rust - 返回对 Arc 和 Mutex 后面的值的可变引用

Question

pub struct ForesterViewModel {
    m_tree_lines: Arc<Mutex<Vec<TreeLine>>>,
}

impl ForesterViewModel {
    pub fn new() -> ForesterViewModel {
        ForesterViewModel {
            m_tree_lines: Arc::new(Mutex::new(vec![])),
        }
    }

    pub fn get_the_forest(&mut self) -> &mut Vec<TreeLine> {
        ???????????????????????????????
    }
}

我需要帮助编写 get_the_forest功能。我尝试过很多不同的东西，但它们都返回编译错误。我需要返回对 Vec<TreeLine> 的可变引用在 self.m_tree_lines 中包裹在 Arc 和 Mutex 后面.

Answer 1

没有办法做到这一点。

您创建了一个具体的 MutexGuard 对象，当您调用 lock 时它会释放互斥锁；您不能将引用移出包含守卫的范围:

pub fn as_mut(&mut self) -> &Whatever {
  let mut guard = self.data.lock().unwrap();
  Ok(guard.deref())
  drop(guard) // <--- implicitly added here, which would invalidate the ref
}

你也不能同时返回互斥保护和引用，原因更复杂(基本上 rust 不能表达)，同样的原因它不能在单个结构中有引用和对象;查看关于 Why can't I store a value and a reference to that value in the same struct? 的讨论

...所以基本上您最好的选择是以下两件事之一:

/// Return the mutex guard itself
pub fn get_the_forest(&mut self) -> Result<MutexGuard<Vec<TreeLine>>, TreeLockError> {
    Ok(self.m_tree_lines.lock()?)
}

/// Pass a function in, which patches the mutable internal value
pub fn patch_forest(&mut self, patch: impl Fn(&mut Vec<TreeLine>)) -> Result<(), TreeLockError>{
    let mut guard = self.m_tree_lines.lock()?;
    patch(&mut guard); // <-- patch happens while guard is still alive
    Ok(())
}

完整代码:

use std::sync::{Arc, Mutex, MutexGuard};
use std::sync::PoisonError;
use std::error::Error;
use std::fmt;
use std::fmt::Formatter;
use std::ops::Deref;

#[derive(Debug, Copy, Clone)]
pub enum TreeLockError {
    FailedToLock
}

impl Error for TreeLockError {}

impl fmt::Display for TreeLockError {
    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
        write!(f, "{:?}", self)
    }
}

impl<T> From<PoisonError<T>> for TreeLockError {
    fn from(_: PoisonError<T>) -> Self {
        TreeLockError::FailedToLock
    }
}

// ---

#[derive(Debug)]
pub struct TreeLine {
    pub value: &'static str
}

pub struct ForesterViewModel {
    m_tree_lines: Arc<Mutex<Vec<TreeLine>>>,
}

impl ForesterViewModel {
    pub fn new() -> ForesterViewModel {
        ForesterViewModel {
            m_tree_lines: Arc::new(Mutex::new(vec![])),
        }
    }

    pub fn get_the_forest(&mut self) -> Result<MutexGuard<Vec<TreeLine>>, TreeLockError> {
        Ok(self.m_tree_lines.lock()?)
    }
    
    pub fn patch_forest(&mut self, patch: impl Fn(&mut Vec<TreeLine>)) -> Result<(), TreeLockError>{
        let mut guard = self.m_tree_lines.lock()?;
        patch(&mut guard);
        Ok(())
    }
}

fn main() -> Result<(), Box<dyn Error>> {
    let mut vm = ForesterViewModel::new();
    
    {
        let mut trees = vm.get_the_forest()?;
        trees.push(TreeLine{ value: "one"});
        trees.push(TreeLine{ value: "two"});
    } // <--- Drop the mutable reference here so you can get it again later
    
    // Patch
    vm.patch_forest(|trees| {
        trees.push(TreeLine{ value: "three"}); 
    });
    
    // ...
    let trees = vm.get_the_forest()?;
    println!("{:?}", trees.deref());
    
    Ok(())
}