egui/egui_demo_lib/src/apps/demo/toggle_switch.rs

//! Source code example of how to create your own widget.
//! This is meant to be read as a tutorial, hence the plethora of comments.

/// iOS-style toggle switch:
///
/// ``` text
///      _____________
///     /       /.....\
///    |       |.......|
///     \_______\_____/
/// ```
pub fn toggle(ui: &mut egui::Ui, on: &mut bool) -> egui::Response {
    // Widget code can be broken up in four steps:
    //  1. Decide a size for the widget
    //  2. Allocate space for it
    //  3. Handle interactions with the widget (if any)
    //  4. Paint the widget

    // 1. Deciding widget size:
    // You can query the `ui` how much space is available,
    // but in this example we have a fixed size widget of the default size for a button:
    let desired_size = ui.spacing().interact_size;

    // 2. Allocating space:
    // This is where we get a region of the screen assigned.
    // We also tell the Ui to sense clicks in the allocated region.
    let (rect, response) = ui.allocate_exact_size(desired_size, egui::Sense::click());

    // 3. Interact: Time to check for clicks!.
    if response.clicked() {
        *on = !*on;
    }

    // 4. Paint!
    // First let's ask for a simple animation from egui.
    // egui keeps track of changes in the boolean associated with the id and
    // returns an animated value in the 0-1 range for how much "on" we are.
    let how_on = ui.ctx().animate_bool(response.id, *on);
    // We will follow the current style by asking
    // "how should something that is being interacted with be painted?".
    // This will, for instance, give us different colors when the widget is hovered or clicked.
    let visuals = ui.style().interact(&response);
    let off_bg_fill = egui::Rgba::from(visuals.bg_fill);
    let on_bg_fill = egui::Rgba::from_rgb(0.0, 0.5, 0.0);
    let bg_fill = egui::lerp(off_bg_fill..=on_bg_fill, how_on);
    // All coordinates are in absolute screen coordinates so we use `rect` to place the elements.
    let rect = rect.expand(visuals.expansion);
    let radius = 0.5 * rect.height();
    ui.painter().rect(rect, radius, bg_fill, visuals.bg_stroke);
    // Paint the circle, animating it from left to right with `how_on`:
    let circle_x = egui::lerp((rect.left() + radius)..=(rect.right() - radius), how_on);
    let center = egui::pos2(circle_x, rect.center().y);
    ui.painter()
        .circle(center, 0.75 * radius, visuals.bg_fill, visuals.fg_stroke);

    // All done! Return the interaction response so the user can check what happened
    // (hovered, clicked, ...) and maybe show a tooltip:
    response
}

/// Here is the same code again, but a bit more compact:
#[allow(dead_code)]
fn toggle_compact(ui: &mut egui::Ui, on: &mut bool) -> egui::Response {
    let desired_size = ui.spacing().interact_size;
    let (rect, response) = ui.allocate_exact_size(desired_size, egui::Sense::click());
    *on ^= response.clicked(); // toggle if clicked

    let how_on = ui.ctx().animate_bool(response.id, *on);
    let visuals = ui.style().interact(&response);
    let off_bg_fill = egui::Rgba::from(visuals.bg_fill);
    let on_bg_fill = egui::Rgba::from_rgb(0.0, 0.5, 0.0);
    let bg_fill = egui::lerp(off_bg_fill..=on_bg_fill, how_on);
    let rect = rect.expand(visuals.expansion);
    let radius = 0.5 * rect.height();
    ui.painter().rect(rect, radius, bg_fill, visuals.bg_stroke);
    let circle_x = egui::lerp((rect.left() + radius)..=(rect.right() - radius), how_on);
    let center = egui::pos2(circle_x, rect.center().y);
    ui.painter()
        .circle(center, 0.75 * radius, visuals.bg_fill, visuals.fg_stroke);

    response
}

pub fn demo(ui: &mut egui::Ui, on: &mut bool) {
    ui.horizontal_wrapped_for_text(egui::TextStyle::Button, |ui| {
        toggle(ui, on).on_hover_text("Click to toggle");
        ui.add(crate::__egui_github_link_file!());
    })
    .response
    .on_hover_text(
        "It's easy to create your own widgets!\n\
        This toggle switch is just one function and 20 lines of code.",
    );
}