如何在成员方法闭包中使用 struct self

Question

如何在闭包中调用方法？ get_access_token 方法可以根据self.get_base_url() 设置新的访问令牌：

fn fetch_access_token(_base_url: &String) -> String {
    String::new()
}

fn get_env_url() -> String {
    String::new()
}

pub struct App {
    pub base_url: Option<String>,
    pub access_token: Option<String>,
}

impl App {
    pub fn new() -> App {
        App {
            base_url: None,
            access_token: None,
        }
    }
    pub fn get_base_url(&mut self) -> &String {
        self.base_url.get_or_insert_with(|| get_env_url())
    }
    pub fn get_access_token(&mut self) -> &String {
        self.access_token
            .get_or_insert_with(|| fetch_access_token(self.get_base_url()))
    }
}

fn main() {}

错误：

Rust 2015

error[E0500]: closure requires unique access to `self` but `self.access_token` is already borrowed
  --> src/main.rs:26:33
   |
25 |         self.access_token
   |         ----------------- borrow occurs here
26 |             .get_or_insert_with(|| fetch_access_token(self.get_base_url()))
   |                                 ^^                    ---- borrow occurs due to use of `self` in closure
   |                                 |
   |                                 closure construction occurs here
27 |     }
   |     - borrow ends here

Rust 2018

error[E0501]: cannot borrow `self.access_token` as mutable because previous closure requires unique access
  --> src/main.rs:25:9
   |
25 | /         self.access_token
26 | |             .get_or_insert_with(|| fetch_access_token(self.get_base_url()))
   | |______________------------------_--____________________----________________^ second borrow occurs here
   |                |                  |                     |
   |                |                  |                     first borrow occurs due to use of `self` in closure
   |                |                  closure construction occurs here
   |                first borrow later used by call

error[E0500]: closure requires unique access to `self` but it is already borrowed
  --> src/main.rs:26:33
   |
24 |       pub fn get_access_token(&mut self) -> &String {
   |                               - let's call the lifetime of this reference `'1`
25 |           self.access_token
   |           -----------------
   |           |
   |  _________borrow occurs here
   | |
26 | |             .get_or_insert_with(|| fetch_access_token(self.get_base_url()))
   | |_________________________________^^____________________----________________- returning this value requires that `self.access_token` is borrowed for `'1`
   |                                   |                     |
   |                                   |                     second borrow occurs due to use of `self` in closure
   |                                   closure construction occurs here

Answer 1

将您的数据和方法拆分为更小的组件，然后您可以在self 上对各个组件进行不相交的借用：

fn fetch_access_token(_base_url: &str) -> String { String::new() }
fn get_env_url() -> String { String::new() }

#[derive(Default)]
struct BaseUrl(Option<String>);

impl BaseUrl {
    fn get(&mut self) -> &str {
        self.0.get_or_insert_with(|| get_env_url())
    }
}

#[derive(Default)]
struct App {
    base_url: BaseUrl,
    access_token: Option<String>,
}

impl App {
    fn new() -> App {
        App::default()
    }

    fn get_access_token(&mut self) -> &str {
        let base_url = &mut self.base_url;
        self.access_token
            .get_or_insert_with(|| fetch_access_token(base_url.get()))
    }
}

fn main() {}

您可以更进一步，为这两个值执行此操作：

fn fetch_access_token(_base_url: &str) -> String { String::new() }
fn get_env_url() -> String { String::new() }

#[derive(Default)]
struct BaseUrl(Option<String>);

impl BaseUrl {
    fn get(&mut self) -> &str {
        self.0.get_or_insert_with(|| get_env_url())
    }
}

#[derive(Default)]
struct AccessToken(Option<String>);

impl AccessToken {
    fn get(&mut self, base_url: &str) -> &str {
        self.0.get_or_insert_with(|| fetch_access_token(base_url))
    }
}

#[derive(Default)]
struct App {
    base_url: BaseUrl,
    access_token: AccessToken,
}

impl App {
    fn new() -> App {
        App::default()
    }

    fn get_access_token(&mut self) -> &str {
        let base_url = self.base_url.get();
        self.access_token.get(base_url)
    }
}

fn main() {}

这让你看到你可以抽象出常见的功能：

fn fetch_access_token(_base_url: &str) -> String { String::new() }
fn get_env_url() -> String { String::new() }

#[derive(Default)]
struct StringCache(Option<String>);

impl StringCache {
    fn get<F>(&mut self, f: F) -> &str
    where
        F: FnOnce() -> String,
    {
        self.0.get_or_insert_with(f)
    }
}

#[derive(Default)]
struct App {
    base_url: StringCache,
    access_token: StringCache,
}

impl App {
    fn new() -> App {
        App::default()
    }

    fn get_access_token(&mut self) -> &str {
        let base_url = self.base_url.get(get_env_url);
        self.access_token.get(|| fetch_access_token(base_url))
    }
}

fn main() {}

然后你意识到抽象可以被通用化：

fn fetch_access_token(_base_url: &str) -> String { String::new() }
fn get_env_url() -> String { String::new() }

#[derive(Default)]
struct Cache<T>(Option<T>);

impl<T> Cache<T> {
    fn get<F>(&mut self, f: F) -> &T
    where
        F: FnOnce() -> T,
    {
        self.0.get_or_insert_with(f)
    }
}

#[derive(Default)]
struct App {
    base_url: Cache<String>,
    access_token: Cache<String>,
}

impl App {
    fn new() -> App {
        App::default()
    }

    fn get_access_token(&mut self) -> &str {
        let base_url = self.base_url.get(get_env_url);
        self.access_token.get(|| fetch_access_token(base_url))
    }
}

fn main() {}

另见：

• Borrowing references to attributes in a struct
• Why is it discouraged to accept a reference to a String (&String), Vec (&Vec) or Box (&Box) as a function argument?
• The Rust Programming Language chapter on closures，它创建了这个缓存结构作为练习的一部分。

Answer 2

在Option<T> 中传递给get_or_insert_with 方法的闭包是FnOnce 类型 - 因此它消耗 或移动 捕获的变量。在这种情况下，self 被捕获，因为在闭包中使用了self.get_base_url()。然而，由于self 已经被借用，闭包不能消耗或移动self 的值以获得唯一访问权。

这可以通过使用get_or_insert 方法来规避，但无论access_token 是否为None，每次调用get_access_token 时，它都需要您执行获取访问令牌的潜在昂贵操作.

Answer 3

我会改用这样的东西：

Playground

fn fetch_access_token(base_url: &str) -> Result<String, ()> {
    let _url = format!("{}/v3/auth/token", base_url);
    // ...
    let token = String::from("test token");
    Ok(token)
}

fn get_env_url() -> String {
    String::from("http://www.test.com")
}

pub struct App {
    // private fields!
    base_url: String,
    access_token: Option<String>,
}

impl App {
    pub fn new() -> App {
        App {
            base_url: get_env_url(),
            access_token: None,
        }
    }

    /// set new base url; clears cached access token
    pub fn set_base_url(&mut self, base_url: String) {
        self.base_url = base_url;
        self.access_token = None;
    }

    pub fn get_base_url(&self) -> &str {
        &self.base_url
    }

    /// retrieve (possibly cached) access token. tries again if previous attempt failed.
    pub fn retrieve_access_token(&mut self) -> Result<&str, ()> {
        if self.access_token.is_none() {
            self.access_token = Some(fetch_access_token(&self.base_url)?);
        }
        Ok(self.access_token.as_ref().unwrap())
    }
}

fn main() {
    let mut app = App::new();
    println!("{}", app.retrieve_access_token().unwrap());
}