When I was implementing the metrics task using tokio, I wanted to save the result JoinHandle in a struct and I saw the type being displayed by the IDE: JoinHandle<?>
What does it mean?
When I looked the definition of the function spawn, that's the code:
pub fn spawn<F>(future: F) -> JoinHandle<F::Output>
where
F: Future + Send + 'static,
F::Output: Send + 'static,
{
// ...
In the Future trait, F::Output refers to the return value of the input function.
Now, the piece of code I have is a long running task. That is the piece of code.
let receiver_task = tokio::spawn(async move {
println!("Starting metrics receiver");
while let Some(event) = rx.recv().await {
if let Err(e) = metrics.add(&event.post_name, &event.origin) {
error!("Error writing access metric for {}: {}", &event.post_name, e);
} else {
debug!("Metric event written for {}", &event.post_name);
}
}
});
As this function returns nothing, the type is Unit, hence for this lambda I can declare my struct as:
pub struct MetricHandler {
receiver_task: JoinHandle<()>,
//...
}
And now you can do:
let receiver_task = tokio::spawn(async move {
// ...
MetricHandler {
receiver_task,
// ...
}
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