How to implement routing in polymer 3 using app-route

I was stuck with implementing routing in polymer 3. I followed the basic guide provided on app-route documentation. But on loading the web page., I don't see any component getting loaded. I checked in shadow DOM and don't see any DOM getting rendered. Not sure what I`m missing. Here is the code.

static get properties() {
      return {
      page:{
        type: String,
        reflectToAttribute: true,
        observer: '_pageChanged'
      }
      };
    }
    _pageChanged(currentPage, oldPage){
    	  console.log('CURRENT - ', currentPage);
    	  console.log('OLD - ', oldPage);
    	  switch(currentPage){
    		  case 'home':
    			import('./home-comp.js').then()
    			break;
    		case 'about':
    			import('./about-comp.js').then()
    			break;
    		case 'contact':
    			import('./contact-comp.js').then()
    			break;
    		default:
    			this.page = 'home';
    	  }
      }

<app-route
          route="{{route}}"
          pattern="/:page"
          data="{{routeData}}"
          tail="{{subroute}}">
</app-route>

<home-comp name="home"></home-comp>
<about-comp name="about"></about-comp>
<contact-comp name="contact"></contact-comp>

I don`t see lot of documentation on Polymer 3 available for checking on issues. After going through Polymer default sample web application, shop., I came across some proper solution. I would like to share it with community for any any one in need of help for same.

Solution

You need to have

app-route: for implementation of routing

Iron pages: Basically page switcher to load required component on demand

In app-route.,

/* observer: Its a simple observer (basically a watch which holds current value & old value) that triggers whenever data changed in page property. We read the observer and calls a function to grab its earlier */

static get properties() {
  return {
  page:{
    type: String,
    reflectToAttribute: true,
    observer: '_pageChanged'
  }
  };
}
_pageChanged(currentPage, oldPage){
	  console.log('CURRENT - ', currentPage);
	  console.log('OLD - ', oldPage);
	  switch(currentPage){
		  case 'home':
			import('./home-comp.js').then()
			break;
		case 'about':
			import('./about-comp.js').then()
			break;
		case 'contact':
			import('./contact-comp.js').then()
			break;
		default:
			this.page = 'home';
	  }
  }

<!-- pattern: reads the href property., hence set the page (pattern="/:page") property in static get property to read its data -->

<app-route route="{{route}}" pattern="/:page" data="{{routeData}}" tail="{{subroute}}"></app-route>
	<ul>
	    <li>
		    <a href="/home">Home</a>
	    </li>
	    <li> 
          <a href="/about">About</a> 
      </li>
	    <li> 
          <a href="/contact">Contact</a> 
      </li>
	</ul>

But for first time loading., page property doe not hold any value and throws undefined.

Hence we can use complex observer to observe such changes

static get observers(){
	return ['_routerChanged(routeData.page)'];
}
_routerChanged(page){
	console.log('CHANGED PAGE - ', page);
	this.page = page || 'home';
}

Changed route data does not load the components unless we have iron-pages. Its basically a component switcher/loader on demand. Wrap all component in main-app under <iron-pages>

<!-- selected: Data binding helps to get changed page value -->
<!-- attr-for-selected: It reads value of name attr defined in each component & matches with selected value and triggers page switch -->

<!-- fallback-selection: for 404., page/component not found handling -->

<iron-pages selected="[[page]]" attr-for-selected="name" selected-attribute="visible" fallback-selection="404">
		<home-comp name="home"></home-comp>
		<about-comp name="about"></about-comp>
		<contact-comp name="contact"></contact-comp>
</iron-pages>

Here is the complete guide for routing implementation in polymer 3 using app-route. Hope this helps click here