Swift 中 Optional 类型是 Swift 作为类型安全的一大亮点, 有同学(婊哥)讲他在项目中 Optional 类型存在耗时的现象
我大概的模拟了下:

let loopCounts = 10000
let parameters = [
    "name": "Just Lee"
    ] as [String: Any]

func loop() {
 
    let dateBeforePrint = Date()
    /// 循环多次
    (0..<loopCounts).forEach { _ in
        /// 1. 直接赋值
        let _ = parameters
    }

    let dateAfterPrint = Date()

    print(
        dateAfterPrint.timeIntervalSince1970 - dateBeforePrint.timeIntervalSince1970
    )

    let anotherDateBeforePrint = Date()
    /// 循环多次
    (0..<loopCounts).forEach { _ in
        /// 2. Cast to Optional Type
        let _ = parameters as? [String: String]
    }

    let anotherDateAfterPrint = Date()

    print(
        anotherDateAfterPrint.timeIntervalSince1970 - anotherDateBeforePrint.timeIntervalSince1970
    )
}

/// 多次输出做比较
(1..<10).forEach { _ in
    loop()
    print("_______________________")
}

====================== Console ======================
0.00882911682128906
0.0256490707397461
_______________________
0.00412702560424805
0.0183320045471191
_______________________
0.00543999671936035
0.0246081352233887
_______________________
0.00349092483520508
0.0193371772766113
_______________________
0.00730085372924805
0.0257163047790527
_______________________
0.00324916839599609
0.0262701511383057
_______________________
0.00549483299255371
0.0178658962249756
_______________________
0.00488090515136719
0.0260119438171387
_______________________
0.00586795806884766
0.0198431015014648
_______________________

可以看到两次耗时时间确实差了几倍
难道 Optional 类型有这么大的性能隐患吗?

于是又做了另一个测试

let anotherParameters: [String: Any]? = parameters

(0..<loopCounts).forEach { _ in
    /// 1. 直接赋值
    let _ = parameters
}

if let _ = anotherParameters {
      /// 2. Unwrapper
    if let _ = anotherParameters {
        let _ = anotherParameters!
    }
}
====================== Console ======================
0.00864982604980469
0.00584006309509277
_______________________
0.00604915618896484
0.00566792488098145
_______________________
0.00336909294128418
0.00624585151672363
_______________________
0.00316119194030762
0.00363922119140625
_______________________
0.00303220748901367
0.00542402267456055
_______________________
0.00536394119262695
0.00592207908630371
_______________________
0.00556802749633789
0.0058138370513916
_______________________
0.00321006774902344
0.00402188301086426
_______________________
0.00303983688354492
0.00353813171386719
_______________________

可以看出如果只是 Unwrapper 并不会有明显的性能损耗, Optional 本身并不会耗时, 它只是一个 enum 类型的容器, 耗时肯定的是 as? 这个关键字, 它的作用是类型转换 T => Optional<T>
我们可以模拟下:

enum WrapperBox<Value> {
    case some(Value)
}

extension WrapperBox {
    var value: Value {
        switch self {
        case .some(let v): return v
        }
    }
}

(0..<loopCounts).forEach { _ in
    /// 1. 直接赋值
    let _ = parameters
}

(0..<loopCounts).forEach { _ in
    /// 2. Cast to WapperBox Type and unwrapper
    let _ = WrapperBox.some(parameters).value
}
====================== Console ======================
0.00923871994018555
1.31787824630737
_______________________
0.00427675247192383
1.69343376159668
_______________________
0.00520586967468262
1.22949695587158
_______________________
0.00712990760803223
1.32245421409607
_______________________
0.00323200225830078
1.45691418647766
_______________________
0.0070960521697998
1.44571399688721
_______________________
0.00644111633300781
1.28946876525879
_______________________
0.00844192504882812
1.18692207336426
_______________________
0.00496792793273926
1.24417400360107
_______________________

耗时差了上百倍

因此在 someValue as? SomeType 时, 额外的耗时是因为 as? 做类型转换产生的, as as! 也是一样的