What is Model-View-Presenter (MVP)?

Hey, welcome back. 0:04 We're gonna take a short break from code to talk about the popular software 0:06 design pattern called model-view-presenter, or MVP, for short. 0:09 Now don't worry if this video seems a little unrelated at first 0:13 to the app that we're working on. 0:16 We don't need to know all the intricacies 0:18 of the model-view-presenter design pattern. 0:20 But the more we know about it and 0:22 keep it in mind, the better software developers we will be. 0:24 We don't necessarily need to follow it for smaller apps like this. 0:27 But good programming patterns and 0:31 practices are invaluable on larger projects or when working with teams. 0:32 By the way, in a previous version of this course, 0:37 we taught this as the model-view-controller design pattern. 0:39 The two are often used interchangeably, but there are subtle differences. 0:42 And the model-view-presenter pattern more closely aligns 0:46 with how Android apps are architected. 0:49 So first off, what is a software design pattern? 0:51 When writing code, 0:54 you'll always end up facing problems that are common to all developers. 0:55 For example, one of those problems is how to architect our apps so 0:59 that our components are largely reusable. 1:02 Solutions to these common problems are known as design patterns. 1:05 And while you weren't explicitly aware of it, we've been working with 1:08 the model-view-presenter design pattern all along. 1:11 Let's start with a very generic example. 1:14 Think of a slideshow presentation you have given at work or 1:16 in a class using a tool like PowerPoint, Google Slides, or Keynote. 1:19 In this case, you are the presenter. 1:23 Your job is to control what is on each slide and when they are displayed. 1:26 These slides are being presented on a large screen, which is the view. 1:30 This is how the audience views your material. 1:34 Lastly, the data used to create your slides is the data model. 1:37 Maybe your presentation is about earnings, or the results of experiments, or 1:41 maybe your favorite episodes of Dr. Who. 1:44 Whatever it is, you collected data and 1:46 formatted it in some way to be viewed on the screen. 1:48 This pattern is the same for Android apps and games. 1:51 The model is the data that you use in the app, 1:53 the view is how it is actually displayed on the screen, and 1:56 the presenter manipulates the data and objects in some way. 1:59 So next, how does this apply to our current app? 2:02 Think for a moment about how this interactive story is set up. 2:05 On the phone, itself, is the screen that the user interacts with. 2:08 The layouts we create for our activities are what are shown on the screen, so 2:11 let's call those the views. 2:14 The layout is a presentational aspect of the app that the user views and 2:16 interacts with. 2:20 Now what piece of our app presents these layouts, these views? 2:21 What part of our code manages how and when things are shown? 2:24 And what should happen when things are tapped? 2:27 That's right, the activities. 2:30 These are the presenters in the MVP pattern in Android. 2:31 As a side note, 2:35 you will sometimes hear the presenter referred to as the controller. 2:35 Since it controls both the view on the screen and the data model. 2:39 We'll stick with presenter, though, as that's more widely used for Android. 2:42 If you're interested in reading more about the exact differences between MVP and MVC, 2:45 check the teacher's notes for some links. 2:49 The presenter's also responsible for interacting with the data model behind 2:51 the scenes, just like how you create and display slides in a presentation. 2:55 Presenter is what adds, changes, or deletes data in the model 2:59 based on what the user is doing on the screen, or the view. 3:02 So that leaves us with the model. 3:06 All the data in our app, 3:07 as well as the logic around that data, goes into model objects. 3:09 It helps to make the connection by thinking of the phrase, data model. 3:12 Now what kind of data are we about to work with? 3:16 That's right, in this case, it's our story. 3:18 The data can really come from anywhere. 3:20 We'll build it right into our app here, but it could be data from someplace 3:22 on the web or even elsewhere on the phone, like from a gallery of photos. 3:25 Wherever we get our data from, the model object's represented, and 3:28 our presenter gets it ready for display. 3:32 Presenter may even adapt the data, like when we take a long time step and 3:34 convert it to a shortened representation that may differ from region to region. 3:38 When we follow this MVP pattern, 3:42 the model does not communicate with the view in any way. 3:44 This way the model object can be separated from the view and 3:47 reused across other views without having to repeat or 3:50 re-implement some of the same functionality. 3:53 This setup is the basic architecture for most Android apps. 3:56 The core Android code that we use to develop apps has been designed in 3:59 such a way that we're kind of forced to follow this pattern. 4:02 The model is whatever data we need to work with. 4:05 The views are the actual user interface of an app. 4:08 The layouts of buttons and other things are the XML layout files. 4:10 And the presenter is the activity where we do stuff behind the scenes and 4:14 update the view or the data model. 4:17 As you go further with Android development, you may even come across 4:19 variations of these patterns that have the same basic ideas, 4:22 but differ in some of the details for very specific reasons. 4:24 We'll eventually talk more about those in later courses and workshops. 4:28 But understanding the basic MVP architecture is some key groundwork. 4:31 All right, so that tells us a bit about what MVP is and how it's used in our apps. 4:35 But we're still missing the why behind it. 4:39 Don't worry, though, that's coming up next in the next video. 4:42