Crate bevy_mod_picking

A flexible set of plugins that add picking functionality to your bevy app, with a focus on modularity, expressiveness, and robustness. Want to drag a UI entity and drop it onto a 3D mesh entity? This plugin allows you to add event listeners to any entity, and works with mouse, touch, or even gamepads. Includes optional integrations for rapier and egui, but is agnostic to the picking backend.

Lightweight

Only compile what you use. All non-critical plugins can be disabled, including highlighting, selection, and any backends not in use.

Expressive

Pointer events make it easy to react to interactions like Click, Over, or Drag (13 pointer events are provided). Reacting to these interaction events on a specific entity is made possible with the On<Event> component. When events are generated, they bubble up the entity hierarchy starting from their target, looking for these event listener components.

This allows you to run callbacks when any children of an entity are interacted with:

fn setup(mut commands: Commands) {
    commands.spawn((
        // Spawn your entity here, e.g. a Mesh.
        // When dragged, mutate the `Transform` component on the dragged target entity:
        On::<Pointer<Drag>>::target_component_mut::<Transform>(|drag, transform| {
            transform.rotate_local_y(drag.delta.x / 50.0)
        }),
        On::<Pointer<Click>>::add_command::<DeleteTarget>(),
        On::<Pointer<Over>>::send_event::<Greeting>(),
    ));
}

Modular

Picking backends run hit tests to determine if a pointer is over any entities. This plugin provides a simple API to write your own backend in about 100 lines of code; it also includes half a dozen backends out of the box. These include rapier, bevy_mod_raycast, and bevy_egui among others. Multiple backends can be used at the same time!

Input Agnostic

Pointers can be controlled with anything, whether its the included mouse or touch inputs, or a custom gamepad input system you write yourself.

Robust

In addition to these features, this plugin also correctly handles multitouch and multiple windows.

Getting Started

Making objects pickable is pretty straightforward. In the most minimal cases, it’s as simple as:

use bevy_mod_picking::prelude::*;
App::new()
    .add_plugins(DefaultPlugins)
    .add_plugins(DefaultPickingPlugins);

commands.spawn((
    PbrBundle::default(),           // The `bevy_picking_raycast` backend works with meshes
    PickableBundle::default(),      // Makes the entity pickable
    RaycastPickTarget::default()    // Marker for the `bevy_picking_raycast` backend
));

commands.spawn((
    Camera3dBundle::default(),
    RaycastPickCamera::default(),   // Enable picking using this camera
));

Next Steps

To learn more, take a look at the examples in the ./examples directory.

The Picking Pipeline

This plugin is designed to be extremely modular. To do so, it works in well-defined stages that form a pipeline, where events are used to pass data between each stage. All the types needed for the pipeline are defined in the bevy_picking_core crate.

Input (bevy_picking_input)

The first stage of the pipeline is to gather inputs and create pointers. This stage is ultimately responsible for generating InputMove and InputPress events. The provided crate does this automatically for mouse, touch, and pen inputs. If you wanted to implement your own pointer, controlled by some other input, you can do that here.

Because pointer positions and presses are driven by these events, you can use them to mock inputs for testing.

After inputs are generated, they are then collected to update the current PointerLocation for each pointer.

Backend (bevy_picking_core::backend)

A picking backend only has one job: reading PointerLocation components, and producing PointerHits.

You will eventually need to choose which picking backend(s) you want to use. This plugin uses bevy_mod_raycast by default; it works with bevy Meshes out of the box and requires no extra dependencies. These qualities make it useful when prototyping, however it is not particularly performant for large meshes. Consider switching to the rapier backend if performance becomes a problem or if you already have the dependency in-tree. For simple or low-poly games, it may never be an issue.

It’s important to understand that you can mix and match backends! For example, you might have a backend for your UI, and one for the 3d scene, with each being specialized for their purpose. This crate provides some backends out of the box, but you can even write your own. It’s been made as easy as possible intentionally; the entire bevy_mod_raycast backend is less than 100 lines of code.

Focus (bevy_picking_core::focus)

The next step is to use the data from the backends, combine and sort the results, and determine what each cursor is hovering over, producing a HoverMap. Note that just because a pointer is over an entity, it is not necessarily hovering that entity. Although multiple backends may be reporting that a pointer is over an entity, the focus system needs to determine which one(s) are actually being hovered based on the pick depth, order of the backend, and the Pickable state of the entity. In other words, if one entity is in front of another, only the topmost one will be hovered, even if the pointer is within the bounds of both entities.

Events (bevy_picking_core::events)

In the final step, the high-level pointer events are generated, such as events that trigger when a pointer hovers or clicks an entity. These simple events are then used to generate more complex events for dragging and dropping. Once all events have been generated, the event bubbling systems propagate events through the entity hierarchy, triggering On<E> callbacks.

Because it is completely agnostic to the the earlier stages of the pipeline, you can easily extend the plugin with arbitrary backends and input methods.

Re-exports

• pub use bevy_picking_core as picking_core;
• pub use bevy_picking_input as input;
• pub use bevy_picking_highlight as highlight;
• pub use bevy_picking_selection as selection;

Modules

• backend
  This module provides a simple interface for implementing a picking backend.
• backends
  Picking backend exports, feature-gated.
• debug
  Text and on-screen debugging tools
• events
  Processes data from input and backends, producing interaction events.
• focus
  Determines which entities are being hovered by which pointers.
• pointer
  Types and systems for pointer inputs, such as position and buttons.
• prelude
  Common imports

Structs

• DefaultPickingPlugins
  A “batteries-included” set of plugins that adds everything needed for picking, highlighting, and multiselect. Backends are automatically added if their corresponding feature is enabled.
• PickableBundle
  Makes an entity pickable.
• PointerBundle
  Bundle of components needed for a fully-featured pointer.