@inject AssignmentDragContainer dragContainer
@inject CoursePlanner planner

@code {
  [Parameter]
  [EditorRequired]
  public LocalAssignment Assignment { get; set; } = new();

  private void dropOnDate(DateTime dropDate)
  {

    if (planner.LocalCourse == null) return;
    var currentModule = planner
      .LocalCourse
      .Modules
      .First(m => 
        m.Assignments
          .Select(a => a.id)
          .Contains(Assignment.id)
      ) ?? throw new Exception("in day callback, could not find module");
    var moduleWithUpdatedAssignment = currentModule with 
    {
      Assignments = currentModule.Assignments.Select(a => 
        a.id != Assignment.id
        ? a
        : a with 
        {
          due_at=dropDate
        }
      )
    };
    var updatedModules = planner.LocalCourse.Modules
      .Select(m => 
        m.Name == moduleWithUpdatedAssignment.Name 
          ? moduleWithUpdatedAssignment 
          : m
      );
    var newCourse = planner.LocalCourse with 
    {
      Modules = updatedModules
    };
    planner.LocalCourse = newCourse;
  }
  private void dropOnModule(LocalModule module)
  {
    if (planner.LocalCourse == null) return;
    var newModules = planner.LocalCourse.Modules.Select(m => 
      m.Name != module.Name
        ? m with 
        {
          Assignments = m.Assignments.Where(a => a.id != Assignment.id)
        }
        : m with 
        {
          Assignments = m.Assignments.Append(Assignment)
        }
    );

    var newCourse = planner.LocalCourse with 
    {
      Modules = newModules
    };
    planner.LocalCourse = newCourse;
  }

  private void DropCallback (DateTime? dropDate, LocalModule? module) 
  {
    if (module == null)
    {
      dropOnDate(dropDate ?? Assignment.due_at);
    } 
    else
    {
      dropOnModule(module);
    }
  }
  private void HandleDragStart()
  {
    dragContainer.DropCallback = DropCallback;
  }

  private void HandleDragEnd()
  {
    dragContainer.DropCallback = null;
  }
}

<div
  draggable="true"
  @ondragstart="HandleDragStart"
  @ondragend="HandleDragEnd"
  role="button"
>
  @Assignment.name
</div>