lightcraft/crates/ui-egui/src/titlebar.rs

//! The integrated title bar's window gestures (macOS: the native title bar is hidden and the top
//! bar takes its place, so dragging and double-click-to-zoom have to be done here).
//!
//! Pure policy, no egui widgets: [`command_for`] maps a gesture on the bar's empty space and the
//! window's current state to the viewport command the host should run.

use egui::ViewportCommand;

/// What the user did on the bar's empty space.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum Gesture {
    /// A primary-button drag began: the window follows the pointer.
    DragStarted,
    /// A primary-button double-click: zoom the window, or restore it when already zoomed.
    DoubleClicked,
}

/// The window as the host reports it.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub struct WindowState {
    /// Zoomed (macOS) / maximized (elsewhere).
    pub maximized: bool,
    /// In the OS's full-screen mode.
    pub fullscreen: bool,
}

/// The viewport command for `gesture`, if any. Full screen has no title bar to drag or zoom.
pub fn command_for(gesture: Gesture, window: WindowState) -> Option<ViewportCommand> {
    if window.fullscreen {
        return None;
    }
    match gesture {
        Gesture::DragStarted => Some(ViewportCommand::StartDrag),
        Gesture::DoubleClicked => Some(ViewportCommand::Maximized(!window.maximized)),
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    const NORMAL: WindowState = WindowState { maximized: false, fullscreen: false };
    const ZOOMED: WindowState = WindowState { maximized: true, fullscreen: false };
    const FULL: WindowState = WindowState { maximized: false, fullscreen: true };

    // Scenario: dragging the title bar moves the window
    #[test]
    fn given_a_normal_window_when_dragging_then_the_window_follows_the_pointer() {
        assert_eq!(command_for(Gesture::DragStarted, NORMAL), Some(ViewportCommand::StartDrag));
    }

    // Scenario: a zoomed window can still be dragged (macOS lets you pull it free)
    #[test]
    fn given_a_zoomed_window_when_dragging_then_the_window_follows_the_pointer() {
        assert_eq!(command_for(Gesture::DragStarted, ZOOMED), Some(ViewportCommand::StartDrag));
    }

    // Scenario: double-click zooms
    #[test]
    fn given_a_normal_window_when_double_clicking_then_it_zooms() {
        assert_eq!(command_for(Gesture::DoubleClicked, NORMAL), Some(ViewportCommand::Maximized(true)));
    }

    // Scenario: double-click again restores
    #[test]
    fn given_a_zoomed_window_when_double_clicking_then_it_is_restored() {
        assert_eq!(command_for(Gesture::DoubleClicked, ZOOMED), Some(ViewportCommand::Maximized(false)));
    }

    // Scenario: full screen ignores both
    #[test]
    fn given_a_full_screen_window_then_neither_gesture_does_anything() {
        assert_eq!(command_for(Gesture::DragStarted, FULL), None);
        assert_eq!(command_for(Gesture::DoubleClicked, FULL), None);
    }
}