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![]() Let's take a look at an implementation of the pattern with a real example. The slight difference from the classic definition is that Observable is used in place of the Subject and is implemented as a class, while the Observer interface remains the same. While some patterns require you to define the interfaces that make the pattern work, the Observer is a case where Java has the work done for you already in the java.util package. The typical real world uses of the pattern all revolve around this type of event handling system. As the button changes state, you can react, if you choose to, in your actionPerformed method. In this case, the ActionListener is the Observer and the button is your Subject. The event listener is the most popular, where you register an ActionListener to a UI control, such a button, and react to action events using the actionPerformed method. The whole concept of listeners is based on this pattern. You'll have seen, and probably used, the Observer many times if you've done any UI programming, especially in Swing. If you have an object that needs to share it's state with others, without knowing who those objects are, the Observer is exactly what you need. In general, you want to use this pattern to reduce coupling. This is a very powerful decoupling - meaning that any object can simply implement the Observer interface and get updates from the Subject. Instead, everything is done through a common interface, and the notify method just calls all the objects out there that have registered their interest. To pass data onto the observers, our subject doesn't need to know who needs to know. #OBZERVER TUTORIAL BZFLAG REGISTRATION#The following sequence diagram illustrates the registration and notification flow in action. When the Observer is no longer interested in the Subject's state, they can simply detatch themselves. #OBZERVER TUTORIAL BZFLAG UPDATE#When something changes in our Subject that the Observer may be interested in, a notify message is sent, which calls the update method in each Observer. The idea behind the pattern is simple - one of more Observers are interested in the state of a Subject and register their interest with the Subject by attaching themselves. ![]() Let's take a look at the classic diagram definition of the observer: The definition provided in the original Gang of Four book on Design Patterns states:ĭefine a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. The Observer is known as a behavioural pattern, as it's used to form relationships between objects at runtime. The Observer pattern is the gold standard in decoupling - the seperation of objects that depend on each other. Of all of the design patterns that are out there, the Observer is one that you've probably used already, even if you weren't aware of it. This is the Observer pattern in action, a publisher/subscriber relationship with one source having many subscribers. Any time that the RSS feed has an update, it will appear in my reader automatically. When I want to get updates from a particular feed, I add it to my feed reader. #OBZERVER TUTORIAL BZFLAG CODE#The Observer In The Real Worldīefore we get into the theory and code behind the Observer, let's take a look at a real world example, such as RSS feeds. To start off, I'll be describing one of the most used design patterns, the Observer pattern.įor a great overview of the most popular design patterns, DZone's Design Patterns Refcard is the best place to start. In this article series, I'll be going through each pattern and describing how it's used and where it's applied in the real world. #OBZERVER TUTORIAL BZFLAG SOFTWARE#More importantly, knowing design patterns gives a common vocabulary for software developers to use when talking about their designs. They illustrate the best design solutions that others have encountered, and allow you to apply the same principle to your own designs. Design patterns are one of the most valuable tools for developers. ![]()
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