Crate bevy_mod_picking

A flexible set of plugins that add picking functionality to your bevy app.

Overview

In the simplest case, this plugin allows you to click on things in the scene. However, it also allows you to express more complex interactions, like detecting when a touch input drags a UI element and drops it on a 3d mesh rendered to a different camera. The crate also provides event listeners, so you can attach On<Click> components to an entity, to run a one-shot bevy system.

The plugin works with any input, including mouse, touch, pens, or virtual pointers controlled by gamepads. It includes (optional) backends for rapier, bevy_xpbd, bevy_mod_raycast, bevy_ui, bevy_sprite, and egui, that can be mixed and matched out of the box, or you can write your own.

At its core, this crate provides a robust abstraction for computing picking state regardless of pointing devices, or what you are hit testing against.

Expressive Events

The plugin provides normal bevy events that can be listened to with EventReaders. These Pointer events allow you to respond to interactions like Click, Over, or Drag (13 pointer events are provided). However, this often causes boilerplate when you try to do something in response to that click, and you want the behavior to be different depending on the entity being clicked on.

This is where event listeners come in. 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. (See bevy_eventlistener for details.)

This allows you to run callbacks when any children of an entity are interacted with, and leads to succinct, expressive code:

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>(),
    ));
}

If you don’t need event bubbling or callbacks, you can respond to pointer events like you would any other bevy event, using EventReader<Pointer<Click>>, EventReader<Pointer<Move>>, etc.

Modularity

Mix and Match Hit Testing Backends

The plugin attempts to handle all the hard parts for you, all you need to do is tell it when a pointer is hitting any entities. Multiple backends can be used at the same time! Use this simple API to write your own backend in about 100 lines of code. The plugin also includes half a dozen backends out of the box. These include rapier, bevy_xpbd, bevy_mod_raycast, bevy_ui, bevy_egui, and bevy_sprite.

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 to control a virtual pointer.

Robustness

In addition to these features, this plugin also correctly handles multitouch, multiple windows, multiple cameras, viewports, and render layers. Using this as a library allows you to write a picking backend that can interoperate with any other picking backend.

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); // Includes a mesh raycasting backend by default

commands.spawn((
    PbrBundle::default(),           // The raycasting backend works with meshes
    PickableBundle::default(),      // Makes the entity pickable, and adds optional features
));

commands.spawn(Camera3dBundle::default());

Next Steps

To learn more, take a look at the examples in the ./examples directory. You can read the next section to understand how the plugin works.

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.

Pointers (bevy_picking_core::pointer)

The first stage of the pipeline is to gather inputs and update 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 can find all documentation and types needed to implement a backend at bevy_picking_core::backend.

You will eventually need to choose which picking backend(s) you want to use. This plugin uses bevy_mod_raycast for 3d 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 might not be performant enough for your use case. Consider switching to the rapier backend if performance becomes a problem or if you already have the dependency in-tree. For many games, it may never be an issue. You can find all the provided backends in the backends module.

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 bevy_mod_raycast backend is 50 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 hitting an entity, the focus system needs to determine which entities are actually being hovered by this pointer 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, usually only the topmost one will be hovered.

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, yet still use all the high level features.

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.