retroglyph_window/

backend.rs

//! [`WindowBackend`]: the generic [`Backend`](retroglyph_core::backend::Backend) implementation for
//! windowed presenters.

use crate::presenter::Presenter;
use retroglyph_core::backend::DrawCell;
use retroglyph_core::backend::{Cursor, Input, Output};
use retroglyph_core::event::{Event, coalesces_with};
use retroglyph_core::grid::Size;
use std::collections::VecDeque;
use std::time::Duration;

/// A [`Backend`](retroglyph_core::backend::Backend) built from a [`Presenter`] plus an input event queue.
///
/// [`Input`] and [`Output`] are independent facets of `Backend`, which does not fit a window as
/// one type: some event loop owns input, while a per-renderer surface owns output.
/// `WindowBackend` reunites the two (implementing `Output` by delegating to `P`, `Input` via
/// its own event queue, and the no-op default `Cursor`), so [`Terminal`](retroglyph_core::terminal::Terminal)
/// gets the full `Backend` it needs, while renderer crates implement only [`Presenter`].
///
/// Because `WindowBackend` owns input, a [`Presenter`] should **not** implement [`Input`] or
/// [`Cursor`] itself for windowed use: those impls would be dead code, silently missing the
/// `Mouse(Moved)` coalescing that [`push_event`](WindowBackend::push_event) applies. See
/// <https://main.retroglyph.dev/book/explanation/architecture.html> for the full data-flow
/// diagram, why the input/output split exists, and when a `Presenter` may still implement
/// `Input` directly (as `retroglyph-software` does for headless pixel tests).
///
/// # Examples
///
/// ```
/// use retroglyph_core::backend::{Backend, DrawCell, Input, Output};
/// use retroglyph_core::event::Event;
/// use retroglyph_core::grid::{Pos, Size};
/// use retroglyph_core::terminal::Terminal;
/// use retroglyph_core::tile::Tile;
/// use retroglyph_window::{Presenter, WindowBackend, WindowHandle};
/// use std::sync::Arc;
/// use std::time::Duration;
///
/// struct NullPresenter;
///
/// impl Output for NullPresenter {
///     type Error = core::convert::Infallible;
///
///     fn draw<'a, I>(&mut self, _content: I) -> Result<(), Self::Error>
///     where
///         I: Iterator<Item = DrawCell<'a>>,
///     {
///         Ok(())
///     }
///
///     fn draw_layers<'a, I>(&mut self, _content: I) -> Result<(), Self::Error>
///     where
///         I: Iterator<Item = DrawCell<'a>>,
///     {
///         Ok(())
///     }
///
///     fn flush(&mut self) -> Result<(), Self::Error> {
///         Ok(())
///     }
///
///     fn size(&self) -> Size {
///         Size::new(4, 2)
///     }
///
///     fn clear(&mut self) -> Result<(), Self::Error> {
///         Ok(())
///     }
///
///     fn resize(&mut self, _size: Size) {}
/// }
///
/// impl Presenter for NullPresenter {
///     type SurfaceError = core::convert::Infallible;
///
///     fn init_surface(&mut self, _window: Arc<dyn WindowHandle>) -> Result<(), Self::SurfaceError> {
///         Ok(())
///     }
///
///     fn resize_surface(&mut self, _width: u32, _height: u32) {}
///
///     fn present(&mut self) -> Result<(), Self::SurfaceError> {
///         Ok(())
///     }
///
///     fn cell_size(&self) -> (u32, u32) {
///         (8, 16)
///     }
/// }
///
/// // A caller driving its own loop (SDL2, tao, a hand-rolled driver) builds
/// // `WindowBackend` directly, no `winit` feature required.
/// let backend = WindowBackend::new(NullPresenter);
/// let mut term = Terminal::new(backend);
///
/// // The loop pushes each translated input event onto the queue...
/// term.backend_mut().push_event(Event::FocusGained);
///
/// // ...and the app drains it through the normal `Terminal` polling API,
/// // which never blocks for `WindowBackend`.
/// while term.poll(Duration::ZERO).is_some() {}
///
/// // Once per frame: `Terminal::present` diffs the grid and drives
/// // `Presenter::flush`, then the caller drives `Presenter::present` itself
/// // to push pixels to the window.
/// term.present().unwrap();
/// term.backend_mut().presenter_mut().present().unwrap();
/// ```
///
/// [`poll_event`](Input::poll_event) never blocks: frame timing is owned by the event loop, not
/// by input waits.
#[derive(Debug)]
pub struct WindowBackend<P: Presenter> {
    presenter: P,
    events: VecDeque<Event>,
}

impl<P: Presenter> WindowBackend<P> {
    /// Wrap a presenter, creating an empty event queue.
    #[must_use]
    pub const fn new(presenter: P) -> Self {
        Self {
            presenter,
            events: VecDeque::new(),
        }
    }

    /// The wrapped presenter.
    #[must_use]
    pub const fn presenter(&self) -> &P {
        &self.presenter
    }

    /// The wrapped presenter, mutably.
    pub const fn presenter_mut(&mut self) -> &mut P {
        &mut self.presenter
    }

    /// Unwrap into the presenter, discarding queued events.
    #[must_use]
    pub fn into_presenter(self) -> P {
        self.presenter
    }
}

impl<P: Presenter> Output for WindowBackend<P> {
    type Error = P::Error;

    fn draw<'a, I>(&mut self, content: I) -> Result<(), Self::Error>
    where
        I: Iterator<Item = DrawCell<'a>>,
    {
        self.presenter.draw(content)
    }

    fn draw_layers<'a, I>(&mut self, content: I) -> Result<(), Self::Error>
    where
        I: Iterator<Item = DrawCell<'a>>,
    {
        self.presenter.draw_layers(content)
    }

    fn flush(&mut self) -> Result<(), Self::Error> {
        self.presenter.flush()
    }

    fn size(&self) -> Size {
        self.presenter.size()
    }

    fn clear(&mut self) -> Result<(), Self::Error> {
        self.presenter.clear()
    }

    fn resize(&mut self, size: Size) {
        self.presenter.resize(size);
    }

    fn needs_full_frame(&self) -> bool {
        self.presenter.needs_full_frame()
    }

    fn composites_layers(&self) -> bool {
        self.presenter.composites_layers()
    }
}

impl<P: Presenter> Input for WindowBackend<P> {
    fn poll_event(&mut self, _timeout: Duration) -> Option<Event> {
        // Non-blocking by design: the caller's event loop drives frame
        // timing, so there is nothing to sleep on here.
        self.events.pop_front()
    }

    fn push_event(&mut self, event: Event) {
        // Coalesce consecutive `Mouse(Moved)` or same-button `Mouse(Drag)` events: winit can
        // deliver `CursorMoved`/drag motion at device polling rate (hundreds/sec) though only the
        // latest position matters once the next frame polls the queue, so replace the queue's
        // tail in place instead of growing it unbounded (retroglyph#294, retroglyph#768). Every
        // other event kind (clicks, scrolls, keys, resize, ...) still pushes in O(1) as before;
        // only a back-to-back `Moved` or same-button `Drag` run collapses. See [`coalesces_with`]
        // for the shared rule (also used by `retroglyph-terminal-wasm` and `Headless`).
        if let Some(back) = self.events.back_mut()
            && coalesces_with(&event, back)
        {
            *back = event;
            return;
        }
        self.events.push_back(event);
    }
}

// No hardware text cursor in windowed mode (games draw their own): the trait's no-op default
// bodies are exactly right here.
impl<P: Presenter> Cursor for WindowBackend<P> {}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::presenter::WindowHandle;
    use retroglyph_core::event::{KeyModifiers, MouseButton, MouseEvent, MouseEventKind};
    use retroglyph_core::grid::Pos;
    use std::sync::Arc;

    struct NullPresenter;

    impl Output for NullPresenter {
        type Error = core::convert::Infallible;

        fn draw<'a, I>(&mut self, _content: I) -> Result<(), Self::Error>
        where
            I: Iterator<Item = DrawCell<'a>>,
        {
            Ok(())
        }

        fn draw_layers<'a, I>(&mut self, _content: I) -> Result<(), Self::Error>
        where
            I: Iterator<Item = DrawCell<'a>>,
        {
            Ok(())
        }

        fn flush(&mut self) -> Result<(), Self::Error> {
            Ok(())
        }

        fn size(&self) -> Size {
            Size::new(4, 2)
        }

        fn clear(&mut self) -> Result<(), Self::Error> {
            Ok(())
        }

        fn resize(&mut self, _size: Size) {}
    }

    impl Presenter for NullPresenter {
        type SurfaceError = core::convert::Infallible;

        fn init_surface(
            &mut self,
            _window: Arc<dyn WindowHandle>,
        ) -> Result<(), Self::SurfaceError> {
            Ok(())
        }

        fn resize_surface(&mut self, _width: u32, _height: u32) {}

        fn present(&mut self) -> Result<(), Self::SurfaceError> {
            Ok(())
        }

        fn cell_size(&self) -> (u32, u32) {
            (8, 16)
        }
    }

    fn moved(x: u16) -> Event {
        Event::Mouse(MouseEvent::new(
            MouseEventKind::Moved,
            Pos { x, y: 0 },
            KeyModifiers::NONE,
        ))
    }

    /// Regression test for retroglyph#294: a burst of consecutive `Moved` events must coalesce
    /// down to the single most recent one instead of growing the queue by one entry per event.
    #[test]
    fn consecutive_moved_events_coalesce_to_one() {
        let mut backend = WindowBackend::new(NullPresenter);
        for x in 0..1_000u16 {
            backend.push_event(moved(x));
        }
        assert_eq!(backend.events.len(), 1);
        assert_eq!(backend.poll_event(Duration::ZERO), Some(moved(999)));
        assert_eq!(backend.poll_event(Duration::ZERO), None);
    }

    /// A non-`Moved` event between two `Moved` bursts must not be swallowed: only *consecutive*
    /// `Moved` events collapse, so interleaving a click still yields three distinct events.
    #[test]
    fn non_moved_event_breaks_coalescing() {
        let mut backend = WindowBackend::new(NullPresenter);
        backend.push_event(moved(1));
        backend.push_event(moved(2));
        backend.push_event(Event::Mouse(MouseEvent::new(
            MouseEventKind::Down(MouseButton::Left),
            Pos { x: 2, y: 0 },
            KeyModifiers::NONE,
        )));
        backend.push_event(moved(3));
        assert_eq!(backend.events.len(), 3);
        assert_eq!(backend.poll_event(Duration::ZERO), Some(moved(2)));
        assert!(matches!(
            backend.poll_event(Duration::ZERO),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Down(MouseButton::Left),
                ..
            }))
        ));
        assert_eq!(backend.poll_event(Duration::ZERO), Some(moved(3)));
    }

    #[test]
    fn satisfies_the_input_contract() {
        retroglyph_core::testing::conformance::assert_input_contract(|| {
            WindowBackend::new(NullPresenter)
        });
    }
}