Swift"as"和"as!"有什么区别？型铸造作业？

Question

为什么在Swift 中有asvs as!vs. as?类型转换？

Answer 1

as 是编译时间

as?并且as!是运行时强制转换

• as? 将施放,如果施放不可能将返回可选(零)
• as! 将转换,如果转换不可能将崩溃与运行时错误

例:

class Music { }
class Pop: Music { }
class Rock: Music { }

Pop() as Music // OK, some might disagree but Music *is* a super class of Pop
Pop() as Rock  // Compile error: 'Pop' is not convertable to 'Rock'

let pop: AnyObject = Pop()

pop as Music // Compile error: 'AnyObject' is not convertible to 'Music'

pop as? Pop // Pop
pop as! Pop // Pop
pop as? Music // Pop
pop as! Music // Pop

pop as? Rock // nil
pop as! Rock // Runtime error signal SIGABRT