结构体 Ref 

pub struct Ref<'a, T> { /* 私有字段 */ }

返回对内部值的引用。

未释放的借用会对内部值保持一个读锁。这意味着长时间存活的借用可能会导致生产者一半被阻塞。建议尽量缩短借用的生命周期。此外，如果运行在允许 !Send future 的环境中，必须确保返回的 Ref 类型不会跨越 .await 点存活，否则可能会导致死锁。

锁的优先级策略取决于底层锁实现，本类型不保证使用任何特定策略。特别是，正在等待通过 send 获取锁的生产者可能会阻塞对 borrow 的并发调用，也可能不会，例如：

Potential deadlock example

// Task 1 (on thread A)    |  // Task 2 (on thread B)
let _ref1 = rx.borrow();   |
                           |  // will block
                           |  let _ = tx.send(());
// may deadlock            |
let _ref2 = rx.borrow();   |

实现

impl<'a, T> Ref<'a, T>

pub fn has_changed(&self) -> bool

指示自上次将借用值标记为已读取以来，该借用值是否被视为已更改。

与 Receiver::has_changed() 不同，此方法在 channel 关闭时不会失败。

从 Sender 借用时，此函数将始终返回 false。

示例

use tokio::sync::watch;

let (tx, mut rx) = watch::channel("hello");

tx.send("goodbye").unwrap();
// The sender does never consider the value as changed.
assert!(!tx.borrow().has_changed());

// Drop the sender immediately, just for testing purposes.
drop(tx);

// Even if the sender has already been dropped...
assert!(rx.has_changed().is_err());
// ...the modified value is still readable and detected as changed.
assert_eq!(*rx.borrow(), "goodbye");
assert!(rx.borrow().has_changed());

// Read the changed value and mark it as seen.
{
    let received = rx.borrow_and_update();
    assert_eq!(*received, "goodbye");
    assert!(received.has_changed());
    // Release the read lock when leaving this scope.
}

// Now the value has already been marked as seen and could
// never be modified again (after the sender has been dropped).
assert!(!rx.borrow().has_changed());

trait 实现

impl<'a, T: Debug> Debug for Ref<'a, T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

使用给定的格式化器格式化此值。 更多信息

impl<T> Deref for Ref<'_, T>

type Target = T

解引用后得到的类型。

fn deref(&self) -> &T

解引用此值。