Observer Design Pattern

Overview Objective Learn how to use the observer design pattern. Requirements Basics of ProdigyView Estimated Time 8 minutes www.prodigyview.com

Follow Along With Code Example Download a copy of the example code at www.prodigyview.com/source. Install the system in an environment you feel comfortable testing in. Proceed to examples/design/Observers.php

What Are Observers Observers are a design pattern, most commonly used in event driven programming, where an object has a list of observers and the observers are notified of a state change. A simpler explanation would be when a function is executed, a list of other functions is automatically executed.

Observers Visual Method doSomething executes. Notify observers execution has taken place. MyObject::doSomething Output Is aware that doSomething has executed Is aware that doSomething has executed Is aware that doSomething has executed DifferentObject1 DifferentObject2 DifferentObject3

PVPatterns and PVStaticPatterns The classes that contain the methods for using observers is the PVPatterns and PVStaticPatterns classes. PVPatterns is for instances and PVStaticPatterns is for static methods. Both PVObject and PVStaticObject extend the pattern classes.

Let's Begin In our example, we are going to pretend you want to integrate a function with social media. So lets start by making two social media classes. Keep note that in our example, one of our classes has a static method. The argument passed from the method observed

Messenger Object Now we are going create a class that has the ability to send messages to objects that are observing this method. Extends PVObject The name of the event that observrs look for Value passed to objects that are observing Implements the ability to call observers

Take Notice! Our class extends PVObject. PVObject extends PVPatterns which has our methods needed to use the observer. The method that will notify other methods than an action has occurred is _notify(). The method notify has the parameter 'new_message'. This is the name of the event that is going to cause the notifications. After the event name, we can add as many parameters as we want but in this example we are only adding one, the message.

Round 1 The first test we are going to do is just sending a message without adding an observer. So lets initialize the object and send a message.

Result The result here will be very simple.

Round 2 Now lets attach our observers. At minimum, the observer requires 3 arguments. The first argument is the name of the event. Our event name has to match to event name set in the notifier, which is 'new_message'. The second argument is the class to be called and the third is the method in the class to be called when the event is executed The last is options, namely for if our method is not static, apply the 'instance’ option here. Code example on next slide =>

Attach the Observers Event Name Class to call Method in class to call Call an instance of an object Event Name Class to call Method in class to call

And the output….

Challenge! Below is an optional challenge to perform that is designed to help you gain a better understanding of the design pattern. Look through ProdigyView’s source code and find the PVSession class. Find the event name for either writing a session or writing a cookie. Create a class that accepts the same parameters that the method in the Session class outputs. Add that class as an observer to PVSession:writeCookie() or PVSession::writeMethod Execute PVSession:writeCookie() or PVSession::writeMethod and print out the results in the class you created.

Summary Add _notify to a function and set the event name. Add as many parameters as you need after the event name. Parameters will be passed to the functions that’s are listening. Attach an observer with _addObserver. Make sure to specify the name of the event, the class and method the even will be calling.

API Reference For a better understanding of the Collections and the Iterator, check out the api at the two links below. PVStaticPatterns PVPatterns www.prodigyview.com More Tutorials For more tutorials, please visit: http://www.prodigyview.com/tutorials