In a moment, you are going to add a ViewModel class to your project. The ViewModel class comes from an Android Jetpack library called lifecycle-extensions, one of many libraries that you will use throughout this book. (You will learn more about Jetpack later in this chapter.) To use the class, you must first include the library in the list of dependencies for your project.

Your project’s dependencies live in a file called build.gradle (recall that Gradle is the Android build tool). In the project tool window, with the Android view enabled, expand the Gradle scripts section and take a look at the contents. Your project actually comes with two build.gradle files: one for the project as a whole and one for your app module. Open the build.gradle file located in your app module. You should see something similar to Listing 4.1.

apply plugin: 'com.android.application'

apply plugin: 'kotlin-android'

apply plugin: 'kotlin-android-extensions'

android {
    ...
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation"org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'androidx.appcompat:appcompat:1.0.0-beta01'
    ...
}

The first line in the dependencies section specifies that the project depends on all of the .jar files in its libs directory. The other lines list libraries that were automatically included in your project based on the settings you selected when you created it.

Gradle also allows for the specification of new dependencies. When your app is compiled, Gradle will find, download, and include the dependencies for you. All you have to do is specify an exact string incantation and Gradle will do the rest.

Add the lifecycle-extensions dependency to your app/build.gradle file, as shown in Listing 4.2. Its exact placement in the dependencies section does not matter, but to keep things tidy it is good to put new dependencies under the last existing implementation dependency.

dependencies {
    ...
    implementation 'androidx.constraintlayout:constraintlayout:1.1.2'
    implementation 'androidx.lifecycle:lifecycle-extensions:2.0.0'
    ...
}

When you make any change to build.gradle, Android Studio will prompt you to sync the file (Figure 4.1).

Figure 4.1  Gradle sync prompt

This sync asks Gradle to update the build based on your changes by either downloading or removing dependencies. To sync, either click Sync Now in the prompt or select File → Sync Project with Gradle Files.