Why didn't Response.is_pointer_button_down_on and Response.dragged work in this case?

[taminhtienhai]

Context

I'm trying to implement a simple drag-and-drop app and ran into this problem. The list of Response from widgets doesn't seem to be reactive with is_pointer_button_down_on and dragged methods whatsoever. Please enlighten me.

Command

RUST_LOG=INFO cargo run --release

Code

use egui::{Align2, Color32, FontId, Id, LayerId, Sense, show_tooltip};
use log::info;

fn main() -> eframe::Result {
    env_logger::init();

    let native_options = eframe::NativeOptions {
        viewport: egui::ViewportBuilder::default()
            .with_inner_size([400.0, 300.0])
            .with_min_inner_size([300.0, 220.0]),
        ..Default::default()
    };

    eframe::run_native(
        "DndApp",
        native_options,
        // Box::new(|cc| Ok(Box::new(Game::new(cc)))),
        Box::new(|cc| Ok(Box::new(MyDnDApp {
            items: vec![
                Potion {
                    id: 1,
                    name: "Healing".to_string(),
                    color: Color32::from_rgb(255, 0, 0),
                },
                Potion {
                    id: 2,
                    name: "Mana".to_string(),
                    color: Color32::from_rgb(0, 0, 255),
                },
                Potion {
                    id: 3,
                    name: "Stamina".to_string(),
                    color: Color32::from_rgb(0, 255, 0),
                },
            ],
            dropped_potion_name: Some("drop zoneee".to_string()),
        }))),
    )
}

#[derive(Clone, PartialEq, Eq, Debug)]
struct Potion {
    id: u32,
    name: String,
    color: Color32,
}

#[derive(Default)]
struct MyDnDApp {
    items: Vec<Potion>,
    dropped_potion_name: Option<String>,
}

impl eframe::App for MyDnDApp {
    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
        egui::CentralPanel::default().show(ctx, |ui| {
            ui.horizontal(|ui| {
                ui.vertical(|ui| {
                    ui.label("Inventory (Drag from here)");
                    for potion in &self.items {
                        let item_id = egui::Id::new(("potion", potion.id));

                        let response = ui
                            .dnd_drag_source(
                                item_id,
                                potion.clone(),
                                |ui| {
                                    egui::Frame::NONE
                                        .fill(potion.color.linear_multiply(0.5))
                                        .inner_margin(8.0)
                                        .show(ui, |ui| {
                                            ui.label(&potion.name);
                                        });
                                },
                            ).response;

                        // if response.is_pointer_button_down_on() {
                        // if response.dragged() {
                        if response.contains_pointer() {
                            info!("Dragging potion: {}", potion.name);
                            show_tooltip(ctx, LayerId::debug(), ui.auto_id_with("tooltip_id"), |ui| {
                                ui.label(format!("A magical {} potion", potion.name));
                            });
                        }
                    }
                });

                ui.vertical(|ui| {
                    ui.label("Cauldron (Drop here)");

                    let (inner, drop_payload) = ui.dnd_drop_zone::<Potion, ()>(
                        egui::Frame::default().inner_margin(16.),
                        |ui| {
                            ui.set_min_size(egui::vec2(150., 150.));
                            if let Some(name) = &self.dropped_potion_name {
                                ui.label(format!("Dropped: {}", name));
                            } else {
                                ui.label("Drop a potion here.");
                            }
                        },
                    );

                    if let Some(potion_payload) = drop_payload {
                        self.dropped_potion_name = Some(potion_payload.name.clone());
                    }

                    let style = ui.style_mut();
                    if inner.response.drag_stopped() {
                        style.visuals.widgets.active.bg_fill = egui::Color32::from_rgb(0, 100, 0);
                    }

                    if inner.response.dragged() {
                        style.visuals.widgets.hovered.bg_fill = egui::Color32::from_rgb(0, 50, 0);
                    }
                });
            });
        });
    }
}

Cargo.toml

[dependencies]
eframe = "0.31.1"
egui = "0.31.1"
egui_extras = "0.31.1"
env_logger = "0.11.8"
log = "0.4.27"
serde = { version = "1.*", features = ["derive"] }

[profile.release]
opt-level = 2

[profile.dev.package."*"]
opt-level = 2

[taminhtienhai]

Ah, after consulting an AI, it became clear: the newly dragged widget wasn't registering with Drag sense, but its origin was. The correct approach to check if the current widget is being dragged is through Context.is_being_dragged.

// Use the egui context to check drag state, not the response
if ctx.is_being_dragged(item_id) {
    info!("Dragging potion: {}", potion.name);
    show_tooltip(ctx, LayerId::debug(), ui.auto_id_with("tooltip_id"), |ui| {
        ui.label(format!("A magical {} potion", potion.name));
    });
}