As your app grows, you might find it useful to place some of your app components in a process other than your app's main process. To test app components in these non-default processes, you can use the functionality of Multiprocess Espresso. This tool, available on Android 8.0 (API level 26) and higher, allows you to seamlessly test your app's UI interactions that cross your app's process boundaries while maintaining Espresso's synchronization guarantees.
When using Multiprocess Espresso, keep the following versioning and scope considerations in mind:
- Your app must target Android 8.0 (API level 26) or higher.
- The tool can only test app components that you include in processes within your app's package. It cannot test external processes.
Using the tool
To test a process within your app using Multiprocess Espresso, add the following
to your app's
<instrumentation>element that defines the process.
<meta-data>element indicating that you want to use Multiprocess Espresso.
The following code snippet shows how to add these elements:
<manifest ... package="android.support.test.mytestapp.tests"> <uses-sdk android:targetSdkVersion="26" android:minSdkVersion="10" /> <instrumentation android:name="android.support.test.runner.AndroidJUnitRunner" android:targetPackage="android.support.mytestapp" android:targetProcesses="*" /> <meta-data android:name="remoteMethod" android:value="android.support.test.espresso.remote.EspressoRemote#remoteInit" /> </manifest>
The previous snippet indicates to the Android framework that you want it to test
every process in your app's package. If you want to test only a subset of your
app's processes, you can specify a comma-separated list within the
targetProcesses element instead:
<instrumentation ... android:targetProcesses= "android.support.test.mytestapp:myFirstAppProcessToTest, android.support.test.mytestapp:mySecondAppProcessToTest" ... />
Understanding the tool's architecture
When you test your app and launch its default process, you might perform a UI interaction, such as a button press, that starts an activity in a secondary process. The system then completes the following steps to enable cross-process testing using Espresso:
- The Android Framework creates and starts a new process to follow your app's
navigational structure. Each
Instrumentationprocess includes a new instance of
AndroidJUnitRunner. At this stage, the 2 instrumentation processes cannot communicate with each other.
AndroidJUnitRunnerinstance registers Espresso as its testing framework.
- The 2 instances of
AndroidJUnitRunnerperform a handshake to establish a connection between each other. At the same time, each
AndroidJUnitRunnerinstance connects all registered clients like Espresso with their respective counterparts in other processes so that these clients can form a direct communication channel between themselves.
AndroidJUnitRunnerinstance continues to look for newly-added instrumentation instances and testing framework clients, establishing additional communication channels as needed.
Figure 1 illustrates the result of this process:
The Test-Driven Development on Android with the Android Testing Support Library session video from Google I/O 2017, beginning at 36:41, also explains this process.