Drag & Drop

Brine2D's drag-and-drop system is canvas-managed. Any IUIComponent can be a drag source; drop zones are UIDropTarget components. The payload is any type that implements IDragPayload.

IDragPayload

IDragPayload is a plain marker interface. Create one record or class per payload type:

C#

public record InventoryItemPayload(InventoryItem Item) : IDragPayload;
public record CardPayload(Card Card, int SourceSlot) : IDragPayload;

Registering a drag source

Any IUIComponent already on the canvas can be made draggable:

C#

var itemIcon = new UIImage(itemTexture, new Vector2(100, 200), new Vector2(48, 48));
_canvas.Add(itemIcon);

_canvas.RegisterDraggable(itemIcon, new InventoryItemPayload(myItem));

The canvas begins a drag when the user holds the left mouse button and moves more than 6 pixels (the drag threshold). During the drag it renders a ghost copy of the source component following the cursor.

Canvas drag events

C#

_canvas.OnDragStarted   += (source, payload) => Debug.Log("Drag started");
_canvas.OnDragCancelled += (source, payload) => Debug.Log("Dropped on nothing");

OnDragCancelled fires when the user releases over no registered drop target.

Registering a drop target

C#

var slot = new UIDropTarget
{
    Position      = new Vector2(300, 200),
    Size          = new Vector2(64, 64),
    IdleColor     = new Color(80, 80, 80, 60),
    HoverColor    = new Color(100, 180, 255, 100),
    BorderColor   = new Color(120, 180, 255, 180),
    BorderThickness = 2f,
};
_canvas.Add(slot);
_canvas.RegisterDropTarget(slot);

slot.OnDrop += payload =>
{
    if (payload is InventoryItemPayload p)
        EquipItem(p.Item);
};

Filtering accepted payloads

C#

slot.AcceptsPayload = payload => payload is InventoryItemPayload;

When AcceptsPayload is null (the default) the target accepts all payload types. When it returns false the cursor does not change to "drop" over that target and OnDrop will not fire.

UIDropTarget properties

Property	Type	Default	Description
`IdleColor`	`Color`	`(80,80,80,60)`	Background when no drag is over it
`HoverColor`	`Color`	`(100,180,255,100)`	Background while a compatible payload hovers
`BorderColor`	`Color`	`(120,180,255,180)`	Border around the zone; set alpha to 0 to hide
`BorderThickness`	`float`	`2`	Border width in pixels
`IsHovered`	`bool`	`false`	Read-only: set by the canvas during drag
`AcceptsPayload`	`Func<IDragPayload, bool>?`	`null`	Optional filter; `null` = accept all

Querying drag state

C#

bool           dragging = _canvas.IsDragging;
IUIComponent?  source   = _canvas.DragSource;

Full example: inventory drag and drop

C#

// Payload
public record SlotPayload(int SlotIndex, InventoryItem Item) : IDragPayload;

// Source slots
for (int i = 0; i < inventory.Slots.Count; i++)
{
    int slotIndex = i;
    var item      = inventory.Slots[i];

    var icon = new UIImage(item.Icon, new Vector2(10 + i * 56, 10), new Vector2(48, 48));
    _canvas.Add(icon);
    _canvas.RegisterDraggable(icon, new SlotPayload(slotIndex, item));
}

// Drop targets
for (int i = 0; i < equipment.Slots.Count; i++)
{
    int slotIndex = i;

    var target = new UIDropTarget
    {
        Position  = new Vector2(300, 10 + i * 56),
        Size      = new Vector2(48, 48),
        HoverColor = new Color(60, 180, 120, 140),
    };
    target.AcceptsPayload = p => p is SlotPayload sp && equipment.Accepts(slotIndex, sp.Item);
    target.OnDrop += payload =>
    {
        if (payload is SlotPayload sp)
            equipment.Equip(slotIndex, sp.Item, sp.SlotIndex);
    };

    _canvas.Add(target);
    _canvas.RegisterDropTarget(target);
}

// Listen for cancelled drags (dropped on nothing)
_canvas.OnDragCancelled += (_, _) => Debug.Log("Drag cancelled");

Notes

The canvas renders the drag ghost automatically using the source component's Render method at the current cursor position.
Drag sources must be added to the canvas with Add before calling RegisterDraggable.
Drop targets must also be added with Add before calling RegisterDropTarget.
There is no built-in multi-select; handle that at the payload level if needed.