Cannot see log from unit test in Android Studio

Question

I'm working on an Android app written in Kotlin. I would like to use Log.* to see the output of some very simple logic in a unit test, rendered as string. I'm using mockk as suggestested in this other question.

I don't get any errors, the test works fine; however, not even a single line is printed in the Logcat of Android Studio. I checked filters and current device with no avail.

Here is my simplified code:

// UnitTest.kt
class MyUnitTest {
  @Before
  fun setup() {
    mockkStatic(Log::class)
    every { Log.d(any(), any()) } returns 0
  }

  @Test
  fun do_test() {
    // ... some logic
    Log.d("TEST_TAG", "Test string")
  }
}

I also tried to:

• put the Log in a function marked as @JvmStatic into a companion object
• adding the following to app level build.gradle as suggested here (though unrecommended)

testOptions {
  unitTests.returnDefaultValues = true
} 

• get rid of everything and use a kotlin implementation of the main answer. Something like this:

// app/src/test/android/util/MyLog.kt
package android.util

class MyLog {
  companion object {
    @JvmStatic
    fun d(tag: String, msg: String): Int {
      println("DEBUG: $tag: $msg")
      return 0
    }
}

Unfortunately none of these worked (with any of the Log levels).

I don't have great experience in writing tests, am I missing something?

Edit (about solution):

I was targeting Logcat, which is wrong, because unit tests run on a local machine.

Answer 1

You need to use println() for logging in unit tests.

Logcat shows messages coming from a device. As unit tests are running on your local machine, it is physically impossible to see messages. println() prints messages to the test "console".