ObserverPattern.md

Observer Pattern

• Wikipedia: Observer pattern

Introduction

Let's say that we have sensors that response us temperature, pressure, and humidity. We can communicate with the sensor using IWeatherDataSetter interface.

The values are set using the setters and measurementsChanged is executed whenever values are changed. Source code.

Current Weather Conditions Display

Now, you want to implement a simple weather station that shows the current conditions. You need:

1. a real object (class), that implements the IWeatherDataSetter interface, let's call it WeatherData;
2. a station that displays current weather conditions, let's call it CurrentConditionsDisplay.

The solution could be the following:

Source code.

Customized Observer Pattern

How does WeatherData should inform CurrentConditionsDisplay about changes? Let's add to WeatherData a list of IDisplay objects and call update method whenever there is a change.

Source code.

Java Observer Pattern

Well, you have just implemented an observer pattern. Java already has an interface Observer (similar to IDisplay) and Observable (similar to the list of IDisplay objects that you have added to WeatherData).

Source code.