rust - 如何测试是否调用了回调闭包？

Question

考虑我的结构 WareHouse，它有一个 watch 函数，它接受一个闭包作为参数。

该结构还有一个 update 函数，它更新一个路径和一个值，然后应该调用传递给 watch 函数的闭包(如果适用)。

我如何验证当我调用 watch(callback) 后更新回调函数被触发？

我试过:

#[test]
fn calls_watcher_on_update() {
    let mut w = WareHouse::new();
    let mut called = false;
    // set a watcher 
    w.watch(move|| {
        println!("watcher called");
        called = true;
    });
    w.update("/foo", "baz").unwrap();
    assert!(called);
}

Answer 1

当您尝试移动实现 Copy 的值时, 它被复制了。所以你的闭包捕获了 called 的值它被创建的那一刻； called闭包内部和外部实际上是两个不同的变量。

您甚至应该收到警告，例如:

warning: value captured by `called` is never read

相反，考虑捕获 Cell<bool>引用。我推断你需要一个 Cell在这里是因为通过可变引用捕获需要 w在您可以读取 assert!() 中的值之前删除闭包否则你会得到一个“不能在已经借用的情况下可变借用”的错误。

use std::cell::Cell;

fn main() {
    // Doesn't work right; called is copied into the closure, not moved.
    {
        let mut called = false;
        let mut closure = move || {
            called = true;
            println!("(A) In closure, called is {called}");
        };
        closure();
        println!("(A) Outside of closure, called is {called}");
    }
    
    // Workes correctly; the closure captures a reference to the cell.
    {
        let called = Cell::new(false);
        let closure = || {
            called.set(true);
            println!("(B) In closure, called is {}", called.get());
        };
        closure();
        println!("(B) Outside of closure, called is {}", called.get());
    }
}

这将输出:

(A) In closure, called is true
(A) Outside of closure, called is false
(B) In closure, called is true
(B) Outside of closure, called is true

( Playground )