async_std/task/
join_handle.rs

1use std::future::Future;
2use std::pin::Pin;
3
4use crate::task::{Context, Poll, Task};
5
6/// A handle that awaits the result of a task.
7///
8/// Dropping a [`JoinHandle`] will detach the task, meaning that there is no longer
9/// a handle to the task and no way to `join` on it.
10///
11/// Created when a task is [spawned].
12///
13/// [spawned]: fn.spawn.html
14#[derive(Debug)]
15pub struct JoinHandle<T> {
16    handle: Option<InnerHandle<T>>,
17    task: Task,
18}
19
20#[cfg(not(target_os = "unknown"))]
21type InnerHandle<T> = async_global_executor::Task<T>;
22#[cfg(target_arch = "wasm32")]
23type InnerHandle<T> = futures_channel::oneshot::Receiver<T>;
24
25impl<T> JoinHandle<T> {
26    /// Creates a new `JoinHandle`.
27    pub(crate) fn new(inner: InnerHandle<T>, task: Task) -> JoinHandle<T> {
28        JoinHandle {
29            handle: Some(inner),
30            task,
31        }
32    }
33
34    /// Returns a handle to the underlying task.
35    ///
36    /// # Examples
37    ///
38    /// ```
39    /// # async_std::task::block_on(async {
40    /// #
41    /// use async_std::task;
42    ///
43    /// let handle = task::spawn(async {
44    ///     1 + 2
45    /// });
46    /// println!("id = {}", handle.task().id());
47    /// #
48    /// # })
49    pub fn task(&self) -> &Task {
50        &self.task
51    }
52
53    /// Cancel this task.
54    #[cfg(not(target_os = "unknown"))]
55    pub async fn cancel(mut self) -> Option<T> {
56        let handle = self.handle.take().unwrap();
57        handle.cancel().await
58    }
59
60    /// Cancel this task.
61    #[cfg(target_arch = "wasm32")]
62    pub async fn cancel(mut self) -> Option<T> {
63        let mut handle = self.handle.take().unwrap();
64        handle.close();
65        handle.await.ok()
66    }
67}
68
69#[cfg(not(target_os = "unknown"))]
70impl<T> Drop for JoinHandle<T> {
71    fn drop(&mut self) {
72        if let Some(handle) = self.handle.take() {
73            handle.detach();
74        }
75    }
76}
77
78impl<T> Future for JoinHandle<T> {
79    type Output = T;
80
81    #[cfg(not(target_os = "unknown"))]
82    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
83        Pin::new(&mut self.handle.as_mut().unwrap()).poll(cx)
84    }
85
86    #[cfg(target_arch = "wasm32")]
87    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
88        match Pin::new(&mut self.handle.as_mut().unwrap()).poll(cx) {
89            Poll::Ready(Ok(t)) => Poll::Ready(t),
90            Poll::Ready(Err(_)) => unreachable!("channel must not be canceled"),
91            Poll::Pending => Poll::Pending,
92        }
93    }
94}
async_std/task/join_handle.rs

async_std/task/
join_handle.rs
