simple_property_observer

A simple Dart package for observing property changes. Also known as property changed notifier.

How to use

This repository consists of 2 packages, simple_property_observer and simple_property_observer_builder. These packages can be used in pure Dart projects as well as Flutter. example is a simple example of using these packages.

Dependency settings

In your pubspec.yaml, write the below settings:

dependencies:
  simple_property_observer: any

dev_dependencies:
  build_runner: any
  simple_property_observer_builder: any

simple_property_observer contains the definitions for the property observable pattern and the auto-generating annotations.

simple_property_observer_builder contains the command line tools for the auto-generating.

Define your class

For example, write the below annotated class:

import 'package:simple_property_observer/simple_property_observer.dart';

part 'example.g.dart';

@propertyObservable
class Example {
  @observable
  String? get observableProperty => null;
}

Dart Analysis will point out the error, but ignore it and proceed to the next step.

Points

  • Annotate your class with @propertyObservable for auto-generating the property observable pattern.
  • Create GETTER and annotate it with @observable.
  • Write part sentence for the auto-generating file.

Then, run the build command in Terminal:

$ flutter packages pub run build_runner build

If you create a pure Dart project, run the below command:

$ dart run build_runner build

After finishing the command, you can see example.g.dart file:

// GENERATED CODE - DO NOT MODIFY BY HAND

part of 'example.dart';

// **************************************************************************
// PropertyObservableGenerator
// **************************************************************************

mixin _ObservableMembers on PropertyObservable {
  String? __observableProperty;
  set observableProperty(String? value) {
    if (__observableProperty == value) {
      return;
    }
    final oldValue = __observableProperty;
    __observableProperty = value;
    notifyPropertyChanged(PropertyChangedInfo(
        'observableProperty', oldValue, __observableProperty));
  }
}

Point

Then, you must modify example.dart as below:

import 'package:simple_property_observer/simple_property_observer.dart';

part 'example.g.dart';

@propertyObservable
class Example with PropertyObservable, _ObservableMembers {
  @observable
  String? get observableProperty => __observableProperty;
}

Add with PropertyObservable, _ObservableMembers and modify the getter from null to __observableProperty.

Observe instances

You can observe the property changes with the registerObserver method:

import 'example.dart';

void main() {
  final example = Example();

  // Observe property changes
  example.registerObserver((info) {
    print('${info.name} changed: from ${info.oldValue} to ${info.newValue}');
  });

  print('set Apple');
  example.observableProperty = 'Apple';

  print('set Banana');
  example.observableProperty = 'Banana';

  // Shows the output below:
  // set Apple
  // observableProperty changed: from null to Apple
  // set Banana
  // observableProperty changed: from Apple to Banana
}

You can also cancel observing with the unregisterObserver method.

Libraries

simple_property_observer