How to mock a class of a library used by the app during instrumented test?

Question

I am working on an Android library and I am writing an instrumented test for it using UI Automator and Mockk.

The library has a class called InstallManager which will install stuff on the device. I want the InstallManager to throw an exception so that I can test if an error notification will be shown.

All I do is finding the Update (Install) button and click on it

        val updateButtonComponent = device.findObject(By.text(updateButtonText))
        updateButtonComponent.click()

How can I mock/manipulate the InstallManager which is being used by the library during the automated test?

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What I tried:

I tried mocking the InstallManager before running the automated test, hoping that UI Automator would magically know that it should use this mocked InstallManager. But (as I thought already) it does not work like that...

private fun breakInstallManager() {
        installManager = spyk(InstallManager(mockk(relaxed = true), nonExistentFile))
        every { installManager.getString(any()) } returns ""
        every { installManager.packageName } returns ""
    }

Answer 1

For mocking InstallManager class in android tests, call MockKAnnotations.init method. Then in the test method you can specify the return value

    @RelaxedMockK
    private lateinit var installManager: InstallManager

    @Before
    fun setUp() {
        MockKAnnotations.init(this)
        // …
    }
    
    @Test
    fun installManagerTest(){
        every { installManager.getString(any()) } returns ""
        every { installManager.packageName } returns ""
        // …
    }