How do I fire a custom event from Polymer Dart?

Question

I want to fire/send/emit a custom event from inside a Polymer element. For example, I want to convert a normal DOM event like "changed" to a more semantic event like "todoupdated".

This is the HTML that I have:

<polymer-element name="todo-item" extends="li" attributes="item">
  <template>
    <style>
      label.done {
        color: gray;
        text-decoration: line-through;
      }
    </style>
    <label class="checkbox {{item.doneClass}}">
      <input type="checkbox" checked="{{item.done}}">
      {{item.text}}
    </label>
  </template>
  <script type="application/dart" src="todo_item.dart"></script>
</polymer-element>

I want the change events on checkbox to bubble out of the custom element as something more... useful. :)

score 30 · Accepted Answer · edited Jan 07 '14 at 17:56

Step 1

Capture the change events on the <input>. Notice the on-change below.

<!-- from inside todo_item.html -->
<input type="checkbox" checked="{{item.done}}" on-change="{{change}}">

Step 2

Handle the change event in the custom element code that contains the checkbox.

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

@CustomTag('todo-item')
class TodoItemElement extends PolymerElement with ObservableMixin {
  @observable Item item;

  bool get applyAuthorStyles => true;

  void change(Event e, var details, Node target) {
    // do stuff here
  }
}

Notice the change event handler. That method is run any time the checkbox state changes.

Step 3

Dispatch a custom event.

  void change(Event e, var details, Node target) {
    dispatchEvent(new CustomEvent('todochange'));
  }

NOTE: the custom event name must not contain dashes.

Step 4

Listen for the custom event in a parent custom element.

    <template repeat="{{item in items}}" >
      <li is="todo-item" class="{{item.doneClass}}" item="{{item}}" on-todochange="todoChanged"></li>
    </template>

Notice the use of on-todochange.

Enjoy!

Am wondering if it's possible to fire an event directly from the element rather than calling an instance method to do it... — Tom Russell, Jan 05 '16 at 04:26

score 9 · Answer 2 · answered Mar 15 '14 at 13:07

Polymer has a helper method that simplifies firing events

// dispatch a custom event
this.fire('polymer-select', detail: {'item': item, 'isSelected': isSelected});

Additional info:
To make the event available to subscriber that want to add a listener programmatically

// getter
async.Stream<dom.CustomEvent> get onPolymerSelect =>
    PolymerSelection._onPolymerSelect.forTarget(this);

// private EventStreamProvider
static const dom.EventStreamProvider<dom.CustomEvent> _onPolymerSelect =
    const dom.EventStreamProvider<dom.CustomEvent>('polymer-select');

subscribe to the event programmatically instead of declaratively:

($['#ps'] as PolymerSelect) // get the children and cast it to its actual type
    .onPolymerSelect.listen((e) => print(e['isSelected'])); // subscribe

score 1 · Answer 3 · edited Jan 20 '15 at 11:20

I managed this using <core-signals> and the polymer helper method fire. This way you are able to listen to events fired from elements that are not children. source.

todochange.html

<!doctype html>

<polymer-element name="todo-item" extends="li">
  <template>
    <style>
      label.done {
        color: gray;
        text-decoration: line-through;
      }
    </style>
    <label class="checkbox {{item.doneClass}}">
      <input type="checkbox" checked="{{item.done}}">
      {{item.text}}
    </label>
  </template>
  <script type="application/dart" src="todo_item.dart"></script>
</polymer-element>

todochange.dart

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

@CustomTag('todo-item')
class TodoItemElement extends PolymerElement {
  @observable Item item;

  void change(Event e, var details, Node target) {
    // the name is the name of your custom event
    this.fire( "core-signal", detail: { "name": "todochange" } );
  }
}

Then any subscriber just has to do this

subscriber.html

...
<link rel="import" href="packages/core_elements/core_signals.html>
...
<template>
  <core-signals on-core-signal-todochange="{{handleToDoChange}}"></core-signals>
...
</template>

subscriber.dart

@CustomTag( "subscriber" )
class Sub extends PolymerElement {
  ...
  void handleToDoChange( Event e, var detail, Node target ) {
    print( "Got event from <todo-item>" );
  }
  ...
}

How do I fire a custom event from Polymer Dart?

3 Answers3

Step 1

Step 2

Step 3

Step 4

Linked