Implementing Drag and Drop

From the blog:
hyperapp - Tiny Library for Frontend Applications - Interview with Jorge Bucaran

Our Kanban application is almost usable now. It looks alright and there's basic functionality in place. In this chapter, we will integrate drag and drop functionality to it as we set up React DnD.

After this chapter, you should be able to sort notes within a lane and drag them from one lane to another. Although this sounds simple, there is quite a bit of work to do as we need to annotate our components the right way and develop the logic needed.

Setting Up React DnD#

As the first step, we need to connect React DnD with our project. We are going to use the HTML5 Drag and Drop based back-end. There are specific back-ends for testing and touch.

In order to set it up, we need to use the DragDropContext decorator and provide the HTML5 back-end to it. To avoid unnecessary wrapping, I'll use Redux compose to keep the code neater and more readable:

app/components/App.jsx

import React from 'react';
import uuid from 'uuid';
import {compose} from 'redux'; import {DragDropContext} from 'react-dnd'; import HTML5Backend from 'react-dnd-html5-backend';
import connect from '../libs/connect'; import Lanes from './Lanes'; import LaneActions from '../actions/LaneActions'; const App = ({LaneActions, lanes}) => { const addLane = () => { LaneActions.create({ id: uuid.v4(), name: 'New lane' }); }; return ( <div> <button className="add-lane" onClick={addLane}>+</button> <Lanes lanes={lanes} /> </div> ); };
export default connect(({lanes}) => ({ lanes }), { LaneActions })(App)
export default compose( DragDropContext(HTML5Backend), connect( ({lanes}) => ({lanes}), {LaneActions} ) )(App)

After this change, the application should look exactly the same as before. We are ready to add some sweet functionality to it now.

Allowing Notes to Be Dragged#

Allowing notes to be dragged is a good first step. Before that, we need to set up a constant so that React DnD can tell different kind of draggables apart. Set up a file for tracking Note as follows:

app/constants/itemTypes.js

export default {
  NOTE: 'note'
};

This definition can be expanded later as we add new types, such as LANE, to the system.

Next, we need to tell our Note that it's possible to drag it. This can be achieved using the DragSource annotation. Replace Note with the following implementation:

app/components/Note.jsx

import React from 'react';
import {DragSource} from 'react-dnd';
import ItemTypes from '../constants/itemTypes';

const Note = ({
  connectDragSource, children, ...props
}) => {
  return connectDragSource(
    <div {...props}>
      {children}
    </div>
  );
};

const noteSource = {
  beginDrag(props) {
    console.log('begin dragging note', props);

    return {};
  }
};

export default DragSource(ItemTypes.NOTE, noteSource, connect => ({
  connectDragSource: connect.dragSource()
}))(Note)

If you try to drag a Note now, you should see something like this at the browser console:

begin dragging note Object {className: "note", children: Array[2]}

Just being able to drag notes isn't enough. We need to annotate them so that they can accept dropping. Eventually this will allow us to swap them as we can trigger logic when we are trying to drop a note on top of another.

In case we wanted to implement dragging based on a handle, we could apply connectDragSource only to a specific part of a Note.
Note that React DnD doesn't support hot loading perfectly. You may need to refresh the browser to see the log messages you expect!

Allowing Notes to Detect Hovered Notes#

Annotating notes so that they can notice that another note is being hovered on top of them is a similar process. In this case we'll have to use a DropTarget annotation:

app/components/Note.jsx

import React from 'react';
import {DragSource} from 'react-dnd';
import {compose} from 'redux'; import {DragSource, DropTarget} from 'react-dnd';
import ItemTypes from '../constants/itemTypes'; const Note = ({
connectDragSource, children, ...props
connectDragSource, connectDropTarget, children, ...props
}) => {
return connectDragSource(
return compose(connectDragSource, connectDropTarget)(
<div {...props}> {children} </div> ); }; const noteSource = { beginDrag(props) { console.log('begin dragging note', props); return {}; } };
const noteTarget = { hover(targetProps, monitor) { const sourceProps = monitor.getItem(); console.log('dragging note', sourceProps, targetProps); } };
export default DragSource(ItemTypes.NOTE, noteSource, connect => ({ connectDragSource: connect.dragSource() }))(Note)
export default compose( DragSource(ItemTypes.NOTE, noteSource, connect => ({ connectDragSource: connect.dragSource() })), DropTarget(ItemTypes.NOTE, noteTarget, connect => ({ connectDropTarget: connect.dropTarget() })) )(Note)

If you try hovering a dragged note on top of another now, you should see messages like this at the console:

dragging note Object {} Object {className: "note", children: Array[2]}

Both decorators give us access to the Note props. In this case, we are using monitor.getItem() to access them at noteTarget. This is the key to making this to work properly.

Developing onMove API for Notes#

Now, that we can move notes around, we can start to define logic. The following steps are needed:

  1. Capture Note id on beginDrag.
  2. Capture target Note id on hover.
  3. Trigger onMove callback on hover so that we can deal with the logic elsewhere. LaneStore would be the ideal place for that.

Based on the idea above we can see we should pass id to a Note through a prop. We also need to set up a onMove callback, define LaneActions.move, and LaneStore.move stub.

Accepting id and onMove at Note#

We can accept id and onMove props at Note like below. There is an extra check at noteTarget as we don't need trigger hover in case we are hovering on top of the Note itself:

app/components/Note.jsx

...

const Note = ({
  connectDragSource, connectDropTarget,
children, ...props
onMove, id, children, ...props
}) => { return compose(connectDragSource, connectDropTarget)( <div {...props}> {children} </div> ); };
const noteSource = { beginDrag(props) { console.log('begin dragging note', props); return {}; } };
const noteSource = { beginDrag(props) { return { id: props.id }; } };
const noteTarget = { hover(targetProps, monitor) { const sourceProps = monitor.getItem(); console.log('dragging note', sourceProps, targetProps); } };
const noteTarget = { hover(targetProps, monitor) { const targetId = targetProps.id; const sourceProps = monitor.getItem(); const sourceId = sourceProps.id; if(sourceId !== targetId) { targetProps.onMove({sourceId, targetId}); } } };
...

Having these props isn't useful if we don't pass anything to them at Notes. That's our next step.

Passing id and onMove from Notes#

Passing a note id and onMove is simple enough:

app/components/Notes.jsx

import React from 'react';
import Note from './Note';
import Editable from './Editable';

export default ({
  notes,
  onNoteClick=() => {}, onEdit=() => {}, onDelete=() => {}
}) => (
  <ul className="notes">{notes.map(({id, editing, task}) =>
    <li key={id}>
<Note className="note" onClick={onNoteClick.bind(null, id)}>
<Note className="note" id={id} onClick={onNoteClick.bind(null, id)} onMove={({sourceId, targetId}) => console.log('moving from', sourceId, 'to', targetId)}>
<Editable className="editable" editing={editing} value={task} onEdit={onEdit.bind(null, id)} /> <button className="delete" onClick={onDelete.bind(null, id)}>x</button> </Note> </li> )}</ul> )

If you hover a note on top of another, you should see console messages like this:

moving from 3310916b-5b59-40e6-8a98-370f9c194e16 to 939fb627-1d56-4b57-89ea-04207dbfb405

Adding Action and Store Method for Moving#

The logic of drag and drop goes as follows. Suppose we have a lane containing notes A, B, C. In case we move A below C we should end up with B, C, A. In case we have another list, say D, E, F, and move A to the beginning of it, we should end up with B, C and A, D, E, F.

In our case, we'll get some extra complexity due to lane to lane dragging. When we move a Note, we know its original position and the intended target position. Lane knows what Notes belong to it by id. We are going to need some way to tell LaneStore that it should perform the logic over the given notes. A good starting point is to define LaneActions.move:

app/actions/LaneActions.js

import alt from '../libs/alt';

export default alt.generateActions(
  'create', 'update', 'delete',
  'attachToLane', 'detachFromLane',
  'move'
);

We should connect this action with the onMove hook we just defined:

app/components/Notes.jsx

import React from 'react';
import Note from './Note';
import Editable from './Editable';
import LaneActions from '../actions/LaneActions';
export default ({ notes, onNoteClick=() => {}, onEdit=() => {}, onDelete=() => {} }) => ( <ul className="notes">{notes.map(({id, editing, task}) => <li key={id}> <Note className="note" id={id} onClick={onNoteClick.bind(null, id)}
onMove={({sourceId, targetId}) => console.log('moving from', sourceId, 'to', targetId)}>
onMove={LaneActions.move}>
<Editable className="editable" editing={editing} value={task} onEdit={onEdit.bind(null, id)} /> <button className="delete" onClick={onDelete.bind(null, id)}>x</button> </Note> </li> )}</ul> )
It could be a good idea to refactor onMove as a prop to make the system more flexible. In our implementation the Notes component is coupled with LaneActions. This isn't particularly nice if you want to use it in some other context.

We should also define a stub at LaneStore to see that we wired it up correctly:

app/stores/LaneStore.js

import LaneActions from '../actions/LaneActions';

export default class LaneStore {
  ...
  detachFromLane({laneId, noteId}) {
    ...
  }
move({sourceId, targetId}) { console.log(`source: ${sourceId}, target: ${targetId}`); }
}

You should see the same log messages as earlier.

Next, we'll need to add some logic to make this work. We can use the logic outlined above here. We have two cases to worry about: moving within a lane itself and moving from lane to another.

Implementing Note Drag and Drop Logic#

Moving within a lane itself is complicated. When you are operating based on ids and perform operations one at a time, you'll need to take possible index alterations into account. As a result, I'm using update immutability helper from React as that solves the problem in one pass.

It is possible to solve the lane to lane case using splice. First, we splice out the source note, and then we splice it to the target lane. Again, update could work here, but I didn't see much point in that given splice is nice and simple. The code below illustrates a mutation based solution:

app/stores/LaneStore.js

import update from 'react-addons-update';
import LaneActions from '../actions/LaneActions'; export default class LaneStore { ...
move({sourceId, targetId}) { console.log(`source: ${sourceId}, target: ${targetId}`); }
move({sourceId, targetId}) { const lanes = this.lanes; const sourceLane = lanes.filter(lane => lane.notes.includes(sourceId))[0]; const targetLane = lanes.filter(lane => lane.notes.includes(targetId))[0]; const sourceNoteIndex = sourceLane.notes.indexOf(sourceId); const targetNoteIndex = targetLane.notes.indexOf(targetId); if(sourceLane === targetLane) { // move at once to avoid complications sourceLane.notes = update(sourceLane.notes, { $splice: [ [sourceNoteIndex, 1], [targetNoteIndex, 0, sourceId] ] }); } else { // get rid of the source sourceLane.notes.splice(sourceNoteIndex, 1); // and move it to target targetLane.notes.splice(targetNoteIndex, 0, sourceId); } this.setState({lanes}); }
}

If you try out the application now, you can actually drag notes around and it should behave as you expect. Dragging to empty lanes doesn't work, though, and the presentation could be better.

It would be nicer if we indicated the dragged note's location more clearly. We can do this by hiding the dragged note from the list. React DnD provides us the hooks we need for this purpose.

Indicating Where to Move#

React DnD provides a feature known as state monitors. Through it we can use monitor.isDragging() and monitor.isOver() to detect which Note we are currently dragging. It can be set up as follows:

app/components/Note.jsx

import React from 'react';
import {compose} from 'redux';
import {DragSource, DropTarget} from 'react-dnd';
import ItemTypes from '../constants/itemTypes';

const Note = ({
connectDragSource, connectDropTarget, onMove, id, children, ...props
connectDragSource, connectDropTarget, isDragging, isOver, onMove, id, children, ...props
}) => { return compose(connectDragSource, connectDropTarget)(
<div {...props}> {children} </div>
<div style={{ opacity: isDragging || isOver ? 0 : 1 }} {...props}>{children}</div>
); }; ... export default compose(
DragSource(ItemTypes.NOTE, noteSource, connect => ({ connectDragSource: connect.dragSource() })), DropTarget(ItemTypes.NOTE, noteTarget, connect => ({ connectDropTarget: connect.dropTarget() }))
DragSource(ItemTypes.NOTE, noteSource, (connect, monitor) => ({ connectDragSource: connect.dragSource(), isDragging: monitor.isDragging() })), DropTarget(ItemTypes.NOTE, noteTarget, (connect, monitor) => ({ connectDropTarget: connect.dropTarget(), isOver: monitor.isOver() }))
)(Note)

If you drag a note within a lane, the dragged note should be shown as blank.

There is one little problem in our system. We cannot drag notes to an empty lane yet.

Dragging Notes to Empty Lanes#

To drag notes to empty lanes, we should allow them to receive notes. Just as above, we can set up DropTarget based logic for this. First, we need to capture the drag on Lane:

app/components/Lane.jsx

import React from 'react';
import {compose} from 'redux'; import {DropTarget} from 'react-dnd'; import ItemTypes from '../constants/itemTypes';
import connect from '../libs/connect'; import NoteActions from '../actions/NoteActions'; import LaneActions from '../actions/LaneActions'; import Notes from './Notes'; import LaneHeader from './LaneHeader'; const Lane = ({
lane, notes, LaneActions, NoteActions, ...props
connectDropTarget, lane, notes, LaneActions, NoteActions, ...props
}) => { ...
return (
return connectDropTarget(
... ); }; function selectNotesByIds(allNotes, noteIds = []) { ... }
const noteTarget = { hover(targetProps, monitor) { const sourceProps = monitor.getItem(); const sourceId = sourceProps.id; // If the target lane doesn't have notes, // attach the note to it. // // `attachToLane` performs necessarly // cleanup by default and it guarantees // a note can belong only to a single lane // at a time. if(!targetProps.lane.notes.length) { LaneActions.attachToLane({ laneId: targetProps.lane.id, noteId: sourceId }); } } };
export default connect( ({notes}) => ({ notes }), { NoteActions, LaneActions } )(Lane)
export default compose( DropTarget(ItemTypes.NOTE, noteTarget, connect => ({ connectDropTarget: connect.dropTarget() })), connect(({notes}) => ({ notes }), { NoteActions, LaneActions }) )(Lane)

After attaching this logic, you should be able to drag notes to empty lanes.

Our current implementation of attachToLane does a lot of the hard work for us. If it didn't guarantee that a note can belong only to a single lane at a time, we would need to adjust our logic. It's good to have these sort of invariants within the state management system.

Fixing Editing Behavior During Dragging#

The current implementation has a small glitch. If you edit a note, you can still drag it around while it's being edited. This isn't ideal as it overrides the default behavior most people are used to. You cannot for instance double-click on an input to select all the text.

Fortunately, this is simple to fix. We'll need to use the editing state per each Note to adjust its behavior. First we need to pass editing state to an individual Note:

app/components/Notes.jsx

import React from 'react';
import Note from './Note';
import Editable from './Editable';
import LaneActions from '../actions/LaneActions';

export default ({
  notes,
  onNoteClick=() => {}, onEdit=() => {}, onDelete=() => {}
}) => (
  <ul className="notes">{notes.map(({id, editing, task}) =>
    <li key={id}>
      <Note className="note" id={id}
editing={editing}
onClick={onNoteClick.bind(null, id)} onMove={LaneActions.move}> <Editable className="editable" editing={editing} value={task} onEdit={onEdit.bind(null, id)} /> <button className="delete" onClick={onDelete.bind(null, id)}>x</button> </Note> </li> )}</ul> )

Next we need to take this into account while rendering:

app/components/Note.jsx

import React from 'react';
import {compose} from 'redux';
import {DragSource, DropTarget} from 'react-dnd';
import ItemTypes from '../constants/itemTypes';

const Note = ({
  connectDragSource, connectDropTarget, isDragging,
isOver, onMove, id, children, ...props
isOver, onMove, id, editing, children, ...props
}) => {
// Pass through if we are editing const dragSource = editing ? a => a : connectDragSource;
return compose(connectDragSource, connectDropTarget)(
return compose(dragSource, connectDropTarget)(
<div style={{ opacity: isDragging || isOver ? 0 : 1 }} {...props}>{children}</div> ); }; ...

This small change gives us the behavior we want. If you try to edit a note now, the input should work as you might expect it to behave normally.

Design-wise it was a good idea to keep editing state outside of Editable. If we hadn't done that, implementing this change would have been a lot harder as we would have had to extract the state outside of the component.

Now we have a Kanban table that is actually useful! We can create new lanes and notes, and edit and remove them. In addition we can move notes around. Mission accomplished!

Conclusion#

In this chapter, you saw how to implement drag and drop for our little application. You can model sorting for lanes using the same technique. First, you mark the lanes to be draggable and droppable, then you sort out their ids, and finally, you'll add some logic to make it all work together. It should be considerably simpler than what we did with notes.

I encourage you to expand the application. The current implementation should work just as a starting point for something greater. Besides extending the DnD implementation, you can try adding more data to the system. You could also do something to the visual outlook. One option would be to try out various styling approaches discussed at the Styling React chapter.

To make it harder to break the application during development, you can also implement tests as discussed at Testing React. Typing with React discussed yet more ways to harden your code. Learning these approaches can be worthwhile. Sometimes it may be worth your while to design your applications test first. It is a valuable approach as it allows you to document your assumptions as you go.

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