Drag and drop | draggable=true demo

[jdu]

Would be nice to see an example for this as I'm struggling pretty badly trying to get drag and drop to work in LitElement at the moment. I can get it working with Vanilla js easily, but LitElement is doing my head in trying to figure it out without having to import the whole Polymer Gesture library.

[thepassle]

<div draggable="true"></div> should work in LitElement, do you have a more specific example/repro I could take a look at?

Additionally:
<div draggable=${true}></div> <- works
<div draggable=${'true'}></div> <- works
<div ?draggable=${true}></div> <- does not work

Apparently the draggable attribute is not an actual boolean attribute, but an enumerated attribute.

More info here: lit/lit-element#565

[jdu]

Neither of the elements in the below become draggable either through setting the draggable attribute through connectedCallback or directly on the custom element. I have drag and drop peppered through the apps I work on in plain vanilla JS and React but for some reason my brain isn't wrapping around using LitElement custom elements...

import {html, css, LitElement} from "lit-element";

class DragTestInElement extends LitElement {
    static get styles() {
        return css`
            div {
                display: block;
                width: 120px;
                height: 40px;
                background: green;
            }
        `;
    }


    connectedCallback() {
        super.connectedCallback();
        this.setAttribute("draggable", true);
    }

    render() {
        return html`
            <div>DragInElement</div>
        `;
    }
}
window.customElements.define("drag-test-in", DragTestInElement);

class DragTestOnElement extends LitElement {
    static get styles() {
        return css`
            div {
                display: block;
                width: 120px;
                height: 40px;
                background: purple;
            }
        `;
    }
    render() {
        return html`
            <div>DragOnElement</div>
        `;
    }
}
window.customElements.define("drag-test-on", DragTestOnElement);

class DragArea extends LitElement {
    render() {
        return html`
            <div class="drag-area">
                <drag-test-on draggable="true"></drag-test-on>
                <drag-test-in></drag-test-in>

            </div>
        `;
    }
}
window.customElements.define("drag-area", DragArea);

let el = document.getElementById("application");
el.innerHTML = "";
let root = document.createElement("drag-area");
el.appendChild(root);

[jdu]

I've also tried a number of different naming variations on the ondragstart event, appending a plain vanilla DOM node with draggables and el.ondragstart = function() {...} works fine in the same example. As well I overrode createRenderRoot within the DragArea component to remove the shadow dom and render DragArea as a plain element tree in case it was something to do with nested Shadow DOMS and even then the drag-test-in and drag-test-on elements won't drag or fire the ondragstart event under those circumstance. Maybe a bug in LitElement/LitHTML not propagating the event or draggable property?

[jdu]

Right I've figured this out after poking and prodding it for a good hour or so.

class DragTestOnElement extends LitElement {
    static get styles() {
        return css`
            :host {
                width: 120px;
                height: 40px;
                background: purple;
                border: 1px solid black;
                display: block;
            }
        `;
    }
    render() {
        return html`
            <div></div>
        `;
    }
}
window.customElements.define("drag-test-on", DragTestOnElement);

class DragArea extends LitElement {
    static get styles() {
        return css`
            .drag-test {
                display: block;
                width: 40px;
                height: 40px;
                background: pink;
                border: 1px solid black;
            }
        `;
    }

    hitTest(e) {
        e.dataTransfer.setData("e", "TEST");
        console.log(e);
    }

    render() {
        return html`
            <style>
                .drag-test {
                    display: block;
                    width: 40px;
                    height: 40px;
                    background: pink;
                    border: 1px solid black;
                }
            </style>
            <div class="drag-area">
                <drag-test-on
                draggable="true"
                @dragstart=${this.hitTest}></drag-test-on>
            </div>
        `;
    }
}
window.customElements.define("drag-area", DragArea);

let el = document.getElementById("application");
let root = document.createElement("drag-area");
el.appendChild(root);

So there was a couple things I was doing wrong, that I'll document here in case others come across this, these may have been clear for someone working in LitElement but coming from Vanilla and React I didn't get it right away:

1. The component that is going to be dragged MUST have :host styles, for instance display: block; width: 40px; height: 40px; even if the inner element has styling and size. @click work on the component without styling the :host node but @dragstart won't for some reason.
2. Use the @ prefixed event markup in lit-html, unprefixed ondragstart, onDragStart, dragStart don't seem to work

I think this is still worth setting up an example of drag and drop as it's a fairly common use case for more complex events than click, hover, etc... and it's clearly been a pain point for me (a moderately experienced developer) to see what the issue in my implementation was.

[thepassle]

Sorry for the delay in getting back to you

I think this could be good to add to the demos under the Advanced section, and additionally it might be good as a write up for the faq like this one: https://open-wc.org/faq/rerender.html as well

Would you be willing to make a PR to add a demo and add the writeup to the faq?

[ernsheong]

See lit/lit#460

Gist: You need to have the drag events mutate the underlying data as you move stuff around. You cannot ever allow the DOM to change due to your drop/move, every DOM change has to be driven from the model.

I'll share some code:

      case "start":
        this._draggedId = itemId;
        e.target.closest(".option-item").classList.add("drag-item");

        break;
      case "enter": {
        if (this._draggedId == null) return; // ignore drag from other places
        e.target.closest(".option-item").classList.add("drag-hover");

        this._moveItem(this._draggedId, itemId);

        // See https://developer.mozilla.org/en-US/docs/Web/API/HTML_Drag_and_Drop_API/Drag_operations#droptargets
        e.preventDefault();
        break;
      }
      case "over": {
        // See https://developer.mozilla.org/en-US/docs/Web/API/HTML_Drag_and_Drop_API/Drag_operations#droptargets
        e.preventDefault();
        break;
      }
      case "leave":
        if (this._draggedId == null) return; // ignore drag from other places
        e.target.closest(".option-item").classList.remove("drag-hover");

        break;
      case "drop": {
        if (this._draggedId == null) return; // ignore drag from other places
        e.target.closest(".option-item").classList.remove("drag-hover");
        e.target.closest(".option-item").classList.remove("drag-item");

        this._moveItem(this._draggedId, itemId, true);
        break;
      }
      case "end":
        if (this._draggedId == null) return; // ignore drag from other places
        e.target.closest(".option-item").classList.remove("drag-item");

        this._draggedId = null;
        break;

Some DOM:

    this._model.map(
          (m, idx) => html`
            <div
              @dragstart="${e => this.dndEvent("start", m.id, e)}"
              @dragenter="${e => this.dndEvent("enter", m.id, e)}"
              @dragover="${e => this.dndEvent("over", m.id, e)}"
              @dragend="${e => this.dndEvent("end", m.id, e)}"
              @dragleave="${e => this.dndEvent("leave", m.id, e)}"
              @drop="${e => this.dndEvent("drop", m.id, e)}"
              draggable="true"
            >Item ${m.id}</div>`;

Here, _moveItem is just mutating the underlying list.

Works well for dragging stuff within a list... still figuring out how to drag and drop between different lists, etc. but the principle is the same: data-driven DOM changes

[caliny97]

@jdu- Thanks, your pointers saved my day. I wanted to contribute some other details to get dnd working.
The dropzone element needs to have handlers for @drop & @dragover, make sure to call preventDefault() in the handler for dragover otherwise the drop won't fire.

[rayanaradha]

Look into this complete sample draggable web-component project which was built using LitElement with JavaScript. I implement this with help of @jdu code. I hope this project will help you on this problem.

https://github.com/rayanaradha/Draggable-web-component

This project contains 3 classes.

DragTestOnElement - Component for Draggable item.
DragContainer - Component for Item Container.
DragArea - UI
In this project Draggable Items can be dragged and dropped between Item Containers.