How to make this RxJs code more elegant? The code recording mouse hover time on a specific area - rxjs

Question

I want to record mouse hover time on a specific area such as a 'div' container box area , by using RxJs.
const element = document.querySelector('#some-div');
let totalHoverTime = 0;
const INTERVAL = 100;
const mousePos = {
x: -1,
y: -1
};
const accumulate = () => {
const x = mousePos.x;
const y = mousePos.y;
const divx1 = element.offsetLeft;
const divy1 = element.offsetTop;
const divx2 = element.offsetLeft + element.offsetWidth;
const divy2 = element.offsetTop + element.offsetHeight;
// out of area
if (x < divx1 || x > divx2 || y < divy1 || y > divy2) {
console.log('out')
} else {
// in area
console.log('in')
totalHoverTime += INTERVAL;
}
};
const accumulateTimer = rx.interval(INTERVAL);
accumulateTimer.subscribe(() => {
accumulate();
});
rx
.fromEvent(element, 'mousemove')
.pipe(rxOp.debounce(() => rx.timer(INTERVAL)))
.subscribe((e: MouseEvent) => {
mousePos.x = e.clientX;
mousePos.y = e.clientY;
});
I'm not very familiar with rxjs, I think this code may can be more elegant to implement.
Optimized code
Thank you very much for your answers. #hugo #der_berni
const element = document.body;
const INTERVAL = 2000;
const withinBounds = ({ x, y }: { x: number; y: number }) => {
const divx1 = element.offsetLeft;
const divy1 = element.offsetTop;
const divx2 = element.offsetLeft + element.offsetWidth;
const divy2 = element.offsetTop + element.offsetHeight;
const outOfBounds = x < divx1 || x > divx2 || y < divy1 || y > divy2;
if (outOfBounds) {
// out of area
console.log('out');
} else {
// in area
console.log('in');
}
return !outOfBounds;
};
const mousePositions = rx
.fromEvent(document, 'mousemove')
.pipe(rxOp.throttleTime(200))
.pipe(rxOp.map((e: MouseEvent) => ({ x: e.pageX, y: e.pageY })));
const mousePositionIsValid = mousePositions
.pipe(rxOp.map(withinBounds))
.pipe(rxOp.distinctUntilChanged());
const hoverTimer = mousePositionIsValid.pipe(rxOp.switchMap(valid => (valid ? accumulateTimer : rx.empty())));
const totalHoverTime = hoverTimer.pipe(rxOp.scan((x, y) => x + INTERVAL, -500)); // The first time mouse moves in, this will be triggered once, so it is set to -500, and the first time it comes in is 0ms.
totalHoverTime.subscribe(hoverTime => {
console.log('totalHoverTime is:', hoverTime);
});
Finally, I found that I still need to use mousemove event combined timer to implement this function. When the mouse is already hovering above the div on page load, the mouseenter event will never triggerd in my page seemly. Maybe only in jsfiddle can be no problem.

Answer 1

I' also only started using RxJS recently, so there might be a better way to solve your problem.
However, a huge improvement over your approach would already be to chain the observables and use the switchMap operator. One thing to keep in mind when working with rxjs is, that you want to avoid manual subscriptions, because you will have to keep track of them and unsubscribe yourself to prevent leaks. When using operators like switchMap, these keep track of the subscriptions to inner observables, and also automatically unsubscribe.
Following code snippet should solve your problem:
Rx.Observable.fromEvent(element, 'mouseenter') // returns Observable<Event>
.map(() => Date.now()) // transform to Observable<number>
.switchMap((startTime) => { // switches to new inner observable
return Rx.Observable.fromEvent(button, 'mouseleave')
// When the observable from mouseleave emmits, calculate the hover time
.map(() => Date.now() - startTime);
})
.subscribe((hoverTime) => {console.log(hoverTime)});
If you want to try it out, see this jsFiddle: https://jsfiddle.net/derberni/hLgw1yvj/3/
EDIT:
Even if your div is very large, and the mouse might never leave it and trigger the mouseleave event, this can be solved with rxjs. You just have to change when the observable emits, and for how long you let it emit before you complete it. The WHEN can be adapted, so that it emits in a set interval, and the UNTIL can be set with the rxjs function takeUntil. takeUntil receives an observable as an argument, and takes values from the source observable, until the 'argument' observable emits.
Check out this code and fiddle, which updates the hover time in 1s steps and when the mouseleave event triggers: https://jsfiddle.net/derberni/3cky0g4e/
let div = document.querySelector('.hover-target');
let text = document.querySelector('.hover-time');
Rx.Observable.fromEvent(div, 'mouseenter')
.map(() => Date.now())
.switchMap((startTime) => {
return Rx.Observable.merge(
Rx.Observable.interval(1000),
Rx.Observable.fromEvent(div, 'mouseleave')
)
.takeUntil(Rx.Observable.fromEvent(div, 'mouseleave'))
.map(() => Date.now() - startTime);
})
//.takeUntil(Rx.Observable.fromEvent(div, 'mouseleave'))
.subscribe((hoverTime) => {
text.innerHTML = "Hover time: " + hoverTime + "ms"
});
At least in the fiddle this works also when the mouse is already hovering above the div on page load, because then the mouseenter event is also triggered.

Answer 2

Point-free
The simplest thing: replace (x => f(x)) with simply f. It's equivalent and will read better in most cases. This:
accumulateTimer.subscribe(() => {
accumulate();
});
Becomes:
accumulateTimer.subscribe(accumulate);
Fat functions:
The accumulate function could be broken down into:
const accumulate = () => {
const x = mousePos.x;
const y = mousePos.y;
if (withinBounds(x, y)) {
totalHoverTime += INTERVAL;
}
};
const withinBounds = ({x, y}) => {
const divx1 = element.offsetLeft;
const divy1 = element.offsetTop;
const divx2 = element.offsetLeft + element.offsetWidth;
const divy2 = element.offsetTop + element.offsetHeight;
const outOfBounds = x < divx1 || x > divx2 || y < divy1 || y > divy2;
if (outOfBounds) {
// out of area
console.log('out')
} else {
// in area
console.log('in')
}
return !outOfBounds;
};
See how we separated withinBounds which is pretty big but performs a simple definite task, purely functionally (no side-effect, one input gives the same output) -- ignoring the debug calls that is. Now we don't have to think so hard about it and we can focus on accumulate.
Avoid side-effects & compose
The most glaring issue is the whole loop relying on a side effect on mousePos:
const mousePositions = rx
.fromEvent(element, 'mousemove')
.pipe(rxOp.debounce(() => rx.timer(INTERVAL)))
//.subscribe((e: MouseEvent) => {
// mousePos.x = e.clientX;
// mousePos.y = e.clientY;
//});
.map((e: MouseEvent) => ({ x: e.clientX, y: e.clientY )));
Don't subscribe and save the value, it breaks the idea of flow behind rxjs. Use the return value, Luke. More specifically, pipe further to refine it until you reach the desired data. Above, we have a stream that emits the mouse positions alone.
// Will emit true when the mouse enters and false when it leaves:
const mousePositionIsValid = mousePositions
.map(withinBounds)
.distinctUntilChanged();
// Fires every INTERVAL, only when mouse is within bounds:
const hoverTimer = mousePositionIsValid
.switchMap(valid => valid ? accumulateTimer : rx.empty())
(edited with switchMap as suggested by #der_berni)
You wrote a function named "accumulate". Whenever you say "accumulate", reduce (and the likes) should come to mind. Reduce emits a single aggregate value when the stream completes. Here we use scan to get a new updated value each time the underlying stream emits:
// For each element produced by hoverTimer, add INTERVAL
const totalHoverTime = hoverTimer.scan((x, y) => x + INTERVAL, 0);
Note that it doesn't add to global each time, but every value it emits is the previous one + INTERVAL. So you can subscribe to that to get your total hover time.