Recommended way of making React component/div draggable
The answer by @codewithfeeling is horribly wrong and lags your page! Here's a version of his code with issues fixed and annotated. This should be the most up to date hook-based answer here now.
import React, { useRef, useState, useEffect, useCallback } from "react";
/// throttle.ts
export const throttle = (f) => {
let token = null,
lastArgs = null;
const invoke = () => {
f(...lastArgs);
token = null;
};
const result = (...args) => {
lastArgs = args;
if (!token) {
token = requestAnimationFrame(invoke);
}
};
result.cancel = () => token && cancelAnimationFrame(token);
return result;
};
/// use-draggable.ts
const id = (x) => x;
// complex logic should be a hook, not a component
const useDraggable = ({ onDrag = id } = {}) => {
// this state doesn't change often, so it's fine
const [pressed, setPressed] = useState(false);
// do not store position in useState! even if you useEffect on
// it and update `transform` CSS property, React still rerenders
// on every state change, and it LAGS
const position = useRef({ x: 0, y: 0 });
const ref = useRef();
// we've moved the code into the hook, and it would be weird to
// return `ref` and `handleMouseDown` to be set on the same element
// why not just do the job on our own here and use a function-ref
// to subscribe to `mousedown` too? it would go like this:
const unsubscribe = useRef();
const legacyRef = useCallback((elem) => {
// in a production version of this code I'd use a
// `useComposeRef` hook to compose function-ref and object-ref
// into one ref, and then would return it. combining
// hooks in this way by hand is error-prone
// then I'd also split out the rest of this function into a
// separate hook to be called like this:
// const legacyRef = useDomEvent('mousedown');
// const combinedRef = useCombinedRef(ref, legacyRef);
// return [combinedRef, pressed];
ref.current = elem;
if (unsubscribe.current) {
unsubscribe.current();
}
if (!elem) {
return;
}
const handleMouseDown = (e) => {
// don't forget to disable text selection during drag and drop
// operations
e.target.style.userSelect = "none";
setPressed(true);
};
elem.addEventListener("mousedown", handleMouseDown);
unsubscribe.current = () => {
elem.removeEventListener("mousedown", handleMouseDown);
};
}, []);
// useEffect(() => {
// return () => {
// // this shouldn't really happen if React properly calls
// // function-refs, but I'm not proficient enough to know
// // for sure, and you might get a memory leak out of it
// if (unsubscribe.current) {
// unsubscribe.current();
// }
// };
// }, []);
useEffect(() => {
// why subscribe in a `useEffect`? because we want to subscribe
// to mousemove only when pressed, otherwise it will lag even
// when you're not dragging
if (!pressed) {
return;
}
// updating the page without any throttling is a bad idea
// requestAnimationFrame-based throttle would probably be fine,
// but be aware that naive implementation might make element
// lag 1 frame behind cursor, and it will appear to be lagging
// even at 60 FPS
const handleMouseMove = throttle((event) => {
// needed for TypeScript anyway
if (!ref.current || !position.current) {
return;
}
const pos = position.current;
// it's important to save it into variable here,
// otherwise we might capture reference to an element
// that was long gone. not really sure what's correct
// behavior for a case when you've been scrolling, and
// the target element was replaced. probably some formulae
// needed to handle that case. TODO
const elem = ref.current;
position.current = onDrag({
x: pos.x + event.movementX,
y: pos.y + event.movementY
});
elem.style.transform = `translate(${pos.x}px, ${pos.y}px)`;
});
const handleMouseUp = (e) => {
e.target.style.userSelect = "auto";
setPressed(false);
};
// subscribe to mousemove and mouseup on document, otherwise you
// can escape bounds of element while dragging and get stuck
// dragging it forever
document.addEventListener("mousemove", handleMouseMove);
document.addEventListener("mouseup", handleMouseUp);
return () => {
handleMouseMove.cancel();
document.removeEventListener("mousemove", handleMouseMove);
document.removeEventListener("mouseup", handleMouseUp);
};
// if `onDrag` wasn't defined with `useCallback`, we'd have to
// resubscribe to 2 DOM events here, not to say it would mess
// with `throttle` and reset its internal timer
}, [pressed, onDrag]);
// actually it makes sense to return an array only when
// you expect that on the caller side all of the fields
// will be usually renamed
return [legacyRef, pressed];
// > seems the best of them all to me
// this code doesn't look pretty anymore, huh?
};
/// example.ts
const quickAndDirtyStyle = {
width: "200px",
height: "200px",
background: "#FF9900",
color: "#FFFFFF",
display: "flex",
justifyContent: "center",
alignItems: "center"
};
const DraggableComponent = () => {
// handlers must be wrapped into `useCallback`. even though
// resubscribing to `mousedown` on every tick is quite cheap
// due to React's event system, `handleMouseDown` might be used
// in `deps` argument of another hook, where it would really matter.
// as you never know where return values of your hook might end up,
// it's just generally a good idea to ALWAYS use `useCallback`
// it's nice to have a way to at least prevent element from
// getting dragged out of the page
const handleDrag = useCallback(
({ x, y }) => ({
x: Math.max(0, x),
y: Math.max(0, y)
}),
[]
);
const [ref, pressed] = useDraggable({
onDrag: handleDrag
});
return (
<div ref={ref} style={quickAndDirtyStyle}>
<p>{pressed ? "Dragging..." : "Press to drag"}</p>
</div>
);
};
See this code live here, a version with improved positioning of cursor with constrained onDrag here and hardcore hook showcase here.
(Previously this answer was about pre-hook React, and told the answer by Jared Forsyth is horribly wrong. It doesn't matter the least now, but it's still in edit history of the answer.)
I should probably turn this into a blog post, but here's pretty solid example.
The comments should explain things pretty well, but let me know if you have questions.
And here's the fiddle to play with: http://jsfiddle.net/Af9Jt/2/
var Draggable = React.createClass({
getDefaultProps: function () {
return {
// allow the initial position to be passed in as a prop
initialPos: {x: 0, y: 0}
}
},
getInitialState: function () {
return {
pos: this.props.initialPos,
dragging: false,
rel: null // position relative to the cursor
}
},
// we could get away with not having this (and just having the listeners on
// our div), but then the experience would be possibly be janky. If there's
// anything w/ a higher z-index that gets in the way, then you're toast,
// etc.
componentDidUpdate: function (props, state) {
if (this.state.dragging && !state.dragging) {
document.addEventListener('mousemove', this.onMouseMove)
document.addEventListener('mouseup', this.onMouseUp)
} else if (!this.state.dragging && state.dragging) {
document.removeEventListener('mousemove', this.onMouseMove)
document.removeEventListener('mouseup', this.onMouseUp)
}
},
// calculate relative position to the mouse and set dragging=true
onMouseDown: function (e) {
// only left mouse button
if (e.button !== 0) return
var pos = $(this.getDOMNode()).offset()
this.setState({
dragging: true,
rel: {
x: e.pageX - pos.left,
y: e.pageY - pos.top
}
})
e.stopPropagation()
e.preventDefault()
},
onMouseUp: function (e) {
this.setState({dragging: false})
e.stopPropagation()
e.preventDefault()
},
onMouseMove: function (e) {
if (!this.state.dragging) return
this.setState({
pos: {
x: e.pageX - this.state.rel.x,
y: e.pageY - this.state.rel.y
}
})
e.stopPropagation()
e.preventDefault()
},
render: function () {
// transferPropsTo will merge style & other props passed into our
// component to also be on the child DIV.
return this.transferPropsTo(React.DOM.div({
onMouseDown: this.onMouseDown,
style: {
left: this.state.pos.x + 'px',
top: this.state.pos.y + 'px'
}
}, this.props.children))
}
})
Thoughts on state ownership, etc.
"Who should own what state" is an important question to answer, right from the start. In the case of a "draggable" component, I could see a few different scenarios.
Scenario 1
the parent should own the current position of the draggable. In this case, the draggable would still own the "am I dragging" state, but would call this.props.onChange(x, y) whenever a mousemove event occurs.
Scenario 2
the parent only needs to own the "non-moving position", and so the draggable would own it's "dragging position" but onmouseup it would call this.props.onChange(x, y) and defer the final decision to the parent. If the parent doesn't like where the draggable ended up, it would just not update it's state, and the draggable would "snap back" to it's initial position before dragging.
Mixin or component?
@ssorallen pointed out that, because "draggable" is more an attribute than a thing in itself, it might serve better as a mixin. My experience with mixins is limited, so I haven't seen how they might help or get in the way in complicated situations. This might well be the best option.
I implemented react-dnd, a flexible HTML5 drag-and-drop mixin for React with full DOM control.
Existing drag-and-drop libraries didn't fit my use case so I wrote my own. It's similar to the code we've been running for about a year on Stampsy.com, but rewritten to take advantage of React and Flux.
Key requirements I had:
- Emit zero DOM or CSS of its own, leaving it to the consuming components;
- Impose as little structure as possible on consuming components;
- Use HTML5 drag and drop as primary backend but make it possible to add different backends in the future;
- Like original HTML5 API, emphasize dragging data and not just “draggable views”;
- Hide HTML5 API quirks from the consuming code;
- Different components may be “drag sources” or “drop targets” for different kinds of data;
- Allow one component to contain several drag sources and drop targets when needed;
- Make it easy for drop targets to change their appearance if compatible data is being dragged or hovered;
- Make it easy to use images for drag thumbnails instead of element screenshots, circumventing browser quirks.
If these sound familiar to you, read on.
Usage
Simple Drag Source
First, declare types of data that can be dragged.
These are used to check “compatibility” of drag sources and drop targets:
// ItemTypes.js
module.exports = {
BLOCK: 'block',
IMAGE: 'image'
};
(If you don't have multiple data types, this libary may not be for you.)
Then, let's make a very simple draggable component that, when dragged, represents IMAGE:
var { DragDropMixin } = require('react-dnd'),
ItemTypes = require('./ItemTypes');
var Image = React.createClass({
mixins: [DragDropMixin],
configureDragDrop(registerType) {
// Specify all supported types by calling registerType(type, { dragSource?, dropTarget? })
registerType(ItemTypes.IMAGE, {
// dragSource, when specified, is { beginDrag(), canDrag()?, endDrag(didDrop)? }
dragSource: {
// beginDrag should return { item, dragOrigin?, dragPreview?, dragEffect? }
beginDrag() {
return {
item: this.props.image
};
}
}
});
},
render() {
// {...this.dragSourceFor(ItemTypes.IMAGE)} will expand into
// { draggable: true, onDragStart: (handled by mixin), onDragEnd: (handled by mixin) }.
return (
<img src={this.props.image.url}
{...this.dragSourceFor(ItemTypes.IMAGE)} />
);
}
);
By specifying configureDragDrop, we tell DragDropMixin the drag-drop behavior of this component. Both draggable and droppable components use the same mixin.
Inside configureDragDrop, we need to call registerType for each of our custom ItemTypes that component supports. For example, there might be several representations of images in your app, and each would provide a dragSource for ItemTypes.IMAGE.
A dragSource is just an object specifying how the drag source works. You must implement beginDrag to return item that represents the data you're dragging and, optionally, a few options that adjust the dragging UI. You can optionally implement canDrag to forbid dragging, or endDrag(didDrop) to execute some logic when the drop has (or has not) occured. And you can share this logic between components by letting a shared mixin generate dragSource for them.
Finally, you must use {...this.dragSourceFor(itemType)} on some (one or more) elements in render to attach drag handlers. This means you can have several “drag handles” in one element, and they may even correspond to different item types. (If you're not familiar with JSX Spread Attributes syntax, check it out).
Simple Drop Target
Let's say we want ImageBlock to be a drop target for IMAGEs. It's pretty much the same, except that we need to give registerType a dropTarget implementation:
var { DragDropMixin } = require('react-dnd'),
ItemTypes = require('./ItemTypes');
var ImageBlock = React.createClass({
mixins: [DragDropMixin],
configureDragDrop(registerType) {
registerType(ItemTypes.IMAGE, {
// dropTarget, when specified, is { acceptDrop(item)?, enter(item)?, over(item)?, leave(item)? }
dropTarget: {
acceptDrop(image) {
// Do something with image! for example,
DocumentActionCreators.setImage(this.props.blockId, image);
}
}
});
},
render() {
// {...this.dropTargetFor(ItemTypes.IMAGE)} will expand into
// { onDragEnter: (handled by mixin), onDragOver: (handled by mixin), onDragLeave: (handled by mixin), onDrop: (handled by mixin) }.
return (
<div {...this.dropTargetFor(ItemTypes.IMAGE)}>
{this.props.image &&
<img src={this.props.image.url} />
}
</div>
);
}
);
Drag Source + Drop Target In One Component
Say we now want the user to be able to drag out an image out of ImageBlock. We just need to add appropriate dragSource to it and a few handlers:
var { DragDropMixin } = require('react-dnd'),
ItemTypes = require('./ItemTypes');
var ImageBlock = React.createClass({
mixins: [DragDropMixin],
configureDragDrop(registerType) {
registerType(ItemTypes.IMAGE, {
// Add a drag source that only works when ImageBlock has an image:
dragSource: {
canDrag() {
return !!this.props.image;
},
beginDrag() {
return {
item: this.props.image
};
}
}
dropTarget: {
acceptDrop(image) {
DocumentActionCreators.setImage(this.props.blockId, image);
}
}
});
},
render() {
return (
<div {...this.dropTargetFor(ItemTypes.IMAGE)}>
{/* Add {...this.dragSourceFor} handlers to a nested node */}
{this.props.image &&
<img src={this.props.image.url}
{...this.dragSourceFor(ItemTypes.IMAGE)} />
}
</div>
);
}
);
What Else Is Possible?
I have not covered everything but it's possible to use this API in a few more ways:
- Use
getDragState(type)andgetDropState(type)to learn if dragging is active and use it to toggle CSS classes or attributes; - Specify
dragPreviewto beImageto use images as drag placeholders (useImagePreloaderMixinto load them); - Say, we want to make
ImageBlocksreorderable. We only need them to implementdropTargetanddragSourceforItemTypes.BLOCK. - Suppose we add other kinds of blocks. We can reuse their reordering logic by placing it in a mixin.
dropTargetFor(...types)allows to specify several types at once, so one drop zone can catch many different types.- When you need more fine-grained control, most methods are passed drag event that caused them as the last parameter.
For up-to-date documentation and installation instructions, head to react-dnd repo on Github.