tower/util/boxed/
layer.rs

1use crate::util::BoxService;
2use std::{fmt, sync::Arc};
3use tower_layer::{layer_fn, Layer};
4use tower_service::Service;
5
6/// A boxed [`Layer`] trait object.
7///
8/// [`BoxLayer`] turns a layer into a trait object, allowing both the [`Layer`] itself
9/// and the output [`Service`] to be dynamic, while having consistent types.
10///
11/// This [`Layer`] produces [`BoxService`] instances erasing the type of the
12/// [`Service`] produced by the wrapped [`Layer`].
13///
14/// # Example
15///
16/// `BoxLayer` can, for example, be useful to create layers dynamically that otherwise wouldn't have
17/// the same types. In this example, we include a [`Timeout`] layer
18/// only if an environment variable is set. We can use `BoxLayer`
19/// to return a consistent type regardless of runtime configuration:
20///
21/// ```
22/// use std::time::Duration;
23/// use tower::{Service, ServiceBuilder, BoxError, util::BoxLayer};
24///
25/// fn common_layer<S, T>() -> BoxLayer<S, T, S::Response, BoxError>
26/// where
27///     S: Service<T> + Send + 'static,
28///     S::Future: Send + 'static,
29///     S::Error: Into<BoxError> + 'static,
30/// {
31///     let builder = ServiceBuilder::new()
32///         .concurrency_limit(100);
33///
34///     if std::env::var("SET_TIMEOUT").is_ok() {
35///         let layer = builder
36///             .timeout(Duration::from_secs(30))
37///             .into_inner();
38///
39///         BoxLayer::new(layer)
40///     } else {
41///         let layer = builder
42///             .map_err(Into::into)
43///             .into_inner();
44///
45///         BoxLayer::new(layer)
46///     }
47/// }
48/// ```
49///
50/// [`Layer`]: tower_layer::Layer
51/// [`Service`]: tower_service::Service
52/// [`BoxService`]: super::BoxService
53/// [`Timeout`]: crate::timeout
54pub struct BoxLayer<In, T, U, E> {
55    boxed: Arc<dyn Layer<In, Service = BoxService<T, U, E>> + Send + Sync + 'static>,
56}
57
58impl<In, T, U, E> BoxLayer<In, T, U, E> {
59    /// Create a new [`BoxLayer`].
60    pub fn new<L>(inner_layer: L) -> Self
61    where
62        L: Layer<In> + Send + Sync + 'static,
63        L::Service: Service<T, Response = U, Error = E> + Send + 'static,
64        <L::Service as Service<T>>::Future: Send + 'static,
65    {
66        let layer = layer_fn(move |inner: In| {
67            let out = inner_layer.layer(inner);
68            BoxService::new(out)
69        });
70
71        Self {
72            boxed: Arc::new(layer),
73        }
74    }
75}
76
77impl<In, T, U, E> Layer<In> for BoxLayer<In, T, U, E> {
78    type Service = BoxService<T, U, E>;
79
80    fn layer(&self, inner: In) -> Self::Service {
81        self.boxed.layer(inner)
82    }
83}
84
85impl<In, T, U, E> Clone for BoxLayer<In, T, U, E> {
86    fn clone(&self) -> Self {
87        Self {
88            boxed: Arc::clone(&self.boxed),
89        }
90    }
91}
92
93impl<In, T, U, E> fmt::Debug for BoxLayer<In, T, U, E> {
94    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
95        fmt.debug_struct("BoxLayer").finish()
96    }
97}
tower/util/boxed/layer.rs

tower/util/boxed/
layer.rs
