Swift - private var/func 与 private final var/func

Question

我知道通过在 Swift 中将属性和方法设置为 final，我可以改善编译时间和应用程序的性能。最近我阅读了 Apple 博客文章:https://developer.apple.com/swift/blog/?id=27 ，它指出使属性或方法(文件)私有(private)(博客文章可以追溯到 2015 年，当时“私有(private)”意味着“文件私有(private)”)使编译器能够通过搜索源文件中的任何覆盖来推断属性或方法的“最终性”。但是“真正的”(Swift 3)私有(private)呢？编译器不必在任何地方搜索潜在的覆盖，所以我的问题是:

这两个声明之间是否存在任何性能和/或编译时间差异？

private final var foo: Int

private var bar: Int

Answer 1

标记为 private 的方法或属性隐式为 final。在 @conarch 提供的测试中，有一个副作用——调度类。

我做了和 fallow 一样的测试:

    class UseFinal {
        private final var foo = 1
    }

    class NoUseFinal {
        private var foo = 1
    }

    var start = Date()
    print(NoUseFinal())
    print(UseFinal())
    print("construct takes \(Date().timeIntervalSince(start))")

    start = Date()
    for _ in 1...1_000_000 {
        let temp = NoUseFinal()
    }
    print("noUseFinal took \(Date().timeIntervalSince(start))")

    start = Date()
    for _ in 1...1_000_000 {
        let temp = UseFinal()
    }
    print("useFinal took   \(Date().timeIntervalSince(start))")

    print("----")
    start = Date()
    for _ in 1...1_000_000 {
        let temp = UseFinal()
    }
    print("useFinal took   \(Date().timeIntervalSince(start))")
    start = Date()
    for _ in 1...1_000_000 {
        let temp = NoUseFinal()
    }
    print("noUseFinal took \(Date().timeIntervalSince(start))")

我得到的结果如下:

NoUseFinal
UseFinal
----
construct takes 0.000900983810424805
noUseFinal took 0.182932019233704
useFinal took   0.182898044586182
----
useFinal took   0.183371007442474
noUseFinal took 0.182500004768372

因此 private 和 private final 调度程序的方法或属性没有区别