Develop Reference

How can I launch an Activity once per test class, not once for each test method?

I have some very granular test methods and it would be a total waste of time to start the Activity under test each time. For this it would be very handy to just start it once. Unfortunately Espresso/JUnit finishes all Activities after each test method. Is there a simple way around this?

There is no way to do that. Espresso test cases start activity for test. Espresso is designed like that.
If you want to test specific scenarios you can have separate methods which you can used it in different tests.
For more info :
Why does Espresso leave the app after the test has finished? How to stop it from doing that

How to call api on background thread in Roboelectric in Android

I'm running a test with Robolectric runner. The code under test verifies it's not executed on the main thread:
if (Looper.getMainLooper().getThread() == java.lang.Thread.currentThread()) {
new IllegalStateException("Method called on the UI thread");
}
The Robolectric test raises this exception, and I don't want that. I tried running the code from a Robolectric.getBackgroundScheduler(), but I'm still getting the exception.
How can my test run in a different thread?

The main idea in testing multithreading code is to make it run in controlled way on a single thread.
What I would do:
Move checking code to some class helper
Inject it and mock it under the test
Pluses of this solution:
It will resolve your issue
It will remove duplication and move you closer to SRP (single class responsibility principle)
Minuses:
It requires proper naming since it will hide functionality behind method
It will give you additional flexibility that you might not need
Success!

How to write an android test which will verify my app hits all required apis

I am heavily testing my application with unit tests and Espresso tests. My next step is to make sure my application hits all required apis. For that reason I am looking for a way to write a test, which will verify the api calls.
I would really appreciate any suggestions.

What you are describing is called a "unit test". Unit tests are meant to test as many lines of code as possible regardless of UI.
Espresso tests are "instrumentation tests" (or "UI tests") intended to check if the app is responding to UI events correctly. They're not meant to verify the correctness of code, but the correctness of the functionality of the app as used by the user.
You can read about both at the official documentation. You'll find that unit tests are very different than instrumentation tests, and often harder to write because they require more engineering of your code to do correctly. You will likely have to "mock" the various parts of your application to make sure their APIs were called exactly as you expected.

There are 2 main goal when I was writing api tests:
First is component based. The goal was to make sure each class / component makes an api call when certain criteria is met (for example calling an api A when onDestroy() is called)
Second, is to make sure the Apis are called is certain order for the analytics purposes.
The first step I achieved by using unit tests with injected mock objects via Mockito and PowerMockito. PowerMockito was used primarily to mock static methods and to make sure the methods were called at least n times.
For the second step, UI test could be used, since it runs the real application. I have implemented the helper class, which was recording the instances when api requests were made. The script in Espresso was validating the order of api calls by referring the helper class.

Robolectric & testing startActivityForResult

I have troubles with my Robolectric unit test.
I can assert without problem that a click started a new activity when the listener uses the method startActivity( Intent )
But it seems Robolectric has trouble when a new activity is started with the method startActivityForResult(Intent, int) : putting some breaks in the code made me figure out that the activity wasn't started ( and just changing for method startActivity( Intent ) made the assert pass).
Is that normal ? It's a pity since the first activity of my app uses startActivityForResult(Intent, int).
Did someone manage to make tests with this way of launching activities ?
Thanks for your help ..

The short Answer to your question is that, due to the way Robolectric converts Android classes to code that executes in the JVM, a lot of their functionality doesn't behave as you'd expect. Many system callbacks won't execute, and you'll have to rely on what Robolectric provides in their Shadow implementation of classes. (See the link provided by #Steven_BDawg).
The long answer: It may be possible to implement this whole flow in one big test, but it's not what the framework is designed for.
Robolectric and Unit Testing in general aren't meant to be used in the way that you describe. The Unit Testing page on wikipedia states that one can view a unit as the smallest testable part of an application. A unit testing suite should contain many lightweight tests, where each test isolates a bit of functionality in your app and ensure it's working properly.
Consider a basic Application that contains two Activities, A and B. Activity A displays some information about a topic, and Activity B allows the user to select which topic to show in A. When the user moves from Activity A to Activity B, B gets called with startActivityForResult() and should return to A with the selected topic.
Now say we want to Unit Test this flow of A getting the result from B and displaying the data. We can break this up into two tests:
Activity Under Test - Activity A. In our test, we'll create a new instance of Activity A. In the Robolectric Test, we create the Intent that we expect B to return to A, and call the shadow method receiveResult() for A, filling out the arguments with a result code of OK and this Intent. After receiveResult(), run your assertions. You now know that Activity A handles the result properly!
Activity Under Test - Activity B. In our test, we'll create a new instance of Activity B, setting it up as if it were started for result from Activity A. In the Robolectric Test, we'll perform all actions needed to select the data, create the intent we'll send back, then run assertions on the intent to ensure it was created correctly.
This is a very simple example. These two steps could probably be broken out into many more tests, as, again, each unit test should only be testing the smallest unit of functionality that your app can be broken into. The example is mainly to help you start thinking in a unit testing kind of way. I've found that as my understand of unit testing deepens, the way I write code has changed. I try to avoid writing methods and classes in such a way that they do too much work and cannot be properly unit tested. As a rule of thumb, code that's easy to unit test performs very specific operations which are readily apparent when reading the code for the first time.
Finally, if you want to take this a step further, mocking frameworks can greatly aid your ability to Unit Test. Mockito is a mocking framework I've had success with in the past. The purpose of a mocking framework is to create stub Objects whose behavior you tightly control. Mockito (or any other Mocking Framework), will allow you to define an object that extends from any type you need and only implement the methods you need. You'll be able to directly control the response to any of these method invocations. This aids Unit Testing because the only real object that you'll need is the Object Under Test; by Mocking all other objects, you'll have a better sense of whether or not the Object Under Test is behaving properly because all other behavior is explicitly defined by you, the tester. (And yes, this does lead to lots of extra code, but such is the life of a good unit tester. However, as previously stated, as you get more comfortable with unit testing, you may find yourself writing methods that require less mocking and are more conducive to writing tests. Some coders will even write their unit tests BEFORE they code, in order to keep their code tight and focused on a single purpose)
Hope this helps!

I did some Google'ing for you. I don't know if this will definitely help you or not, but I think it is a good start!
Roboelectric: Testing startActivityForResult() and onActivityResult()

Android Unit testing - Have a test wait on a response

So I've got an Android library that I need to do unit testing on. You pass the library some handlers, and I need to unit test based on the messages sent to those handlers from the library.
The problem is, I don't know how to tell a test to wait until I get a response back. I have a timer checking the response every 10 seconds in my test method and if it sees a response is does an assert, but the test method schedules the timer task and then finishes as successful instead of waiting for an assert in the timer task.
Is there a way to have the test explicitly wait until it runs into an assert before finishing? Or some other way of accomplishing this?
Thanks!

You can create mocks of the handlers. Mocks can/should replace the actual handlers in the unit test of the library. You have full control over mocks, and avoid threading issues (unit testing is damn hard with threading involved).