Rust 的 闭包（closures）是可以保存进变量或作为参数传递给其他函数的匿名函数。可以在一个地方创建闭包，然后在不同的上下文中执行闭包运算。不同于函数，闭包允许捕获调用者作用域中的值。我们将展示闭包的这些功能如何复用代码和自定义行为。

定义一个闭包

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let expensive_closure = |num| {
        println!("calculating slowly...");
        thread::sleep(Duration::from_secs(2));
        num
    };

官方例子：

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use std::thread;
use std::time::Duration;

fn main() {
    generate_workout(25, 43);
}

// fn simulated_expensive_calculation(intensity: u32) -> u32 {
//     println!("calculating slowly...");
//     thread::sleep(Duration::from_secs(2));

//     intensity
// }

fn generate_workout(intensity: u32, random_number: u32) {
    let expensive_closure = |num| {
        println!("calculating slowly...");
        thread::sleep(Duration::from_secs(2));
        num
    };
    // let expensive_result = simulated_expensive_calculation(intensity);

    if intensity < 25 {
        println!("Today, do {} pushups!", expensive_closure(intensity));
        println!("Next, do {} situps!", expensive_closure(intensity));
    } else {
        if random_number == 3 {
            println!("Take a break today! Remember to stay hydrated!");
        } else {
            println!("Today, run for {} minutes!", expensive_closure(intensity));
        }
    }
}

闭包类型推断和注解

在’|’符号中添加闭包参数的类型和 ‘{‘ 前添加返回值增加类型。这类似与函数中的类型注解。

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let expensive_closure = |num: u32| -> u32 {
        println!("calculating slowly...");
        thread::sleep(Duration::from_secs(2));
        num
    };

几个行为相同的闭包：

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fn  add_one_v1   (x: u32) -> u32 { x + 1 }
let add_one_v2 = |x: u32| -> u32 { x + 1 };
let add_one_v3 = |x|             { x + 1 }; // 没有添加闭包参数类型和返回值类型，在没有使用时会编译出错
let add_one_v4 = |x|               x + 1  ;

带有泛型和 Fn trait 的闭包

例子：

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use std::thread;
use std::time::Duration;

struct Cacher<T>
where
    T: Fn(u32) -> u32,
{
    calculation: T,
    value: Option<u32>,
}

impl<T> Cacher<T>
where
    T: Fn(u32) -> u32,
{
    fn new(calculation: T) -> Cacher<T> {
        Cacher {
            calculation,
            value: None,
        }
    }

    fn value(&mut self, arg: u32) -> u32 {
        match self.value {
            Some(v) => {
                if v == arg {
                    v
                } else {
                    let v = (self.calculation)(arg);
                    self.value = Some(v);
                    v
                }
            }
            None => {
                let v = (self.calculation)(arg);
                self.value = Some(v);
                v
            }
        }
    }
}

fn main() {
    // generate_workout(22, 7);
    let mut expensive_result = Cacher::new(|num| {
        println!("calculating slowly...");
        thread::sleep(Duration::from_secs(2));
        num
    });

    println!("{}", expensive_result.value(13));
    println!("{}", expensive_result.value(13));
    println!("{}", expensive_result.value(13));
    println!("{}", expensive_result.value(13));
    println!("{}", expensive_result.value(3));
    println!("{}", expensive_result.value(3));
    println!("{}", expensive_result.value(3));
}

fn generate_workout(intensity: u32, random_number: u32) {
    let mut expensive_result = Cacher::new(|num| {
        println!("calculating slowly...");
        thread::sleep(Duration::from_secs(2));
        num
    });

    if intensity < 25 {
        println!("Today, do {} pushups!", expensive_result.value(intensity));
        println!("Next, do {} situps!", expensive_result.value(intensity));
    } else {
        if random_number == 3 {
            println!("Take a break today! Remember to stay hydrated!");
        } else {
            println!(
                "Today, run for {} minutes!",
                expensive_result.value(intensity)
            );
        }
    }
}