1 files changed, 182 insertions, 0 deletions
diff --git a/core/src/subscription.rs b/core/src/subscription.rs
new file mode 100644
index 00000000..8de6cae8
--- /dev/null
+++ b/core/src/subscription.rs
@@ -0,0 +1,182 @@
+//! Listen to external events in your application.
+
+/// A request to listen to external events.
+///
+/// Besides performing async actions on demand with [`Command`], most
+/// applications also need to listen to external events passively.
+///
+/// A [`Subscription`] is normally provided to some runtime, like a [`Command`],
+/// and it will generate events as long as the user keeps requesting it.
+///
+/// For instance, you can use a [`Subscription`] to listen to a WebSocket
+/// connection, keyboard presses, mouse events, time ticks, etc.
+///
+/// This type is normally aliased by runtimes with a specific `Input` and/or
+/// `Hasher`.
+///
+/// [`Command`]: ../struct.Command.html
+/// [`Subscription`]: struct.Subscription.html
+pub struct Subscription<Hasher, Input, Output> {
+    recipes: Vec<Box<dyn Recipe<Hasher, Input, Output = Output>>>,
+}
+
+impl<H, I, O> Subscription<H, I, O>
+where
+    H: std::hash::Hasher,
+{
+    /// Returns an empty [`Subscription`] that will not produce any output.
+    ///
+    /// [`Subscription`]: struct.Subscription.html
+    pub fn none() -> Self {
+        Self {
+            recipes: Vec::new(),
+        }
+    }
+
+    /// Creates a [`Subscription`] from a [`Recipe`] describing it.
+    ///
+    /// [`Subscription`]: struct.Subscription.html
+    /// [`Recipe`]: trait.Recipe.html
+    pub fn from_recipe(
+        recipe: impl Recipe<H, I, Output = O> + 'static,
+    ) -> Self {
+        Self {
+            recipes: vec![Box::new(recipe)],
+        }
+    }
+
+    /// Batches all the provided subscriptions and returns the resulting
+    /// [`Subscription`].
+    ///
+    /// [`Subscription`]: struct.Subscription.html
+    pub fn batch(
+        subscriptions: impl Iterator<Item = Subscription<H, I, O>>,
+    ) -> Self {
+        Self {
+            recipes: subscriptions
+                .flat_map(|subscription| subscription.recipes)
+                .collect(),
+        }
+    }
+
+    /// Returns the different recipes of the [`Subscription`].
+    ///
+    /// [`Subscription`]: struct.Subscription.html
+    pub fn recipes(self) -> Vec<Box<dyn Recipe<H, I, Output = O>>> {
+        self.recipes
+    }
+
+    /// Transforms the [`Subscription`] output with the given function.
+    ///
+    /// [`Subscription`]: struct.Subscription.html
+    pub fn map<A>(
+        mut self,
+        f: impl Fn(O) -> A + Send + Sync + 'static,
+    ) -> Subscription<H, I, A>
+    where
+        H: 'static,
+        I: 'static,
+        O: 'static,
+        A: 'static,
+    {
+        let function = std::sync::Arc::new(f);
+
+        Subscription {
+            recipes: self
+                .recipes
+                .drain(..)
+                .map(|recipe| {
+                    Box::new(Map::new(recipe, function.clone()))
+                        as Box<dyn Recipe<H, I, Output = A>>
+                })
+                .collect(),
+        }
+    }
+}
+
+impl<I, O, H> std::fmt::Debug for Subscription<I, O, H> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("Subscription").finish()
+    }
+}
+
+/// The description of a [`Subscription`].
+///
+/// A [`Recipe`] is the internal definition of a [`Subscription`]. It is used
+/// by runtimes to run and identify subscriptions. You can use it to create your
+/// own!
+///
+/// [`Subscription`]: struct.Subscription.html
+/// [`Recipe`]: trait.Recipe.html
+pub trait Recipe<Hasher: std::hash::Hasher, Input> {
+    /// The events that will be produced by a [`Subscription`] with this
+    /// [`Recipe`].
+    ///
+    /// [`Subscription`]: struct.Subscription.html
+    /// [`Recipe`]: trait.Recipe.html
+    type Output;
+
+    /// Hashes the [`Recipe`].
+    ///
+    /// This is used by runtimes to uniquely identify a [`Subscription`].
+    ///
+    /// [`Subscription`]: struct.Subscription.html
+    /// [`Recipe`]: trait.Recipe.html
+    fn hash(&self, state: &mut Hasher);
+
+    /// Executes the [`Recipe`] and produces the stream of events of its
+    /// [`Subscription`].
+    ///
+    /// It receives some generic `Input`, which is normally defined by runtimes.
+    ///
+    /// [`Subscription`]: struct.Subscription.html
+    /// [`Recipe`]: trait.Recipe.html
+    fn stream(
+        self: Box<Self>,
+        input: Input,
+    ) -> futures::stream::BoxStream<'static, Self::Output>;
+}
+
+struct Map<Hasher, Input, A, B> {
+    recipe: Box<dyn Recipe<Hasher, Input, Output = A>>,
+    mapper: std::sync::Arc<dyn Fn(A) -> B + Send + Sync>,
+}
+
+impl<H, I, A, B> Map<H, I, A, B> {
+    fn new(
+        recipe: Box<dyn Recipe<H, I, Output = A>>,
+        mapper: std::sync::Arc<dyn Fn(A) -> B + Send + Sync + 'static>,
+    ) -> Self {
+        Map { recipe, mapper }
+    }
+}
+
+impl<H, I, A, B> Recipe<H, I> for Map<H, I, A, B>
+where
+    A: 'static,
+    B: 'static,
+    H: std::hash::Hasher,
+{
+    type Output = B;
+
+    fn hash(&self, state: &mut H) {
+        use std::hash::Hash;
+
+        std::any::TypeId::of::<B>().hash(state);
+        self.recipe.hash(state);
+    }
+
+    fn stream(
+        self: Box<Self>,
+        input: I,
+    ) -> futures::stream::BoxStream<'static, Self::Output> {
+        use futures::StreamExt;
+
+        let mapper = self.mapper;
+
+        self.recipe
+            .stream(input)
+            .map(move |element| mapper(element))
+            .boxed()
+    }
+}