Shows an example of wrapping shared preferences and how we can use a fake to test the value.

FakePreferences.kt

class FakePreferences : Preferences {
    var storedLongCallCount = 0
        private set
    
    var getLongCallCount = 0
        private set

    /**
     * In this fake, we're just incrementing the number of calls to get long, but we could be more 
     * thorough and track the key that was called.
     */
    override fun getLong(key: String, default: Long): Long {
        getLongCallCount++
        return 0L
    }

    override fun storeLong(key: String, value: Long) {
        storedLongCallCount++
    }
}

Preferences.kt

interface Preferences {
    fun getLong(key: String, default: Long): Long
    fun storeLong(key: String, value: Long)
}

class AndroidPreferences(private val preferences: SharedPreferences): Preferences {
    override fun getLong(key: String, default: Long): Long {
        return preferences.getLong(key, default)
    }

    override fun storeLong(key: String, value: Long) {
        preferences.edit().putLong(key, value).apply()
    }
}

UserProvider.kt

class UserProvider(
    private val preferences: Preferences
) {
    var userId: Long
        get() = preferences.getLong("USER_ID", 0L)
        set(value) {
            preferences.storeLong("USER_ID", value)
        }
}

UserProviderTest.kt

class UserProviderTest {
    fun testStoredId() {
        val fakePreferences = FakePreferences()
        val userProvider = UserProvider(fakePreferences)
        userProvider.userId = 123L
        
        assertThat(fakePreferences.storedLongCallCount).isEqualTo(1)
    }
}