How to implement drag and drop in Blazor?

Question

I know Blazor is a new technology. Its current release states v0.5.1

However I'm currently implementing a PoC for a new web application. We'd like to have the feature drag&drop in the app. I tried to implement it the Blazor way but it does not work.

My droptarget:

<div class="col" ondragstart="@AllowDragOver" ondrop="@Add">

And the draggable item:

<span class="badge badge-warning" draggable="true">îtem 1</span>

The Blazor C# code:

@functions {

void Add()
{
     Items.Add("hello");
}

void AllowDragOver(UIDragEventArgs e)
{
}

}

The problem is that the drop target does not show in the browser as a drop target:

What I've read so far is that when attaching an event handler to a Blazor C# function (e.g. ondragstart) than the default behavior is the well-known "e.preventDefault()" which should make the drop target droppable.

Does anyone know how to fix this?

Sven

Answer 1

I'd like to post my solution.

What I've found out so far is that dataTransfer.setData currently does not work in Blazor 0.5.1. I can workaround this with saving the dragged element in a shared C# class that is injected as a DI service. The container needs to have the call to "e.preventDefault()" to be a valid drop target. Though this is not possible in C# you easily can call this as pure Javascript:

<div class="col-sm-1" dropzone="move" ondragenter="@(e => OnContainerDragEnter(e))" ondragover="event.preventDefault();" ondragleave="@(e => OnContainerDragLeave(e))"
 ondrop="@(e => OnContainerDrop(e, Date))" style="@_highlightColor;@_highlightDropTargetStyle">

Drag and drop works pretty good with C# and very smooth without flickering and interruptions. I will create a simple working example in the next days.

UPDATE:

Sorry, my example is already outdated and currently I haven't enough time to maintain this demo along with its source. So I'd like to add a link to a working demo from Chris Sainty: https://chrissainty.com/investigating-drag-and-drop-with-blazor/