Creating a custom Polymer element

Issue

I’m trying to create a custom Polymer element that extends paper-shadow to display a Tweet.

Here is my implementation:

tweet_element.html

<link rel="import" href="packages/paper_elements/paper_shadow.html">
<link rel="import" href="packages/polymer/polymer.html">
<link rel="stylesheet" href="tweet_element.css">

<polymer-element name="tweet-element" extends="paper-shadow">

  <template>
    <div id="header">
      <div id="user-image">
        <img src="{{userImageUrl}}">
      </div>
      <div id="details">
        <div id="user">{{user}}</div>
        <div id="date-published">{{datePublished}}</div>
      </div>
    </div>

    <div id="content">
      <div id="text">{{text}}</div>
      <div id="photos">{{photos}}</div>
    </div>
  </template>

  <script type="application/dart" src="twitter.dart"></script>

</polymer-element>

twitter.dart

import 'package:polymer/polymer.dart';

@CustomTag('tweet-element')
class TweetElement extends PolymerElement {
  @Observable String userImageUrl;
  @Observable String user;
  @Observable String datePublished;
  @Observable String text;

  TweetElement.created() : super.created();

  void update(Tweet tweet) {
    userImageUrl = tweet.user.profileImage;
    user = '${tweet.user.name} (@${tweet.user.screenName})';
    datePublished = _parseDate(tweet.date);
    text = tweet.text;
  }

  ...    

}

And finally the code that creates a TweetElement and tries to add it to the DOM:

import 'dart:html';
import 'package:polymer/polymer.dart';
import 'twitter.dart';

...

var mainContent = querySelector('#main-content');
var element;
for (var tweet in tweets) {
  element = new TweetElement.created();
  element.update(tweet);
  mainContent.children.add(element);
}

And when I run this I get:

Exception: Uncaught Error: created called outside of custom element creation.

So then I tried to change twitter.dart to:

TweetElement.created() : super.created();

TweetElement (Tweet tweet) {
  TweetElement.created();
  userImageUrl = tweet.user.profileImage;
  user = '${tweet.user.name} (@${tweet.user.screenName})';
  datePublished = _parseDate(tweet.date);
  text = tweet.text;
}

And add the element to the DOM like this:

var mainContent = querySelector('#main-content');
var element;
for (var tweet in tweets) {
  element = new TweetElement(tweet);
  mainContent.children.add(element);
}

And now I got this error:

Internal error: unresolved implicit call to super constructor 'PolymerElement()' TweetElement (Tweet tweet) {

Solution

Ralph.

You can do that in two ways:

Option A

If you want to create a Polymer Element in your Dart code, you just have to create that tag this way:

var myTweetElement = new Element.tag ('tweet-element');
mainContent.childern.add(myTweetElement);

Also, check that you have imported the html library:

import 'dart:html';

Option B: (the coolest one)

You can do it extra convenient (and more object oriented) by adding this factory to your twitter.dart:

factory TweetElement () => new Element.tag('tweet-element');

Then, the creation of the tag in your dart code, is more readable:

var myTweetElement = new TweetElement (); // This uses the factory transparently
mainContent.children.add(myTweetElement);

Additionally, you can set the values of your properties. As those are observable, the bindings will update automagically:

myTweetElement.userImageUrl   = "foo"
myTweetElement.user           = "bar"
myTweetElement.datePublished  = "baz"
myTweetElement.text           = "qux"

Or if you like the cascade operator, you can write this instead:

myTweetElement..userImageUrl  = "foo"
              ..user          = "bar"
              ..datePublished = "baz"
              ..text          = "qux";

Hope this helps.

Answered By – Gonzalo Pezzi

Answer Checked By – Marilyn (FlutterFixes Volunteer)

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