helix/helix-term/src/compositor.rs

// Features:
// Tracks currently focused component which receives all input
// Event loop is external as opposed to cursive-rs
// Calls render on the component and translates screen coords to local component coords
//
// TODO:
// Q: where is the Application state stored? do we store it into an external static var?
// A: probably makes sense to initialize the editor into a `static Lazy<>` global var.
//
// Q: how do we composit nested structures? There should be sub-components/views
//
// Each component declares it's own size constraints and gets fitted based on it's parent.
// Q: how does this work with popups?
// cursive does compositor.screen_mut().add_layer_at(pos::absolute(x, y), <component>)

use crossterm::event::Event;
use helix_core::Position;
use smol::Executor;
use tui::buffer::Buffer as Surface;
use tui::layout::Rect;

pub type Callback = Box<dyn FnOnce(&mut Compositor, &mut Editor)>;

// --> EventResult should have a callback that takes a context with methods like .popup(),
// .prompt() etc. That way we can abstract it from the renderer.
// Q: How does this interact with popups where we need to be able to specify the rendering of the
// popup?
// A: It could just take a textarea.
//
// If Compositor was specified in the callback that's then problematic because of

// Cursive-inspired
pub enum EventResult {
    Ignored,
    Consumed(Option<Callback>),
}

use helix_view::{Editor, View};
// shared with commands.rs
pub struct Context<'a> {
    pub editor: &'a mut Editor,
    pub executor: &'static smol::Executor<'static>,
}

pub trait Component {
    /// Process input events, return true if handled.
    fn handle_event(&mut self, event: Event, ctx: &mut Context) -> EventResult;
    // , args: ()

    /// Should redraw? Useful for saving redraw cycles if we know component didn't change.
    fn should_update(&self) -> bool {
        true
    }

    fn render(&self, area: Rect, frame: &mut Surface, ctx: &mut Context);

    fn cursor_position(&self, area: Rect, ctx: &Editor) -> Option<Position> {
        None
    }
}

// For v1:
// Child views are something each view needs to handle on it's own for now, positioning and sizing
// options, focus tracking. In practice this is simple: we only will need special solving for
// splits etc

// impl Editor {
//     fn render(&mut self, surface: &mut Surface, args: ()) {
//         // compute x, y, w, h rects for sub-views!
//         // get surface area
//         // get constraints for textarea, statusbar
//         // -> cassowary-rs

//         // first render textarea
//         // then render statusbar
//     }
// }

// usecases to consider:
// - a single view with subviews (textarea + statusbar)
// - a popup panel / dialog with it's own interactions
// - an autocomplete popup that doesn't change focus

pub struct Compositor {
    layers: Vec<Box<dyn Component>>,
}

impl Compositor {
    pub fn new() -> Self {
        Self { layers: Vec::new() }
    }

    pub fn push(&mut self, layer: Box<dyn Component>) {
        self.layers.push(layer);
    }

    pub fn pop(&mut self) {
        self.layers.pop();
    }

    pub fn handle_event(&mut self, event: Event, cx: &mut Context) -> bool {
        // propagate events through the layers until we either find a layer that consumes it or we
        // run out of layers (event bubbling)
        for layer in self.layers.iter_mut().rev() {
            match layer.handle_event(event, cx) {
                EventResult::Consumed(Some(callback)) => {
                    callback(self, cx.editor);
                    return true;
                }
                EventResult::Consumed(None) => return true,
                EventResult::Ignored => false,
            };
        }
        false
    }

    pub fn render(&self, area: Rect, surface: &mut Surface, cx: &mut Context) {
        for layer in &self.layers {
            layer.render(area, surface, cx)
        }
    }

    pub fn cursor_position(&self, area: Rect, editor: &Editor) -> Position {
        for layer in self.layers.iter().rev() {
            if let Some(pos) = layer.cursor_position(area, editor) {
                return pos;
            }
        }
        panic!("No layer returned a position!");
    }
}