，使用as?，能确定成功，可以使用as!

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let animal: Animal = Cat()
if let cat = animal as? Cat {
    print("cat is not nil")
} else {
    print("cat is nil")
}