Add hook/event system

1 files changed, 198 insertions, 3 deletions

diff --git a/helix-event/src/lib.rs b/helix-event/src/lib.rs
index 9c082b93..894de5e8 100644
--- a/helix-event/src/lib.rs
+++ b/helix-event/src/lib.rs
@@ -1,8 +1,203 @@
 //! `helix-event` contains systems that allow (often async) communication between
-//! different editor components without strongly coupling them. Currently this
-//! crate only contains some smaller facilities but the intend is to add more
-//! functionality in the future ( like a generic hook system)
+//! different editor components without strongly coupling them. Specifically
+//! it allows defining synchronous hooks that run when certain editor events
+//! occur.
+//!
+//! The core of the event system are hook callbacks and the [`Event`] trait. A
+//! hook is essentially just a closure `Fn(event: &mut impl Event) -> Result<()>`
+//! that gets called every time an appropriate event is dispatched. The implementation
+//! details of the [`Event`] trait are considered private. The [`events`] macro is
+//! provided which automatically declares event types. Similarly the `register_hook`
+//! macro should be used to (safely) declare event hooks.
+//!
+//! Hooks run synchronously which can be advantageous since they can modify the
+//! current editor state right away (for example to immediately hide the completion
+//! popup). However, they can not contain their own state without locking since
+//! they only receive immutable references. For handler that want to track state, do
+//! expensive background computations or debouncing an [`AsyncHook`] is preferable.
+//! Async hooks are based around a channels that receive events specific to
+//! that `AsyncHook` (usually an enum). These events can be sent by synchronous
+//! hooks. Due to some limitations around tokio channels the [`send_blocking`]
+//! function exported in this crate should be used instead of the builtin
+//! `blocking_send`.
+//!
+//! In addition to the core event system, this crate contains some message queues
+//! that allow transfer of data back to the main event loop from async hooks and
+//! hooks that may not have access to all application data (for example in helix-view).
+//! This include the ability to control rendering ([`lock_frame`], [`request_redraw`]) and
+//! display status messages ([`status`]).
+//!
+//! Hooks declared in helix-term can furthermore dispatch synchronous jobs to be run on the
+//! main loop (including access to the compositor). Ideally that queue will be moved
+//! to helix-view in the future if we manage to detach the compositor from its rendering backend.
 
+use anyhow::Result;
+pub use cancel::{cancelable_future, cancelation, CancelRx, CancelTx};
+pub use debounce::{send_blocking, AsyncHook};
 pub use redraw::{lock_frame, redraw_requested, request_redraw, start_frame, RenderLockGuard};
+pub use registry::Event;
 
+mod cancel;
+mod debounce;
+mod hook;
 mod redraw;
+mod registry;
+#[doc(hidden)]
+pub mod runtime;
+pub mod status;
+
+#[cfg(test)]
+mod test;
+
+pub fn register_event<E: Event + 'static>() {
+    registry::with_mut(|registry| registry.register_event::<E>())
+}
+
+/// Registers a hook that will be called when an event of type `E` is dispatched.
+/// This function should usually not be used directly, use the [`register_hook`]
+/// macro instead.
+///
+///
+/// # Safety
+///
+/// `hook` must be totally generic over all lifetime parameters of `E`. For
+/// example if `E` was a known type `Foo<'a, 'b>`, then the correct trait bound
+/// would be `F: for<'a, 'b, 'c> Fn(&'a mut Foo<'b, 'c>)`, but there is no way to
+/// express that kind of constraint for a generic type with the Rust type system
+/// as of this writing.
+pub unsafe fn register_hook_raw<E: Event>(
+    hook: impl Fn(&mut E) -> Result<()> + 'static + Send + Sync,
+) {
+    registry::with_mut(|registry| registry.register_hook(hook))
+}
+
+/// Register a hook solely by event name
+pub fn register_dynamic_hook(
+    hook: impl Fn() -> Result<()> + 'static + Send + Sync,
+    id: &str,
+) -> Result<()> {
+    registry::with_mut(|reg| reg.register_dynamic_hook(hook, id))
+}
+
+pub fn dispatch(e: impl Event) {
+    registry::with(|registry| registry.dispatch(e));
+}
+
+/// Macro to declare events
+///
+/// # Examples
+///
+/// ``` no-compile
+/// events! {
+///     FileWrite(&Path)
+///     ViewScrolled{ view: View, new_pos: ViewOffset }
+///     DocumentChanged<'a> { old_doc: &'a Rope, doc: &'a mut Document, changes: &'a ChangeSet  }
+/// }
+///
+/// fn init() {
+///    register_event::<FileWrite>();
+///    register_event::<ViewScrolled>();
+///    register_event::<DocumentChanged>();
+/// }
+///
+/// fn save(path: &Path, content: &str){
+///     std::fs::write(path, content);
+///     dispatch(FileWrite(path));
+/// }
+/// ```
+#[macro_export]
+macro_rules! events {
+    ($name: ident<$($lt: lifetime),*> { $($data:ident : $data_ty:ty),* } $($rem:tt)*) => {
+        pub struct $name<$($lt),*> { $(pub $data: $data_ty),* }
+        unsafe impl<$($lt),*> $crate::Event for $name<$($lt),*> {
+            const ID: &'static str = stringify!($name);
+            const LIFETIMES: usize = $crate::events!(@sum $(1, $lt),*);
+            type Static = $crate::events!(@replace_lt $name, $('static, $lt),*);
+        }
+        $crate::events!{ $($rem)* }
+    };
+    ($name: ident { $($data:ident : $data_ty:ty),* } $($rem:tt)*) => {
+        pub struct $name { $(pub $data: $data_ty),* }
+        unsafe impl $crate::Event for $name {
+            const ID: &'static str = stringify!($name);
+            const LIFETIMES: usize = 0;
+            type Static = Self;
+        }
+        $crate::events!{ $($rem)* }
+    };
+    () => {};
+    (@replace_lt $name: ident, $($lt1: lifetime, $lt2: lifetime),* ) => {$name<$($lt1),*>};
+    (@sum $($val: expr, $lt1: lifetime),* ) => {0 $(+ $val)*};
+}
+
+/// Safely register statically typed event hooks
+#[macro_export]
+macro_rules! register_hook {
+    // Safety: this is safe because we fully control the type of the event here and
+    // ensure all lifetime arguments are fully generic and the correct number of lifetime arguments
+    // is present
+    (move |$event:ident: &mut $event_ty: ident<$($lt: lifetime),*>| $body: expr) => {
+        let val = move |$event: &mut $event_ty<$($lt),*>| $body;
+        unsafe {
+            // Lifetimes are a bit of a pain. We want to allow events being
+            // non-static. Lifetimes don't actually exist at runtime so its
+            // fine to essentially transmute the lifetimes as long as we can
+            // prove soundness. The hook must therefore accept any combination
+            // of lifetimes. In other words fn(&'_ mut Event<'_, '_>) is ok
+            // but examples like fn(&'_ mut Event<'_, 'static>) or fn<'a>(&'a
+            // mut Event<'a, 'a>) are not. To make this safe we use a macro to
+            // forbid the user from specifying lifetimes manually (all lifetimes
+            // specified are always function generics and passed to the event so
+            // lifetimes can't be used multiple times and using 'static causes a
+            // syntax error).
+            //
+            // There is one soundness hole tough: Type Aliases allow
+            // "accidentally" creating these problems. For example:
+            //
+            // type Event2  = Event<'static>.
+            // type Event2<'a>  = Event<'a, a>.
+            //
+            // These cases can be caught by counting the number of lifetimes
+            // parameters at the parameter declaration site and then at the hook
+            // declaration site. By asserting the number of lifetime parameters
+            // are equal we can catch all bad type aliases under one assumption:
+            // There are no unused lifetime parameters. Introducing a static
+            // would reduce the number of arguments of the alias by one in the
+            // above example Event2 has zero lifetime arguments while the original
+            // event has one lifetime argument. Similar logic applies to using
+            // a lifetime argument multiple times. The ASSERT below performs a
+            // a compile time assertion to ensure exactly this property.
+            //
+            // With unused lifetime arguments it is still one way to cause unsound code:
+            //
+            // type Event2<'a, 'b> = Event<'a, 'a>;
+            //
+            // However, this case will always emit a compiler warning/cause CI
+            // failures so a user would have to introduce #[allow(unused)] which
+            // is easily caught in review (and a very theoretical case anyway).
+            // If we want to be pedantic we can simply compile helix with
+            // forbid(unused). All of this is just a safety net to prevent
+            // very theoretical misuse. This won't come up in real code (and is
+            // easily caught in review).
+            #[allow(unused)]
+            const ASSERT: () = {
+                if <$event_ty as $crate::Event>::LIFETIMES != 0 + $crate::events!(@sum $(1, $lt),*){
+                    panic!("invalid type alias");
+                }
+            };
+            $crate::register_hook_raw::<$crate::events!(@replace_lt $event_ty, $('static, $lt),*)>(val);
+        }
+    };
+    (move |$event:ident: &mut $event_ty: ident| $body: expr) => {
+        let val = move |$event: &mut $event_ty| $body;
+        unsafe {
+            #[allow(unused)]
+            const ASSERT: () = {
+                if <$event_ty as $crate::Event>::LIFETIMES != 0{
+                    panic!("invalid type alias");
+                }
+            };
+            $crate::register_hook_raw::<$event_ty>(val);
+        }
+    };
+}