Struct relm4::JoinHandle

source ·
pub struct JoinHandle<T> { /* private fields */ }
Expand description

An owned permission to join on a task (await its termination).

This can be thought of as the equivalent of std::thread::JoinHandle for a Tokio task rather than a thread. You do not need to .await the JoinHandle to make the task execute — it will start running in the background immediately.

A JoinHandle detaches the associated task when it is dropped, which means that there is no longer any handle to the task, and no way to join on it.

This struct is created by the task::spawn and task::spawn_blocking functions.

Cancel safety

The &mut JoinHandle<T> type is cancel safe. If it is used as the event in a tokio::select! statement and some other branch completes first, then it is guaranteed that the output of the task is not lost.

If a JoinHandle is dropped, then the task continues running in the background and its return value is lost.

Examples

Creation from task::spawn:

use tokio::task;

let join_handle: task::JoinHandle<_> = task::spawn(async {
    // some work here
});

Creation from task::spawn_blocking:

use tokio::task;

let join_handle: task::JoinHandle<_> = task::spawn_blocking(|| {
    // some blocking work here
});

The generic parameter T in JoinHandle<T> is the return type of the spawned task. If the return value is an i32, the join handle has type JoinHandle<i32>:

use tokio::task;

let join_handle: task::JoinHandle<i32> = task::spawn(async {
    5 + 3
});

If the task does not have a return value, the join handle has type JoinHandle<()>:

use tokio::task;

let join_handle: task::JoinHandle<()> = task::spawn(async {
    println!("I return nothing.");
});

Note that handle.await doesn’t give you the return type directly. It is wrapped in a Result because panics in the spawned task are caught by Tokio. The ? operator has to be double chained to extract the returned value:

use tokio::task;
use std::io;

#[tokio::main]
async fn main() -> io::Result<()> {
    let join_handle: task::JoinHandle<Result<i32, io::Error>> = tokio::spawn(async {
        Ok(5 + 3)
    });

    let result = join_handle.await??;
    assert_eq!(result, 8);
    Ok(())
}

If the task panics, the error is a JoinError that contains the panic:

use tokio::task;
use std::io;
use std::panic;

#[tokio::main]
async fn main() -> io::Result<()> {
    let join_handle: task::JoinHandle<Result<i32, io::Error>> = tokio::spawn(async {
        panic!("boom");
    });

    let err = join_handle.await.unwrap_err();
    assert!(err.is_panic());
    Ok(())
}

Child being detached and outliving its parent:

use tokio::task;
use tokio::time;
use std::time::Duration;

let original_task = task::spawn(async {
    let _detached_task = task::spawn(async {
        // Here we sleep to make sure that the first task returns before.
        time::sleep(Duration::from_millis(10)).await;
        // This will be called, even though the JoinHandle is dropped.
        println!("♫ Still alive ♫");
    });
});

original_task.await.expect("The task being joined has panicked");
println!("Original task is joined.");

// We make sure that the new task has time to run, before the main
// task returns.

time::sleep(Duration::from_millis(1000)).await;

Implementations§

Abort the task associated with the handle.

Awaiting a cancelled task might complete as usual if the task was already completed at the time it was cancelled, but most likely it will fail with a cancelled JoinError.

use tokio::time;

#[tokio::main]
async fn main() {
   let mut handles = Vec::new();

   handles.push(tokio::spawn(async {
      time::sleep(time::Duration::from_secs(10)).await;
      true
   }));

   handles.push(tokio::spawn(async {
      time::sleep(time::Duration::from_secs(10)).await;
      false
   }));

   for handle in &handles {
       handle.abort();
   }

   for handle in handles {
       assert!(handle.await.unwrap_err().is_cancelled());
   }
}

Checks if the task associated with this JoinHandle has finished.

Please note that this method can return false even if abort has been called on the task. This is because the cancellation process may take some time, and this method does not return true until it has completed.

use tokio::time;

let handle1 = tokio::spawn(async {
    // do some stuff here
});
let handle2 = tokio::spawn(async {
    // do some other stuff here
    time::sleep(time::Duration::from_secs(10)).await;
});
// Wait for the task to finish
handle2.abort();
time::sleep(time::Duration::from_secs(1)).await;
assert!(handle1.is_finished());
assert!(handle2.is_finished());

Trait Implementations§

Formats the value using the given formatter. Read more
Executes the destructor for this type. Read more
The type of value produced on completion.
Attempt to resolve the future to a final value, registering the current task for wakeup if the value is not yet available. Read more

Blanket Implementations§

Gets the TypeId of self. Read more
Immutably borrows from an owned value. Read more
Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Map this future’s output to a different type, returning a new future of the resulting type. Read more
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Wrap this future in an Either future, making it the right-hand variant of that Either. Read more
Convert this future into a single element stream. Read more
Flatten the execution of this future when the output of this future is itself another future. Read more
Flatten the execution of this future when the successful result of this future is a stream. Read more
Fuse a future such that poll will never again be called once it has completed. This method can be used to turn any Future into a FusedFuture. Read more
Do something with the output of a future before passing it on. Read more
Catches unwinding panics while polling the future. Read more
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Turn this future into a future that yields () on completion and sends its output to another future on a separate task. Read more
Wrap the future in a Box, pinning it. Read more
Wrap the future in a Box, pinning it. Read more
A convenience for calling Future::poll on Unpin future types.
Evaluates and consumes the future, returning the resulting output if the future is ready after the first call to Future::poll. Read more
Instruments this type with the provided Span, returning an Instrumented wrapper. Read more
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Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The output that the future will produce on completion.
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The type returned in the event of a conversion error.
Performs the conversion.
The type of successful values yielded by this future
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Poll this TryFuture as if it were a Future. Read more
Flattens the execution of this future when the successful result of this future is a Sink. Read more
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Do something with the success value of a future before passing it on. Read more
Do something with the error value of a future before passing it on. Read more
Flatten the execution of this future when the successful result of this future is another future. Read more
Flatten the execution of this future when the successful result of this future is a stream. Read more
Unwraps this future’s output, producing a future with this future’s Ok type as its Output type. Read more
Wraps a TryFuture into a type that implements Future. Read more
A convenience method for calling TryFuture::try_poll on Unpin future types. Read more
The type returned in the event of a conversion error.
Performs the conversion.
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