@using Sandbox;
@using Sandbox.UI;
@inherits PanelComponent
@namespace Sandbox

<root>

	@if ( Current is not null )
	{
		var mouse = Mouse.Position;
		var left = mouse.x * Panel.ScaleFromScreen - 32;
		var top = mouse.y * Panel.ScaleFromScreen - 32;
		var iconStyle = string.IsNullOrEmpty( Current.Icon ) ? "" : $"background-image: url({Current.Icon}); ";
		<div class="dragging" style="@(iconStyle)left: @(left)px; top: @(top)px;" @ref="DragVisual">
			@if ( string.IsNullOrEmpty( Current.Icon ) )
			{
				<div class="drag-title">@Current.Title</div>
			}
		</div>
	}

</root>

@code
{
    public static DragHandler Instance { get; private set; }

    /// <summary>
    /// The data currently being dragged, or null.
    /// </summary>
    public static DragData Current { get; private set; }

    /// <summary>
    /// True if something is actively being dragged.
    /// </summary>
    public static bool IsDragging => Current is not null;

    Panel DragVisual { get; set; }
    private RootPanel _rootPanel;

    protected override void OnStart()
    {
        Instance = this;
    }

    protected override void OnDestroy()
    {
        if ( Instance == this )
            Instance = null;
    }

    /// <summary>
    /// Start dragging with the given data.
    /// </summary>
    public static void StartDragging( DragData data )
    {
        Current = data;
        data.Source.UserData = data;

        Instance?.StateHasChanged();
    }

    /// <summary>
    /// Stop dragging and clear all state.
    /// </summary>
    public static void StopDragging()
    {
        if (!IsDragging) return;

        Current = null;
        Instance?.StateHasChanged();
    }

    protected override void OnUpdate()
    {
        if ( !IsDragging ) return;

        if ( !_rootPanel.WantsMouseInput() )
        {
            StopDragging();
            return;
        }

        if ( DragVisual is not null )
        {
            var mouse = Mouse.Position;
            DragVisual.Style.Left = Length.Pixels( mouse.x * Panel.ScaleFromScreen - 32 );
            DragVisual.Style.Top = Length.Pixels( mouse.y * Panel.ScaleFromScreen - 32 );
        }
    }

    protected override void OnTreeBuilt()
    {
        _rootPanel = Panel.FindRootPanel();
    }

	protected override int BuildHash() => HashCode.Combine( Current );
}