HostListener and Angular Universal - angularjs

Question

I'm trying to listen to a MessageEvent sent with postMessage in my Angular 2 component.
My first attempt was simply doing:
window.addEventListener("message", this.handlePostMessage.bind(this));
And then in ngOnDestroy:
window.removeEventListener("message", this.handlePostMessage.bind(this));
However this didn't work as expected. If I navigated to another route and back, there would be two event listeners registered.
So instead I've been trying to decorate the method with HostListener, but I can't get this working when using prerendering (Angular Universal with .NET Core using the asp-prerender-module).
#HostListener('window:message', ['$event'])
private handlePostMessage(msg: MessageEvent) {
...
}
That gives me the following error on page load:
Exception: Call to Node module failed with error: Prerendering failed because of error: ReferenceError: MessageEvent is not defined
Is there a workaround for this?

Answer 1

You're getting this error because MessageEvent is not defined. You must import whatever file defines this.
My #HostListeners look like this:
#HostListener("window:savePDF", ["$event"]) savePDF(event) {
this.savePDFButtonPushed();
}
and you can read more about them here:
https://angular.io/guide/attribute-directives
However, I'm currently experiencing the same issue you are -- that if I navigate to another route and back, I now receive two events. And that is using #HostListener. :-( However I haven't upgraded Angular in a while (currently using 4.4.6), so maybe they've fixed it since that release.
**Edit: Just upgraded to Angular 5.1.0. The 'duplicate events' #HostListener issue remains. :-(
Edit #2: I tried also using window.addEventListener like you tried, and also had the same issue, despite using window.removeEventListener in ngOnDestroy().
This lead me to dig a little deeper, where I found some code I had added to listen to messages from a child iFrame. Any chance you have something similar in your code?
var eventMethod = window.addEventListener ? "addEventListener" : "attachEvent";
var eventer = window[eventMethod];
var messageEvent = eventMethod == "attachEvent" ? "onmessage" : "message";
// Listen to messages from child window ("unsign" and "savePDF") and pass those along as events to Angular can pick them up in its context
eventer(messageEvent,function(e) {
window.dispatchEvent( new Event( e.data ) );
},false);
This had been in my page's constructor. I protected it so it only executed the first time the page was constructor, and now all is well.