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Meet Observer

5:51 with Craig Dennis

Let's explore the concepts behind the Observer Design Pattern. The implementation in Java has evolved with the language, let's explore some use cases.

Teacher's Notes
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Pre-requisites

Introduction to Design Patterns
Learn Java track or a good grasp on the basics of the Java language and JDK.

Definition

Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Code

The Java Observer Patterns repository on GitHub.

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[SOUND] Hello I'm Craig and I'm a developer. 0:00

In this workshop we're gonna explore the extremely popular design pattern, 0:06

Observer. 0:10

We'll explore what makes the pattern so popular and 0:11

we'll also walk through the why, the how, and the when you should use it. 0:13

First though, 0:18

just want to remind you that there are speed controls on the video player. 0:19

Make use of those to speed me up or slow me down to your tastes. 0:22

Always remember to check the teacher's notes on each video. 0:25

I've drop some prerequisites on this one. 0:28

So take a moment and 0:30

make sure that we're on the same page before we get started here. 0:30

And finally, don't forget to hit up the community to discuss these ideas more. 0:33

I'm excited to take the time to walk through the Observer pattern with you. 0:37

It's a very powerful and commonly used pattern. 0:41

It's ability to make code more extensible is what makes it so popular. 0:43

So you'll see it used all over the place as a solution. 0:47

Now, I guarantee that you've interacted with this pattern already. 0:49

It' everywhere. 0:52

I mean, you're literally seeing it at play right now. 0:54

See the timeline on this video? 0:56

The toolbar is observing where you are in this video. 0:58

As you progress through this video, it changes, right? 1:02

It updates both the number of seconds as well as the rendering of the progress bar. 1:05

So in the video example, the video itself represents the master object, or 1:10

subject as it's often referred to. 1:14

It's the one portion of the one-to-many. 1:17

The many side of things, the dependent objects or observers as they're usually 1:20

called, those are the number of seconds displayed and the progress bar. 1:24

So the subject changes its state and 1:28

its observers are notified and update themselves accordingly. 1:31

So with that in mind, you can probably think 1:35

of a couple other examples where you can interact with this pattern at work today. 1:37

Now, for instance, I was just using a spreadsheet. 1:41

When I updated the cell with my percentage complete on a project that I'm working on, 1:44

the graph changed. 1:47

And so did several other fields that were observing that value. 1:48

Those other objects were notified by the changes I made in my cells. 1:52

Now the Observer pattern is at play in the compound design pattern MVC. 1:55

The V in MVC, or view layer, makes heavy use of this pattern. 1:59

The view observes the model. 2:04

Changes to the model will notify the view. 2:06

Now UI libraries like the ones that you've interacted with, like JavaFX or 2:09

Android apps, rely heavily on this pattern to display the current state of the model. 2:12

Now, this pattern is often referred to as Pub/Sub which is short for 2:17

Publish/Subscribe. 2:21

Now this might help things stick. 2:23

A newspaper is published and delivered to all of it's subscribers, right? 2:24

And that makes sense. 2:28

You get the paper because you have a subscription, and 2:29

it only comes when a new issue is published. 2:31

All right, all right, enough analogies. 2:34

So let's go take a quick look at the UML so 2:36

you can see the suggested implementation. 2:38

So if we do a quick search for the Observer pattern, 2:41

we'll see here that the first result here is from Wikipedia. 2:44

So let's click this. 2:49

Let's scroll down to the UML. 2:50

I'm going to go ahead I'm going to click this. 2:51

It's gonna open it up and I'm gonna zoom in on it. 2:52

So we can take a more detailed look here. 2:55

Okay, so we see that there's an Observer interface, and 2:57

it has a method called notify. 3:00

There's several concrete implementations here, right? 3:03

So Concrete A and B, and they also implement, have a method called notify. 3:06

So there's a little diamond here on a class called subject, 3:11

and what that means is there's an aggregation of these observers here and 3:15

they're being stored in this observer collection. 3:19

And they get added by calling a thing called registerObserver, and 3:20

you pass in the observer or unregister to remove it. 3:24

And then there is a method called notifyObservers, and 3:26

this is the pseudo code here. 3:29

Basically it says for each one of these observers, loop through and 3:31

call the notify method. 3:34

So basically what this is saying is that any object that 3:36

implements the observer interface can be notified. 3:39

So, now don't worry, you'll get your head wrapped around it here in a bit. 3:43

Just remember that this is a suggestion of how to implement the pattern. 3:46

There are numerous ways of achieving this pattern in code and 3:50

we'll hit them up separately here in a bit. 3:52

Now these names aren't required, but they are common. 3:56

So you can start to recognize them when you see them. 3:58

While we're here, why don't we go get the starter files for application installed? 4:00

So in the teacher's notes there's a link to a rebo. 4:05

I went ahead and copied that and pasted it. 4:07

Here we are on GitHub. 4:09

I'm gonna choose clone or download, I'm gonna grab the URL here for SSH, and 4:11

I'm gonna pop over to my InteliJ, which you'll see is running 2016.2.1. 4:17

And I'm gonna check out from version control, use GitHub, 4:23

I'm gonna put in the password for this. 4:28

So I'm gonna come here and I'm going to paste in. 4:35

And want to choose clone. 4:39

And it says would you like to open? 4:44

I'm going to say yes. 4:45

It says it's an unlinked Gradle project. 4:47

I'm gonna say go ahead and import the Gradle project. 4:48

And I'm gonna click it again. 4:50

And just click all the defaults here, okay. 4:53

Okay, so I've built this was several different modules where 5:07

we can break up the different use cases that we're going to do. 5:10

So go ahead and just leave all of those checked by default and click OK. 5:13

When you take a look here under Project, you can see that the different ones here 5:20

are in bold are the modules that we'll be looking at here just a bit. 5:24

So now that we've seen it have a pretty basic understanding, 5:28

what do you say we put it into practice? 5:30

This pattern and its usage has evolved along with Java. 5:32

I think a good approach is to walk through a few of the iterations 5:36

since you'll encounter them in your journeys. 5:39

We'll discuss the pros and cons of each approach. 5:41

Let's start with the original implementation. 5:44

It's been around since version 1.1. 5:46

So let's dust it off and dive into it next. 5:48

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