Introduce keyboard::on_key_press and on_key_release

Also rename `subscription::events*` to `event::listen*`.

futures/src/keyboard.rs

@@ -0,0 +1,61 @@
+//! Listen to keyboard events.
+use crate::core;
+use crate::core::keyboard::{Event, KeyCode, Modifiers};
+use crate::subscription::{self, Subscription};
+use crate::MaybeSend;
+
+/// Listens to keyboard key presses and calls the given function
+/// map them into actual messages.
+///
+/// If the function returns `None`, the key press will be simply
+/// ignored.
+pub fn on_key_press<Message>(
+    f: fn(KeyCode, Modifiers) -> Option<Message>,
+) -> Subscription<Message>
+where
+    Message: MaybeSend + 'static,
+{
+    #[derive(Hash)]
+    struct OnKeyPress;
+
+    subscription::filter_map((OnKeyPress, f), move |event, status| {
+        match (event, status) {
+            (
+                core::Event::Keyboard(Event::KeyPressed {
+                    key_code,
+                    modifiers,
+                }),
+                core::event::Status::Ignored,
+            ) => f(key_code, modifiers),
+            _ => None,
+        }
+    })
+}
+
+/// Listens to keyboard key releases and calls the given function
+/// map them into actual messages.
+///
+/// If the function returns `None`, the key release will be simply
+/// ignored.
+pub fn on_key_release<Message>(
+    f: fn(KeyCode, Modifiers) -> Option<Message>,
+) -> Subscription<Message>
+where
+    Message: MaybeSend + 'static,
+{
+    #[derive(Hash)]
+    struct OnKeyPress;
+
+    subscription::filter_map((OnKeyPress, f), move |event, status| {
+        match (event, status) {
+            (
+                core::Event::Keyboard(Event::KeyReleased {
+                    key_code,
+                    modifiers,
+                }),
+                core::event::Status::Ignored,
+            ) => f(key_code, modifiers),
+            _ => None,
+        }
+    })
+}