# Swift语法对编译速度的影响

使用Swift初期会惊艳于它强大的表达力，发现原来很啰嗦的逻辑可以用一两句话写完时，工具的强大导致自己会情不自禁的写一些“聪明”的代码。但是Swift毕竟是一个婴儿时期的语言，编译器对于这些极简的代码的处理能力是一个容易被忽略的副作用。

在混编Swift两年后，工程的编译速度到了忍无可忍的程度。通过网上搜寻“如何提高Xcode编译速度”把能做的优化都做了之后，还是没有得到根本的提升。直到发现了这篇文章[Regarding Swift build time optimizations](https://link.juejin.im?target=https%3A%2F%2Fmedium.com%2F%40RobertGummesson%2Fregarding-swift-build-time-optimizations-fc92cdd91e31)，它采用了一个[开源工具](https://link.juejin.im?target=https%3A%2F%2Fgithub.com%2FRobertGummesson%2FBuildTimeAnalyzer-for-Xcode)来检测每一个Swift方法的编译时间，从而定位耗时。我通过对耗时语法的修改，成功的把一个超过4年多庞大工程的编译速度缩短了30%。

下面介绍一下这次优化中遇到的有坑的语法：

## 1. 避免在Array/Dictionary literals里使用复杂运算，如`??`(Nil Coalescing Operator), 随元素个数耗时成指数式增长

```
var a: String?
var b: String?
var c: String?
var d: String?
var e: String?
var f: String?
var g: String?

let _ = [
    "a": a ?? "", // 1 ms
    "b": b ?? "", // 3 ms
    "c": c ?? "", // 11 ms
    "d": d ?? "", // 48 ms
    "e": e ?? "", // 274 ms
    "f": f ?? "", // 1339 ms
    "g": g ?? "", // 6550 ms
]
```

**解决方案:** 用传统的键值对赋值

```
var dict = [String: String]()
dict["a"] = a ?? "" // 30 ms
dict["b"] = b ?? "" // 27 ms
dict["c"] = c ?? "" // 25 ms
dict["d"] = d ?? "" // 27 ms
dict["e"] = e ?? "" // 26 ms
dict["f"] = f ?? "" // 26 ms
dict["g"] = g ?? "" // 28 ms
```

## 2. 浮点数参与的数学函数后追加算数运算, 耗时增长幅度匪夷所思

```
min(1, 2.0)
    + 1 // 10 ms
    + 2 // 35 ms
    + 3 // 352 ms
    + 4 // 6092 ms
```

**解决方案:** 数学函数独立运算，用结果做算数运算

```
let a = min(1, 2.0)
a + 1 + 2 + 3 + 4 // 11 ms
```

## 3. 避免在有??的情况下使用 + 来拼接字符串

```
var a: String?

let _ = (a ?? "")
    + "1" // 27 ms
    + "2" // 101 ms
    + "3" // 1043 ms
    + "4" // 编译超时
```

**解决方案:** 采用`\()`(String Interpolation)

```
let _ = "\(a ?? "")1234" // 73 ms
```

## 另外介绍一个hack的设置

在User-Defined里添加`SWIFT_WHOLE_MODULE_OPTIMIZATION = YES`可以在debug选择Onone的前提下[享受到优化](https://link.juejin.im/?target=https%3A%2F%2Fjobs.zalando.com%2Ftech%2Fblog%2Fimproving-swift-compilation-times-from-12-to-2-minutes%2F)


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