Swift【优化Swift项目编译时间】

前言
Swift编译时间过长，是件难以忍受的事情。你能忍心看着几行看似简单的代码，慢吞吞的编译吗？╮(╯_╰)╭。下面来介绍几种减少编译时间的方法。
查看每个函数编译时间
若要对症下药，首先要抓住作祟的小鬼。有2种方法。
1、在build setting中，设置Other Swift Flags为-Xfrontend -debug-time-function-bodies。这样编译时清楚的看到每个函数的编译时间，找到耗时较多的地方。
点击右边的展开按钮，可详细查看时间。
2、Xcode plugin，https://github.com/RobertGummesson/BuildTimeAnalyzer-for-Xcode
方法
明确指明类型，不要让编译器去推断
经过测试，发现，在ViewDidLoad中添加如下代码，build的话，在我的机器上一直卡着不动。原因不明，不知道是机器还是xcode问题，还是本身就很慢。但是在原文章说，耗时630ms。总之看起来是比较耗时的。
let myCompany = [   "employees": [        "employee 1": ["attribute": "value"],        "employee 2": ["attribute": "value"],        "employee 3": ["attribute": "value"],        "employee 4": ["attribute": "value"],        "employee 5": ["attribute": "value"],        "employee 6": ["attribute": "value"],        "employee 7": ["attribute": "value"],        "employee 8": ["attribute": "value"],        "employee 9": ["attribute": "value"],        "employee 10": ["attribute": "value"],        "employee 11": ["attribute": "value"],        "employee 12": ["attribute": "value"],        "employee 13": ["attribute": "value"],        "employee 14": ["attribute": "value"],        "employee 15": ["attribute": "value"],        "employee 16": ["attribute": "value"],        "employee 17": ["attribute": "value"],        "employee 18": ["attribute": "value"],        "employee 19": ["attribute": "value"],        "employee 20": ["attribute": "value"],    ]]


但是将类型声明之后，
let myCompany: Dictionary<String, AnyObject> = ...
ViewDidLoad消耗的时间立马降到十几ms。

4.png
nil判断
将nil判断写成if let方式解包。
class CMExpandingTextField: UITextField {    func textFieldEditingChanged() {        invalidateIntrinsicContentSize()    }    override func intrinsicContentSize() -> CGSize {        if isFirstResponder(), let text = text {            let size = text.sizeWithAttributes(typingAttributes)            // Build time: 5238.3ms            return CGSize(width: size.width + (rightView?.bounds.width ?? 0) + (leftView?.bounds.width ?? 0) + 22, height: bounds.height)             // Build time: 32.4ms        var padding: CGFloat = 22        if let rightView = rightView {            padding += rightView.bounds.width        }        if let leftView = leftView {            padding += leftView.bounds.width        }        return CGSizeMake(size.width + padding, bounds.height)        }        return super.intrinsicContentSize()    }}
array+[]
将array+[data]，变成array.append(data)
// Build time: 1250.3mslet systemOptions = [ 7, 14, 30, -1 ]let systemNames = (0...2).map{ String(format: localizedFormat, systemOptions[$0]) } + [NSLocalizedString("everything", comment: "")]// Some code in-between labelNames = Array(systemNames[0..<count]) + [systemNames.last!]// Build time: 25.5mslet systemOptions = [ 7, 14, 30, -1 ]var systemNames = systemOptions.dropLast().map{ String(format: localizedFormat, $0) }systemNames.append(NSLocalizedString("everything", comment: ""))// Some code in-betweenlabelNames = Array(systemNames[0..<count])labelNames.append(systemNames.last!)
三段式判断 a > b ? a : b
// Build time: 239.0mslet labelNames = type == 0 ? (1...5).map{type0ToString($0)} : (0...2).map{type1ToString($0)}// Build time: 16.9msvar labelNames: [String]if type == 0 {    labelNames = (1...5).map{type0ToString($0)}} else {    labelNames = (0...2).map{type1ToString($0)}}
CGFloat-->CGFloat类型转换
// Build time: 3431.7 msreturn CGFloat(M_PI) * (CGFloat((hour + hourDelta + CGFloat(minute + minuteDelta) / 60) * 5) - 15) * unit / 180// Build time: 3.0msreturn CGFloat(M_PI) * ((hour + hourDelta + (minute + minuteDelta) / 60) * 5 - 15) * unit / 180
Round()
// Build time: 1433.7mslet expansion = a — b — c + round(d * 0.66) + e// Build time: 34.7mslet expansion = a — b — c + d * 0.66 + e
总结
减少编译时间归结于几点：
1、明确指明类型，而不要让编译器去推断代码中你所使用的类型，这样会节省大部分的时间。
2、减少让编译器懵逼的代码，尽量写的简单，明了，不要绕圈子。
Whether or not you have a problem with slow build times, it is still useful to build an understanding of what confuses the compiler. I’m sure you’ll find a few surprises yourself
一句话，要善意的对待编译器，不要整各种来虐待它~ o(￣▽￣)ブ~