fix(router-outlet): support relative router links

[daem0ndev]

Short description of what this resolves:

This PR resolves an issue where relative router links break after a forward and back navigation to a component that has already been created in the ion router outlet stack.

Changes proposed in this pull request:

• Create activated route proxy for each component instance activated in the outlet
• Update the underlying observables on the proxy so consumers need not be aware of implementation details

Ionic Version:
4.x

Fixes: #16534, #16736, #16954

[liamdebeasi]

So I did some testing and took a brief look over the code. The issue is fixed and the code looks good! Only thing I'd suggest is maybe adding more comments setupProxyObservables.
I am going to work on some automated tests today/tomorrow.

[mhartington]

Only comment (more of code style) is to not format with prettier. But everything else looks good.

[mhartington]

Merged! Thanks @daem0ndev for all your work on this and doing the investigation.

[liamdebeasi]

This will be in the next release of Ionic. I am not certain of the version number yet.

[rgolea]

Great! Some minor breaking changes I've got as I just tested it, is that if you were using something like the example below, you will get route.params as undefined and it will get instanciated afterwards. You will have to translate that code inside ngOnInit (no biggie, just to write it down in breaking changes):

@Component({
  ....
})
export class MyComponent(){
    public id$ = this.route.params.pipe(map(params => params.id));

    constructor(
         private readonly route: ActivatedRoute
    ){}
}

This will have to be translated to this

@Component({
  ....
})
export class MyComponent implements OnInit(){
    public id$:Observable<string> = null;

    constructor(
         private readonly route: ActivatedRoute
    ){}

    ngOnInit(){
          this.id$ =  this.route.params.pipe(map(params => params.id));
    }
}

[mhartington]

I wouldn't consider this a breaking change actually, as the example above is really an anti-pattern/not-recommended. We can make a note, but it should have been avoided to begin with.

[rgolea]

@mhartington sure! Just a note will go a long way. Thank you for the fix @daem0ndev!

[daem0ndev]

@rgolea @mhartington I created this PR to resolve the breaking change. I do agree its breaking since in a vanilla angular app, you can bind to the observables prior to ngOnInit. #17914