android viewmodel tutorial

For the project, we are going to follow a beginner version of MVVM. To use this Property Delegation, the following dependency needs to be added to the build.gradle (module-level). Now, create a Kotlin class ViewModelFactory inside the base package and add the following code. Let us briefly think about what this word requirements actually means: I would love to write an article where I discuss in fine detail how MVVM works with respect to the requirements and concerns listed above. Others may also point out that it promotes reusability of ViewModels, but this is exactly where things break down with this pattern.

For now, AndroidViewModel is good for me. Some of us do not like filling View classes with Logic. In the layout folder, update the activity_main.xml with the following code: Add item_layout.xml in the layout folder and add the following code: Create package - adapter inside the main package. While the app was running, we experienced first-hand the kind of problems that occur when developing apps in this way when the data displayed on a TextView widget was lost during a device rotation. Make ApiService Singleton and reuse the same instance for all the features. Once unpublished, all posts by vtsen will become hidden and only accessible to themselves. My goal in this article is to explain why the Model-View-ViewModel architectural pattern presents a very awkward separation of concerns in some situations with regard to the presentation logic of a GUI architecture. If what I am saying still seems vague, I promise it will be clear once I describe the other approach. If you're not familiar Kotlin, you can go through some quick examples here first to understand some important concepts such as "Delegation". In the chapter that followed, an AVD emulator was created and used to run the app. It essentially does the same thing as by lazy without the need to specify the ViewModelProvider. So it may not work in the activity class. Below is the code for the MainActivity.ktfile. As our activity destroyed our activity data has also vanished. This content will typically take the form of a Fragment consisting of an XML layout resource file and corresponding class file. Now just click on the button 3 to 4 times you will see the incremented number on the screen. Here, we have used LiveData, learn more about it here. Sometimes you will run in to situations where you must find some kind of half-measure between the distribution of presentation logic between Views and ViewModels, which does not strictly follow either of these approaches. Now, create a Kotlin class MainAdapter inside the same adapter package and add the following code. Within the Project tool window, locate the file, double-click on it to load it into the code editor and modify the class so that it reads as follows: The class declares variables to store the current dollar string value and the converted amount together with getter and setter methods to provide access to those data values. In this chapter we revisited the AndroidSample project created earlier in the book and created a new version of the project structured to comply with the Android Jetpack architectural guidelines. The best architecture (or at least a good choice) for a given application depends strongly on the requirements at hand. In simple terms, there are two ways you can achieve this re-usability: The second point may sound vague or counter-intuitive (how can it know anything about something which it does not reference? Begin by launching Android Studio and, if necessary, closing any currently open projects using the File -> Close Project menu option so that the Welcome screen appears. The fragment class now needs to be updated to react to button clicks and to interact with the data values stored in the ViewModel. Sometimes not having fine-grained control over your Views (which is a consequence of prioritizing re-usability of ViewModels), actually kind of sucks. It saves my time to figure out how to pass data to different fragments. Once unsuspended, vtsen will be able to comment and publish posts again. An activity must extend the ViewModel class to create a view model: class MainActivityViewModel : ViewModel() {. We need a utility class that will be responsible to communicate the current state of Network Call to the UI Layer. Kotlin, Java, C/C++, Javascript, Android mobile and web mostly. To avoid these issues, it is recommended to store all UI data in the ViewModel instead of an activity. So this is just for your reference and knowledge. With the view selected in the layout refer to the Attributes tool window and change the id to dollarText. Ultimately, I think the two elements which make for a great architecture come down to the following considerations: Firstly, play with several approaches until you find one which you prefer. Secondly, understand that preferences aside, different styles will tend to emphasize different benefits in exchange for different deficits. Now, create a class MainRepository inside the repository package and add the following code. When the button is clicked, the onClick() method needs to read the current value from the EditText view, confirm that the field is not empty and then call the setAmount() method of the ViewModel instance. Step 1: Create a Kotlin class file MainActivityViewModel.kt. This will consist of data class User like below. Go to the MainActivity.ktfile and update the following code. Now, in this section, we will setup UI, build the project and run on the device. If you read this far, tweet to the author to show them you care. Also, add the following dependency to the build.gradle(Module:app) file. There are drawbacks being discussed over the internet, but I do not fully understand this part yet. To overcome this problem we use ViewModels which holds the data even after configuration changes like the rotation of the screen. Finally, add the Internet Permission in your project. We are naming that as Resource. The main user interface layout for the activity is contained within the app -> res -> layout -> main_activity.xml file and provides an empty container space in the form of a FrameLayout (highlighted in Figure 32-2) in which screen content will appear: The FrameLayout container is just a placeholder which will be replaced at runtime by the content of the first screen that is to appear when the app launches. ViewModel is one of the most critical class of the Android Jetpack Architecture Component that support data for UI components. The above image is taken from another MindOrks blog. Our package in the project will look like below: We need the enum to represent the UI State. The method will then need to call the ViewModels getResult() method and display the converted value on the TextView widget. Jetpack essentially defines a set of recommendations describing how an Android app project should be structured while providing a set of libraries and components that make it easier to conform with these guidelines with the goal of developing reliable apps with less coding and fewer errors. Name: MVVM-Architecture-Android-Beginners, Package name: com.mindorks.framework.mvvm. Are you sure you want to hide this comment? The layout resource file for this fragment can be found at app -> res -> layout -> main_fragment.xml and will appear as shown in Figure 32-3 when loaded into the layout editor: By default, the fragment simply contains a TextView displaying text which reads MainFragment but is otherwise ready to be modified to contain the layout of the first app screen. The chapter outlined the structure of the Fragment + ViewModel project template and explained the concept of basing an app on a single Activity using Fragments to present different screens within a single Activity layout. As we have done some simplifications in this project for the Beginners level, so, we can improve this project to go to the Advanced level, a few of the things which we can improve are as follows: If you are preparing for your next Android Interview, Join our Android Professional Course to learn the latest in Android and land a job at top tech companies. It will become hidden in your post, but will still be visible via the comment's permalink. Also, I usually use by activityViewModels instead of by viewModels which allows me to share data across different fragments. [Updated - Nov 7, 2021]: you can also use by lazy and ViewModelProvider() instead of by viewModels and it should still work. When the activity or fragment is created, ViewModelProvider is smart enough to figure out to reuse the first created ViewModel instance. Please use, Accordingly, we need to create our data class. The code to do this can be added to the onActivityCreated() method of the file as follows: With the listener added, any code placed within the onClick() method will be called whenever the button is clicked by the user. ), so I think it is time to look at some code: While this is a very simplified example, the point is that the only thing which this particular ViewModel exposes publicly (other than the handleEvent function), is a simple Note object: With this particular approach, the ViewModel is well and truly decoupled from not just a particular View, but also the details, and by extension, presentation logic of any particular View. The previous chapter introduced the key concepts of Android Jetpack and outlined the basics of modern Android app architecture. This method inflates the main_fragment.xml layout file so that it is displayed within the container area of the main activity layout: The ViewModel for the activity is contained within the class file located at app -> java -> ui.main -> MainViewModel. This page was last modified on 11 January 2019, at 18:27. With that being said, one cannot simply add a reference back in to the ViewModel in order to regain this fine-grained control over the View. If vtsen is not suspended, they can still re-publish their posts from their dashboard. We are adding these two dependencies because to avoid using findViewById() in our MainActivity.kt file. You can also checkout Android Studio project code from our, 7. That would basically just be MVP + memory leaks (assuming you are still using ViewModel from AAC). Eventually, you will be able to pick good choices based on your understanding of the project requirements rather than blind faith. As you have probably noticed, we are probably not going to be re-using this ViewModel anywhere else. How to add opt-in compiler argument in build.gradle. [Updated - July 15, 2022]: I managed to try hilt to inject the dependencies ito view model. If you are preparing for your next Android Interview, Join our Android Professional Course to learn the latest in Android and land job at top tech companies. On the form factors screen, enable the Phone and Tablet option and set the minimum SDK setting to API 26: Android 8.0 (Oreo) before proceeding to the Activity selection screen. Create base classes such as BaseActivity. They can still re-publish the post if they are not suspended. By not referencing a specific View. Understand the differences, make you a better programmer. Android Location Google Play Services, 71. To use val variable, you use by lazy property initialization. While this is an improvement on the original AndroidSample app, there is much more that can be achieved to simplify the project by making use of LiveData and data binding, both of which are the topics of the next chapter. Once the layout has loaded, select the existing TextView widget and use the Attributes tool window to change the id property to resultText. Drag a Number (Decimal) view from the palette and position it above the existing TextView. generate link and share the link here. Create Interfaces for the classes wherever required. We accomplish this by creating thousands of videos, articles, and interactive coding lessons - all freely available to the public. The getResult() method, on the other hand, simply returns the current value assigned to the result variable. Ok to get the answer we have to get some knowledge about the Lifecycle of a ViewModel. Remaining in the file, implement these requirements as follows in the onActivityCreated() method: With this phase of the project development completed, build and run the app on the simulator or a physical device, enter a dollar value and click on the Convert button. But make no mistake that plenty of cash cow applications have been written by people who do not have the faintest clue about what that means. When a project is created using the Fragment + ViewModel template, the structure of the project differs in a number of ways from the Basic Activity used when the AndroidSample project was created. The provider will then either create a new instance of that ViewModel class, or return an existing instance: Edit the file and verify that Android Studio has already included this step within the onActivityCreated() method (albeit performing the operation in a single line of code for brevity): With access to the model view, code can now be added to the Fragment to begin working with the data model. Should the View observe a field for every individual widget/control it possesses, or should it observe one field which publishes a single. Now, we need to set up our Network Layer. When you rotate your phone, an activity or fragment is destroyed and recreated. A-143, 9th Floor, Sovereign Corporate Tower, We use cookies to ensure you have the best browsing experience on our website. I mean that it breaks down into (at least) two different approaches which have very distinct appearances, benefits, and consequences. Having a custom constructor parameter for ViewModel is a bit complicated. Check if Email Address is Valid or not in Java, Java Concurrency yield(), sleep() and join() Methods,,,, Helps in data management during configuration changes. Add the following in the AndroidManifest.xml: Now, build the project and run the app on the device. Create package - repository inside the data. Tweet a thanks, Learn to code for free. The version is just an example, you can use later or latest version. Android architecture components are the components that are used to build robust, clean, and scalable apps. So, create a Kotlin data class Resource inside the same utils package and add the following code. Android Architecture Components provides the ViewModel helper class for the UI controller that is responsible for preparing data for the UI. It can't be used to replace by activityViewModels because the created ViewModelwon't be shared across different fragments. Now, we need to complete our MainActivity class. I also use AndroidViewModel by default instead of ViewModel because I usually need to access string resources and system services from the Application context. Step 3: Working with the MainActivity.kt file. This chapter will implement the same currency converter app, this time using the ViewModel component and following the Google app architecture guidelines to avoid Activity lifecycle complications. For custom AndroidViewModel factory, you can inherit from ViewModelProvider.AndroidViewModelFactory. Handle all the API errors at a single place in a better way. Self taught programmer specializing in Object Oriented Design and Clean Software Architecture. Come write articles for us and get featured, Learn and code with the best industry experts. We will cover the following in this tutorial: MVVM architecture is a Model-View-ViewModel architecture that removes the tight coupling between each component. Below is the code for the MainActivity.ktfile.

Then, create an interface ApiService inside the api package and add the following code. Here, we are going to set up the Android Project. Once unpublished, this post will become invisible to the public This is because the ViewModel remained in memory as the Fragment was destroyed and recreated and code was added to the onActivityCreated() method to update the TextView with the result data value from the ViewModel each time the Fragment re-started. The first step in this exercise is to create the new project. I made this mistake because I did not understand the reason of usingViewModelProvider to create ViewModel. DEV Community A constructive and inclusive social network for software developers. There are few ways to create ViewModel and AndroidViewModel. A ViewModelProvider instance is created via a call to the ViewModelProviders.of() method from within the Fragment. This is where things get very muddy and dependent on project requirements. To create ViewModel with your custom constructor parameter, you use by viewModels delegate property. Moreover, its main function is to maintain the integrity and allows data to service during configuration changes like screen rotations. As far as I can tell, most people who implement MVVM make it a goal to promote re-usability of ViewModels, so that they may be reused for n number of different Views (many-to-one ratio). Create package - base inside the ui package. The class will also need references to the three views in the user interface layout to react to button clicks, extract the current dollar value and to display the converted currency amount. Create package - main inside the ui package, Create package - view inside the main package, Move the MainActivity to the view package, Create package - viewmodel inside the main package. The ViewModel exists from when you first request a ViewModel until the activity is finished and destroyed. DEV Community 2016 - 2022. This defeats the purpose of ViewModel architecture. This article shows you the Kotlin examples of creating them. A more generally applicable name for this approach is the Humble Object Pattern. Examples can be found here. Now our project is ready with dependencies. MVVM (Model View ViewModel) Architecture Pattern in Android, Fragment to Fragment Communication in Android using Shared ViewModel, Unit Testing of ViewModel with Kotlin Coroutines and LiveData in Android, Assisted Dependency Injection in ViewModel with Dagger-Hilt in Android, Jetpack Architecture Components in Android, Lifecycle in Android Architecture Components, LiveData in Android Architecture Components, Overview of WorkManager in Android Architecture Components, Overview of Data Binding in Android Architecture Components, Overview of Room in Android Architecture Components, Overview of Navigation in Android Architecture Components, Overview of Paging in Android Architecture Components, Difference Between MVC, MVP and MVVM Architecture Pattern in Android, MVC (Model View Controller) Architecture Pattern in Android with Example, MVP (Model View Presenter) Architecture Pattern in Android with Example, Difference Between MVC and MVP Architecture Pattern in Android, Difference Between MVC and MVVM Architecture Pattern in Android, Difference Between MVP and MVVM Architecture Pattern in Android, Android MVP Architecture Extension with Interactors and Repositories, Complete Interview Preparation- Self Paced Course. Get started, freeCodeCamp is a donor-supported tax-exempt 501(c)(3) nonprofit organization (United States Federal Tax Identification Number: 82-0779546). This article is accompanied by a video tutorial covering many of the same ideas here: When I said break down in the previous section, I do not mean to say that the pattern literally breaks. You usually request a ViewModel for the first time when the system calls an activity objects onCreate() method. Convert View Model to Use Hilt Dependency Injection, My Most Used Android Studio Shortcut Keys, Free and Useful Tools for Android Development and Blogging. For my friends who cannot read old English: You may not reference view classes.". These are my common practices. We will explore two variants of MVVM (there is not just one way to do it), and the reasons why you may prefer one variant over another, based on project requirements. So, the image credit goes to the writer of that blog. For this project, however, the Fragment + ViewModel template will be used. Part of JournalDev IT Services Private Limited. Made with love and Ruby on Rails. As this is for beginners, I have done some simplifications. Select this option as shown in Figure 32-1 before clicking the Next button: On the final setup screen, accept the default file names and create the project using the Finish button. Associating the Fragment with the View Model, Creating an Example Java Android App in Android Studio,, Modern Android App Architecture with Jetpack. Before we look at the code, please note that I personally do not use LiveData in my own production code. Show your love by sharing this blog with your fellow developers. Once suspended, vtsen will not be able to comment or publish posts until their suspension is removed. Finally, the chapter showed how the ViewModel approach avoids some of the problems of handling Fragment and Activity lifecycles. I Amit Shekhar, writing this article to share my knowledge on MVVM Architecture. Donations to freeCodeCamp go toward our education initiatives, and help pay for servers, services, and staff. But first, let us start with the general idea. For example, using Bundleto share data between fragments. You can use by activityViewModels. The above image is showing the ViewModel scope, even with any configuration changes the data is persistent. It should load the data into the UI. This article was originally published at on Sept 18, 2021. To make me even less enthusiastic about applying the previous approach indiscriminately, I find that I often do not need to reuse a ViewModel. With the button still selected, change the id property to convertButton. Now, create a class ApiHelper inside the api package and add the following code. Set up UI layer, build and run the project. The key components of the project are as follows: The first point to note is that the user interface of the main activity has been structured so as to allow a single activity to act as a container for all of the screens that will eventually be needed for the completed app. In truth, we could get more specific about small differences even within these two approaches. I'm not sure other Android developers agree with me.

This is all about the MVVM, now let's move to the implementation part of it. To help re-enforce and clarify the information provided in the previous chapter, this chapter will step through the creation of an example app project that makes use of the ViewModel component. The code examples here are used in fragment class. At this point, the layout should resemble that illustrated in Figure 32-4: Click on the Infer constraints button (Figure 32-5) to add any missing layout constraints: Finally, click on the warning icon in the top right-hand corner of the layout editor and convert the hardcoded strings to resources. Locate the main_fragment.xml file in the Project tool window and double click on it to load it into the layout editor. Maybe we could avoid having to make our ViewModels one-to-one, while keeping our Views as Humble Objects, simply by adding something like a Presenter or Controller to the mix. freeCodeCamp's open source curriculum has helped more than 40,000 people get jobs as developers. Any kind of configuration change in Android devices tends to recreate the whole activity of the application. Posted on Oct 8, 2021 Our mission: to help people learn to code for free. This is declared as a sub-class of the ViewModel Android architecture component class and is ready to be modified to store the data model for the app: The next step is to design the layout of the fragment. Clearly, there needs to be some way for the fragment to obtain a reference to the ViewModel in order to be able to access the model and observe data changes. Add the following dependencies in your app level build.gradle. It is worth taking some time at this point to look at the code that has already been generated by Android Studio to display this fragment within the activity container area. A new instance of ViewModel or AndroidViewModel is created again. In Martin Fowlers naming conventions, this is known as Passive View/Screen. Change the data supplied to the ListView through ArrayList. To get around this limitation, we will need to add some code to the Fragment class to set up an onClick listener on the button. How to build a simple Calculator app using Android Studio? Built on Forem the open source software that powers DEV and other inclusive communities.

Also, Lets become friends on Twitter, Linkedin, Github, Quora, and Facebook. Navigate to the app > res > layout > activity_main.xml and add the below code to that file. All Rights Reserved. Proof of ownership: Amazon Case #08987793. I am not advocating one approach over another, but rather encouraging you to be flexible in your approach, based on the requirements at hand. The following code is awesome! Refer to this article: How to Create Classes in Android Studio? Why, you may ask? In order to achieve this, you must essentially have your ViewModel possess an observable field (however you achieve that data binding, Rx, LiveData, whatever) for every control or widget present in the View: Subsequently, the View will still need to wire itself up to the ViewModel, but the functions required to do so become trivially simple to write: You can find the full code for this example here. Repeat this test now with the ViewModelDemo app and note that the current euro value is retained after the rotation. The system may call onCreate() several times throughout the life of an activity, such as when a device screen is rotated. In any case, the approach we will discuss next, while having its own side effects, once again removes the presentation logic from the View. The fun thing about programming is there are many ways to do the same thing. It is very common to pass additional objects to the ViewModel constructor. Although my earlier heading, ViewLogic + ViewModel is not meant to be used or taken seriously, I mean that by having very decoupled and re-usable ViewModels, we are now depending on the View itself to do the work of figuring out how to render/bind this Note object on screen. Set up a new project with Kotlin and other dependencies required. Now, create a Kotlin class MainViewModel inside the same viewmodel package and add the following code. If you want to share your ViewModel across different fragments within the same activity. The converted amount should appear on the TextView indicating that the UI controller and ViewModel re-structuring appears to be working as expected. Since LiveData is not yet being used in the project, it will also be necessary to get the latest result value from the ViewModel each time the Fragment is created. Within this class file is the onCreateView() method which is called when the fragment is created. Updated on Jul 16 For further actions, you may consider blocking this person and/or reporting abuse. ViewModel objects are automatically retained during configuration changes we will see that in the below example. Apart from using the name ViewModel (which itself is confusing if the class is full of logic), the one iron-clad rule of MVVM architecture is that you may never reference a View, from ViewModel. With you every step of your journey. The following example is passing Repository object into the MyViewModel and MyAndroidViewModel.