Drag and Drop System

The drag and drop system allows players to move game objects like cards between different zones using intuitive mouse-based interactions.

Components

Draggable

The Draggable component marks entities that can be dragged and contains:

#![allow(unused)]
fn main() {
pub struct Draggable {
    /// Whether the entity is currently being dragged
    pub dragging: bool,
    /// Offset from the mouse cursor to the entity's origin
    pub drag_offset: Vec2,
    /// Z-index for rendering order
    pub z_index: f32,
}
}

Core System

The drag system is implemented in drag_system which handles:

Detecting when a draggable entity is clicked
Starting the drag operation with the appropriate offset
Moving the entity with the mouse cursor
Handling release events to end the drag operation

The system automatically manages z-indices to ensure that dragged objects appear on top of other game elements. When multiple draggable entities overlap, the one with the highest z-index is selected for dragging.

Integration Points

Card Integration

The drag and drop system integrates with the card visualization system to allow players to:

Move cards between hand, battlefield, and other zones
Organize cards within a zone
Reveal cards or interact with them in specific ways

Targeting Integration

Drag and drop operations are used in conjunction with the targeting system to select targets for spells and abilities. This creates a fluid interaction where players can drag a card to play it and then naturally continue to select targets.

Plugin Setup

To enable drag and drop functionality, the DragPlugin must be added to your Bevy app:

#![allow(unused)]
fn main() {
app.add_plugins(DragPlugin);
}

Implementation Example

Below is a simplified example of how to make an entity draggable:

#![allow(unused)]
fn main() {
// Spawn a card entity with a draggable component
commands.spawn((
    card_bundle,
    Draggable {
        dragging: false,
        drag_offset: Vec2::ZERO,
        z_index: 1.0,
    },
));
}

Customization

The drag and drop system can be customized by:

Adding different visual feedback during dragging
Implementing custom drop target detection
Creating drag constraints for certain game zones

For more information on implementing custom drag behaviors, see the game state integration documentation.

Rummage Documentation