Here's another solution. I wrote it in React, but I'll explain it at the end if you want to rebuild it in plain JS. It's similar to other answers here, but perhaps slightly more refined.
import React from 'react';
import styled from '@emotion/styled';
import BodyEnd from "./BodyEnd";
const DropTarget = styled.div`
position: fixed;
top: 0;
right: 0;
bottom: 0;
left: 0;
pointer-events: none;
background-color:rgba(0,0,0,.5);
`;
function addEventListener<K extends keyof DocumentEventMap>(type: K, listener: (this: Document, ev: DocumentEventMap[K]) => any, options?: boolean | AddEventListenerOptions) {
document.addEventListener(type, listener, options);
return () => document.removeEventListener(type, listener, options);
}
function setImmediate(callback: (...args: any[]) => void, ...args: any[]) {
let cancelled = false;
Promise.resolve().then(() => cancelled || callback(...args));
return () => {
cancelled = true;
};
}
function noop(){}
function handleDragOver(ev: DragEvent) {
ev.preventDefault();
ev.dataTransfer!.dropEffect = 'copy';
}
export default class FileDrop extends React.Component {
private listeners: Array<() => void> = [];
state = {
dragging: false,
}
componentDidMount(): void {
let count = 0;
let cancelImmediate = noop;
this.listeners = [
addEventListener('dragover',handleDragOver),
addEventListener('dragenter',ev => {
ev.preventDefault();
if(count === 0) {
this.setState({dragging: true})
}
++count;
}),
addEventListener('dragleave',ev => {
ev.preventDefault();
cancelImmediate = setImmediate(() => {
--count;
if(count === 0) {
this.setState({dragging: false})
}
})
}),
addEventListener('drop',ev => {
ev.preventDefault();
cancelImmediate();
if(count > 0) {
count = 0;
this.setState({dragging: false})
}
}),
]
}
componentWillUnmount(): void {
this.listeners.forEach(f => f());
}
render() {
return this.state.dragging ? <BodyEnd><DropTarget/></BodyEnd> : null;
}
}
So, as others have observed, the dragleave event fires before the next dragenter fires, which means our counter will momentarily hit 0 as we drag files (or whatever) around the page. To prevent that, I've used setImmediate to push the event to the bottom of JavaScript's event queue.
setImmediate isn't well supported, so I wrote my own version which I like better anyway. I haven't seen anyone else implement it quite like this. I use Promise.resolve().then to move the callback to the next tick. This is faster than setImmediate(..., 0) and simpler than many of the other hacks I've seen.
Then the other "trick" I do is to clear/cancel the leave event callback when you drop a file just in case we had a callback pending -- this will prevent the counter from going into the negatives and messing everything up.
That's it. Seems to work very well in my initial testing. No delays, no flashing of my drop target.
Can get the file count too with ev.dataTransfer.items.length
ENTER [ENTER LEAVE]* [LEAVE|DROP]is the correct sequence. The events should be registered on thewindowusing capturing (setting the third parameter totrue) instead of being on the html/document/body elements so that none of the events are missed if someone else callse.stopPropagation()on something deeper in the tree. – CubicleSoft