Clear dangling EventListeners and Detached Nodes when a controller is removed from the DOM #1
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… removed from the DOM
`eventListenerMaps` maps HTML elements handled by Stimulus to `eventListenerMap`, a map of Stimulus
EventListeners in this way: `{ HTMLelement: { eventName: EventListener } }`.
When a controller HTML is removed from the DOM the dangling EventListeners aren't removed,
moreover, the removed HTML elements are still referenced via the `eventListenerMaps` keys.
This leads to having multiple detached HTMLelements and eventListeners which can't be GCed
hence a memory leak. The leak is fixed by removing the dangling eventListeners and clearing
unused `eventListenerMaps` keys. When the HTMLelement is attached again to the DOM,
the `eventListenerMaps` and the related `eventListeners` are automatically re-created
by Stimulus via the MutationObservers, no data is lost by doing this.
Below is an example to reproduce the issue:
```
<html>
<head>
<title>EventListenerMaps memory leak</title>
<script type="module">
import { Application, Controller } from "https://unpkg.com/@hotwired/stimulus/dist/stimulus.js"
window.Stimulus = Application.start()
Stimulus.register("boosts", class extends Controller {
doClick() { process(500) }
})
</script>
<script>
function process(count) {
let i = 0
let handler = setInterval(function(){
if (++i > count) {
clearInterval(handler)
} else {
document.body.replaceWith(document.body.cloneNode(true))
}
}, 1)
}
</script>
</head>
<body>
<div data-controller="boosts">
To reproduce:
<ul>
<li>Check heap snapshot</li>
<li>Click "trigger leak" button</li>
<li>Check heap snapshot again</li>
</ul>
<button data-action="click->boosts#doClick">trigger leak</button>
</div>
</body>
</html>
```
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eventListenerMapsmaps HTML elements handled by Stimulus toeventListenerMap, a map of Stimulus EventListeners in this way:{ HTMLelement: { eventName: EventListener } }. When a controller HTML is removed from the DOM the dangling EventListeners aren't removed, moreover, the removed HTML elements are still referenced via theeventListenerMapskeys. This leads to having multiple detached HTMLelements and eventListeners which can't be GCed hence a memory leak. The leak is fixed by removing the dangling eventListeners and clearing unusedeventListenerMapskeys. When the HTMLelement is attached again to the DOM, theeventListenerMapsand the relatedeventListenersare automatically re-created by Stimulus via the MutationObservers, no data is lost by doing this.Below is an example to reproduce the issue:
heap snapshot before:
heap snapshot after:
Related hotwired#489 hotwired#547