How to observe simple variable in dart language

Question

How to observe simple variable in dart language

8.7k Views Asked by gblaszczyk At 05 April 2025 at 11:05

My question is: How to observe changes on simple variable like String or num? I know that you can easy observe object like this:

observe(t, (e) => print ("Value changed"));

but How to do this on simple variable?

Original Q&A

There are 2 best solutions below

Shailen Tuli On 30 August 2013 at 14:13

Here is an example of the binding of a string value that is object attribute:

<!DOCTYPE html>

<html>
  <head>
    <title>index</title>
    <script src="packages/polymer/boot.js"></script>
  </head>

  <body>
    <template id="_template" bind>
      <input type="text" value="{{name}}">
      <p>The name is {{name}}</p>
    </template>

    <script type="application/dart" src="index.dart"></script>
  </body>
</html>

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

class Person extends Object with ObservableMixin {
  @observable
  String name;
  Person(this.name);
}

main() {
  query("#_template").model = new Person('Bob');
}

**Seth Ladd** · Accepted Answer

(This answer applies to Polymer.dart.)

The observe package includes a wrapper for single observable values: ObservableBox.

import 'package:observe/observe.dart';
import 'dart:async';

void main() {
  ObservableBox myValue = new ObservableBox('hello');

  myValue.changes.listen((List<ChangeRecord> records) {
    PropertyChangeRecord record = records[0] as PropertyChangeRecord;

    print('${record.field} changed, it is now ${myValue.value}');
  });

  new Timer.periodic(const Duration(seconds: 1), (t) {
    myValue.value = new DateTime.now();
  });
}

There is no way to observe a top-level or function-scope single string, boolean, int, or double without using ObservableBox.

If the string, boolean, int, or double is a field of a class, you can use ObservableMixin and the @observable annotation.

class Foo extends Object with ObservableMixin {
  @observable String bar = '';
}

You can then get notified when an instance of Foo changes:

foo.changes.listen((List<ChangeRecord> records) {
  // ...
});

How to observe simple variable in dart language

There are 2 best solutions below

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