"Hello, Jetpack": Build your first Jetpack app

This document walks you through the basic process of building a simple, Jetpack-based app. This example is based on a sample Kotlin app called ComponentsBasicSample.

For guidance about how to create apps in general, see Build Your First App. To learn more about the classes that this document covers, see Android Architecture Components.

Set up your app

Perform the following steps to set up your app to use Jetpack:

Launch Android Studio 3.2 or higher, and enter information in the Create Android Project and Target Android Devices as always.

If you are writing your app in Kotlin, remember to check the Include Kotlin support box on the Create Android Project screen.

After the Create Android Project screen, the Add an Activity to Mobile screen appears, offering you a variety of templates to use in starting your project. The Activity & Fragment + ViewModel template, shown in figure 1, is designed to make it easy to incorporate Jetpack into your app. Click that template, and then click Next.

Figure 1. The Activity & Fragment + ViewModel template

On the Configure Activity screen, enter names for the initial activity, fragment, and ViewModel object with which to start your project. Optionally, you can also enter a fragment package path. Then, click Finish.

figure 2. The Configure Activity screen for the Activity & Fragment + ViewModel template

Opening the java folder in your project reveals, as shown in figure 3, that the project initially contains three classes: StartActivity, StartFragment, and StartViewModel.

Figure 3. Initial classes that the Activity & Fragment + ViewModel template places into a project

StartActivity is your app's entry point. It is the stub of an Activity to serve as the container for the fragments that appear in your app's initial screen.
StartFragment is the stub of an initial Fragment for you to use in your app.
StartViewModel is the stub of an initial ViewModel for you to use in your app.

Take advantage of Jetpack

With these pieces in place, you might choose to implement your ViewModel object, as in the following example:

    class StartViewModel : ViewModel() {
        private val _data = MutableLiveData<String>()
        val data: LiveData<String>
            get() = _data

        init {
            _data.value = "Hello, Jetpack!"
        }
    }

If your app contains more than one screen, you can add the Navigation class to implement navigation triggers for your fragments. The following code is an example of implementing a navigation trigger:

    // Set up a click listener on the login button
    view?.findViewById<Button>(R.id.navigate_bt)?.setOnClickListener {
       // Navigate to the login destination
       view?.let { Navigation.findNavController(it).navigate(R.id.end_action) }

If your app requires local access to SQLite data, you might also add the Room persistence library. If your app needs to display large volumes of data on a single screen, you should consider using the Paging library.

Configure your Gradle file

In order to use Jetpack, you must remember to add the appropriate lines to your gradle file. Because this app uses ViewModel, LiveData, and NavigationController, its Gradle file contains these lines:

       // LiveData + ViewModel

       implementation "android.arch.lifecycle:extensions:$rootProject.archLifecycleVersion"

       // Navigation

       implementation 'androidx.navigation:navigation-fragment:' + rootProject.navigationVersion
       implementation 'androidx.navigation:navigation-ui:' + rootProject.navigationVersion

Learn more

To learn more about the sample and classes discussed in this document, see the following resources: