windows/event_loop/runner.rs

use std::{
    any::Any,
    cell::{Cell, RefCell},
    collections::{HashSet, VecDeque},
    mem, panic, ptr,
    rc::Rc,
    time::Instant,
};

use winapi::{
    shared::{minwindef::DWORD, windef::HWND},
    um::winuser,
};

use crate::{
    dpi::PhysicalSize,
    event::{Event, StartCause, WindowEvent},
    event_loop::ControlFlow,
    platform_impl::platform::util,
    window::WindowId,
};

pub(crate) type EventLoopRunnerShared<T> = Rc<EventLoopRunner<T>>;
pub(crate) struct EventLoopRunner<T: 'static> {
    // The event loop's win32 handles
    thread_msg_target: HWND,
    wait_thread_id: DWORD,

    control_flow: Cell<ControlFlow>,
    runner_state: Cell<RunnerState>,
    last_events_cleared: Cell<Instant>,

    event_handler: Cell<Option<Box<dyn FnMut(Event<'_, T>, &mut ControlFlow)>>>,
    event_buffer: RefCell<VecDeque<BufferedEvent<T>>>,

    owned_windows: Cell<HashSet<HWND>>,

    panic_error: Cell<Option<PanicError>>,
}

pub type PanicError = Box<dyn Any + Send + 'static>;

/// See `move_state_to` function for details on how the state loop works.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
enum RunnerState {
    /// The event loop has just been created, and an `Init` event must be sent.
    Uninitialized,
    /// The event loop is idling.
    Idle,
    /// The event loop is handling the OS's events and sending them to the user's callback.
    /// `NewEvents` has been sent, and `MainEventsCleared` hasn't.
    HandlingMainEvents,
    /// The event loop is handling the redraw events and sending them to the user's callback.
    /// `MainEventsCleared` has been sent, and `RedrawEventsCleared` hasn't.
    HandlingRedrawEvents,
    /// The event loop has been destroyed. No other events will be emitted.
    Destroyed,
}

enum BufferedEvent<T: 'static> {
    Event(Event<'static, T>),
    ScaleFactorChanged(WindowId, f64, PhysicalSize<u32>),
}

impl<T> EventLoopRunner<T> {
    pub(crate) fn new(thread_msg_target: HWND, wait_thread_id: DWORD) -> EventLoopRunner<T> {
        EventLoopRunner {
            thread_msg_target,
            wait_thread_id,
            runner_state: Cell::new(RunnerState::Uninitialized),
            control_flow: Cell::new(ControlFlow::Poll),
            panic_error: Cell::new(None),
            last_events_cleared: Cell::new(Instant::now()),
            event_handler: Cell::new(None),
            event_buffer: RefCell::new(VecDeque::new()),
            owned_windows: Cell::new(HashSet::new()),
        }
    }

    pub(crate) unsafe fn set_event_handler<F>(&self, f: F)
    where
        F: FnMut(Event<'_, T>, &mut ControlFlow),
    {
        let old_event_handler = self.event_handler.replace(mem::transmute::<
            Option<Box<dyn FnMut(Event<'_, T>, &mut ControlFlow)>>,
            Option<Box<dyn FnMut(Event<'_, T>, &mut ControlFlow)>>,
        >(Some(Box::new(f))));
        assert!(old_event_handler.is_none());
    }

    pub(crate) fn reset_runner(&self) {
        let EventLoopRunner {
            thread_msg_target: _,
            wait_thread_id: _,
            runner_state,
            panic_error,
            control_flow,
            last_events_cleared: _,
            event_handler,
            event_buffer: _,
            owned_windows: _,
        } = self;
        runner_state.set(RunnerState::Uninitialized);
        panic_error.set(None);
        control_flow.set(ControlFlow::Poll);
        event_handler.set(None);
    }
}

/// State retrieval functions.
impl<T> EventLoopRunner<T> {
    pub fn thread_msg_target(&self) -> HWND {
        self.thread_msg_target
    }

    pub fn wait_thread_id(&self) -> DWORD {
        self.wait_thread_id
    }

    pub fn redrawing(&self) -> bool {
        self.runner_state.get() == RunnerState::HandlingRedrawEvents
    }

    pub fn take_panic_error(&self) -> Result<(), PanicError> {
        match self.panic_error.take() {
            Some(err) => Err(err),
            None => Ok(()),
        }
    }

    pub fn control_flow(&self) -> ControlFlow {
        self.control_flow.get()
    }

    pub fn handling_events(&self) -> bool {
        self.runner_state.get() != RunnerState::Idle
    }

    pub fn should_buffer(&self) -> bool {
        let handler = self.event_handler.take();
        let should_buffer = handler.is_none();
        self.event_handler.set(handler);
        should_buffer
    }
}

/// Misc. functions
impl<T> EventLoopRunner<T> {
    pub fn catch_unwind<R>(&self, f: impl FnOnce() -> R) -> Option<R> {
        let panic_error = self.panic_error.take();
        if panic_error.is_none() {
            let result = panic::catch_unwind(panic::AssertUnwindSafe(f));

            // Check to see if the panic error was set in a re-entrant call to catch_unwind inside
            // of `f`. If it was, that error takes priority. If it wasn't, check if our call to
            // catch_unwind caught any panics and set panic_error appropriately.
            match self.panic_error.take() {
                None => match result {
                    Ok(r) => Some(r),
                    Err(e) => {
                        self.panic_error.set(Some(e));
                        None
                    }
                },
                Some(e) => {
                    self.panic_error.set(Some(e));
                    None
                }
            }
        } else {
            self.panic_error.set(panic_error);
            None
        }
    }
    pub fn register_window(&self, window: HWND) {
        let mut owned_windows = self.owned_windows.take();
        owned_windows.insert(window);
        self.owned_windows.set(owned_windows);
    }

    pub fn remove_window(&self, window: HWND) {
        let mut owned_windows = self.owned_windows.take();
        owned_windows.remove(&window);
        self.owned_windows.set(owned_windows);
    }

    pub fn owned_windows(&self, mut f: impl FnMut(HWND)) {
        let mut owned_windows = self.owned_windows.take();
        for hwnd in &owned_windows {
            f(*hwnd);
        }
        let new_owned_windows = self.owned_windows.take();
        owned_windows.extend(&new_owned_windows);
        self.owned_windows.set(owned_windows);
    }
}

/// Event dispatch functions.
impl<T> EventLoopRunner<T> {
    pub(crate) unsafe fn poll(&self) {
        self.move_state_to(RunnerState::HandlingMainEvents);
    }

    pub(crate) unsafe fn send_event(&self, event: Event<'_, T>) {
        if let Event::RedrawRequested(_) = event {
            if self.runner_state.get() != RunnerState::HandlingRedrawEvents {
                warn!("RedrawRequested dispatched without explicit MainEventsCleared");
                self.move_state_to(RunnerState::HandlingRedrawEvents);
            }
            self.call_event_handler(event);
        } else {
            if self.should_buffer() {
                // If the runner is already borrowed, we're in the middle of an event loop invocation. Add
                // the event to a buffer to be processed later.
                self.event_buffer
                    .borrow_mut()
                    .push_back(BufferedEvent::from_event(event))
            } else {
                self.move_state_to(RunnerState::HandlingMainEvents);
                self.call_event_handler(event);
                self.dispatch_buffered_events();
            }
        }
    }

    pub(crate) unsafe fn main_events_cleared(&self) {
        self.move_state_to(RunnerState::HandlingRedrawEvents);
    }

    pub(crate) unsafe fn redraw_events_cleared(&self) {
        self.move_state_to(RunnerState::Idle);
    }

    pub(crate) unsafe fn loop_destroyed(&self) {
        self.move_state_to(RunnerState::Destroyed);
    }

    unsafe fn call_event_handler(&self, event: Event<'_, T>) {
        self.catch_unwind(|| {
            let mut control_flow = self.control_flow.take();
            let mut event_handler = self.event_handler.take()
                .expect("either event handler is re-entrant (likely), or no event handler is registered (very unlikely)");

            if control_flow != ControlFlow::Exit {
                event_handler(event, &mut control_flow);
            } else {
                event_handler(event, &mut ControlFlow::Exit);
            }

            assert!(self.event_handler.replace(Some(event_handler)).is_none());
            self.control_flow.set(control_flow);
        });
    }

    unsafe fn dispatch_buffered_events(&self) {
        loop {
            // We do this instead of using a `while let` loop because if we use a `while let`
            // loop the reference returned `borrow_mut()` doesn't get dropped until the end
            // of the loop's body and attempts to add events to the event buffer while in
            // `process_event` will fail.
            let buffered_event_opt = self.event_buffer.borrow_mut().pop_front();
            match buffered_event_opt {
                Some(e) => e.dispatch_event(|e| self.call_event_handler(e)),
                None => break,
            }
        }
    }

    /// Dispatch control flow events (`NewEvents`, `MainEventsCleared`, `RedrawEventsCleared`, and
    /// `LoopDestroyed`) as necessary to bring the internal `RunnerState` to the new runner state.
    ///
    /// The state transitions are defined as follows:
    ///
    /// ```text
    ///    Uninitialized
    ///          |
    ///          V
    ///  HandlingMainEvents
    ///   ^            |
    ///   |            V
    /// Idle <--- HandlingRedrawEvents
    ///   |
    ///   V
    /// Destroyed
    /// ```
    ///
    /// Attempting to transition back to `Uninitialized` will result in a panic. Attempting to
    /// transition *from* `Destroyed` will also reuslt in a panic. Transitioning to the current
    /// state is a no-op. Even if the `new_runner_state` isn't the immediate next state in the
    /// runner state machine (e.g. `self.runner_state == HandlingMainEvents` and
    /// `new_runner_state == Idle`), the intermediate state transitions will still be executed.
    unsafe fn move_state_to(&self, new_runner_state: RunnerState) {
        use RunnerState::{
            Destroyed, HandlingMainEvents, HandlingRedrawEvents, Idle, Uninitialized,
        };

        match (
            self.runner_state.replace(new_runner_state),
            new_runner_state,
        ) {
            (Uninitialized, Uninitialized)
            | (Idle, Idle)
            | (HandlingMainEvents, HandlingMainEvents)
            | (HandlingRedrawEvents, HandlingRedrawEvents)
            | (Destroyed, Destroyed) => (),

            // State transitions that initialize the event loop.
            (Uninitialized, HandlingMainEvents) => {
                self.call_new_events(true);
            }
            (Uninitialized, HandlingRedrawEvents) => {
                self.call_new_events(true);
                self.call_event_handler(Event::MainEventsCleared);
            }
            (Uninitialized, Idle) => {
                self.call_new_events(true);
                self.call_event_handler(Event::MainEventsCleared);
                self.call_redraw_events_cleared();
            }
            (Uninitialized, Destroyed) => {
                self.call_new_events(true);
                self.call_event_handler(Event::MainEventsCleared);
                self.call_redraw_events_cleared();
                self.call_event_handler(Event::LoopDestroyed);
            }
            (_, Uninitialized) => panic!("cannot move state to Uninitialized"),

            // State transitions that start the event handling process.
            (Idle, HandlingMainEvents) => {
                self.call_new_events(false);
            }
            (Idle, HandlingRedrawEvents) => {
                self.call_new_events(false);
                self.call_event_handler(Event::MainEventsCleared);
            }
            (Idle, Destroyed) => {
                self.call_event_handler(Event::LoopDestroyed);
            }

            (HandlingMainEvents, HandlingRedrawEvents) => {
                self.call_event_handler(Event::MainEventsCleared);
            }
            (HandlingMainEvents, Idle) => {
                warn!("RedrawEventsCleared emitted without explicit MainEventsCleared");
                self.call_event_handler(Event::MainEventsCleared);
                self.call_redraw_events_cleared();
            }
            (HandlingMainEvents, Destroyed) => {
                self.call_event_handler(Event::MainEventsCleared);
                self.call_redraw_events_cleared();
                self.call_event_handler(Event::LoopDestroyed);
            }

            (HandlingRedrawEvents, Idle) => {
                self.call_redraw_events_cleared();
            }
            (HandlingRedrawEvents, HandlingMainEvents) => {
                warn!("NewEvents emitted without explicit RedrawEventsCleared");
                self.call_redraw_events_cleared();
                self.call_new_events(false);
            }
            (HandlingRedrawEvents, Destroyed) => {
                self.call_redraw_events_cleared();
                self.call_event_handler(Event::LoopDestroyed);
            }

            (Destroyed, _) => panic!("cannot move state from Destroyed"),
        }
    }

    unsafe fn call_new_events(&self, init: bool) {
        let start_cause = match (init, self.control_flow()) {
            (true, _) => StartCause::Init,
            (false, ControlFlow::Poll) => StartCause::Poll,
            (false, ControlFlow::Exit) | (false, ControlFlow::Wait) => StartCause::WaitCancelled {
                requested_resume: None,
                start: self.last_events_cleared.get(),
            },
            (false, ControlFlow::WaitUntil(requested_resume)) => {
                if Instant::now() < requested_resume {
                    StartCause::WaitCancelled {
                        requested_resume: Some(requested_resume),
                        start: self.last_events_cleared.get(),
                    }
                } else {
                    StartCause::ResumeTimeReached {
                        requested_resume,
                        start: self.last_events_cleared.get(),
                    }
                }
            }
        };
        self.call_event_handler(Event::NewEvents(start_cause));
        self.dispatch_buffered_events();
        winuser::RedrawWindow(
            self.thread_msg_target,
            ptr::null(),
            ptr::null_mut(),
            winuser::RDW_INTERNALPAINT,
        );
    }

    unsafe fn call_redraw_events_cleared(&self) {
        self.call_event_handler(Event::RedrawEventsCleared);
        self.last_events_cleared.set(Instant::now());
    }
}

impl<T> BufferedEvent<T> {
    pub fn from_event(event: Event<'_, T>) -> BufferedEvent<T> {
        match event {
            Event::WindowEvent {
                event:
                    WindowEvent::ScaleFactorChanged {
                        scale_factor,
                        new_inner_size,
                    },
                window_id,
            } => BufferedEvent::ScaleFactorChanged(window_id, scale_factor, *new_inner_size),
            event => BufferedEvent::Event(event.to_static().unwrap()),
        }
    }

    pub fn dispatch_event(self, dispatch: impl FnOnce(Event<'_, T>)) {
        match self {
            Self::Event(event) => dispatch(event),
            Self::ScaleFactorChanged(window_id, scale_factor, mut new_inner_size) => {
                dispatch(Event::WindowEvent {
                    window_id,
                    event: WindowEvent::ScaleFactorChanged {
                        scale_factor,
                        new_inner_size: &mut new_inner_size,
                    },
                });
                util::set_inner_size_physical(
                    (window_id.0).0,
                    new_inner_size.width as _,
                    new_inner_size.height as _,
                );
            }
        }
    }
}