setup Robolectric for tests

app/build.gradle

@@ -21,7 +21,9 @@ android {
 
 dependencies {
     compile fileTree(dir: 'libs', include: ['*.jar'])
-    testCompile 'junit:junit:4.12'
     compile 'com.android.support:appcompat-v7:23.1.1'
     compile 'com.jakewharton:butterknife:7.0.1'
+
+    testCompile 'junit:junit:4.12'
+    testCompile "org.robolectric:robolectric:3.0"
 }

app/src/main/project.properties

@@ -0,0 +1,2 @@
+android.library.reference.1=../../build/intermediates/exploded-aar/com.android.support/support-v4/23.1.1
+android.library.reference.2=../../build/intermediates/exploded-aar/com.android.support/appcompat-v7/23.1.1

app/src/test/java/calculator/simplemobiletools/com/simple_calculator/ExampleUnitTest.java

@@ -1,15 +0,0 @@
-package calculator.simplemobiletools.com.simple_calculator;
-
-import org.junit.Test;
-
-import static org.junit.Assert.*;
-
-/**
- * To work on unit tests, switch the Test Artifact in the Build Variants view.
- */
-public class ExampleUnitTest {
-    @Test
-    public void addition_isCorrect() throws Exception {
-        assertEquals(4, 2 + 2);
-    }
-}

app/src/test/java/calculator/simplemobiletools/com/simple_calculator/MainActivityTest.java

@@ -0,0 +1,17 @@
+package calculator.simplemobiletools.com.simple_calculator;
+
+import org.junit.Before;
+import org.junit.runner.RunWith;
+import org.robolectric.Robolectric;
+import org.robolectric.annotation.Config;
+
+@RunWith(MyTestRunner.class)
+@Config(constants = BuildConfig.class, manifest = "app/src/main/AndroidManifest.xml", sdk = 21)
+public class MainActivityTest {
+    MainActivity activity;
+
+    @Before
+    public void setUp() {
+        activity = Robolectric.setupActivity(MainActivity.class);
+    }
+}

app/src/test/java/calculator/simplemobiletools/com/simple_calculator/MyTestRunner.java

@@ -0,0 +1,22 @@
+package calculator.simplemobiletools.com.simple_calculator;
+
+import org.junit.runners.model.InitializationError;
+import org.robolectric.RobolectricTestRunner;
+import org.robolectric.annotation.Config;
+import org.robolectric.manifest.AndroidManifest;
+import org.robolectric.res.Fs;
+
+public class MyTestRunner extends RobolectricTestRunner {
+
+    public MyTestRunner(Class<?> testClass) throws InitializationError {
+        super(testClass);
+    }
+
+    @Override
+    protected AndroidManifest getAppManifest(Config config) {
+        return new AndroidManifest(
+                Fs.fileFromPath("../app/src/main/AndroidManifest.xml"),
+                Fs.fileFromPath("../app/src/main/res"),
+                Fs.fileFromPath("../app/src/main/assets"));
+    }
+}