For perf-frontend-issues#33: Add HotStartPerformanceMonitorTest.

We really don't want our startup tests to subtlely change so I thought
it was worth writing some quick tests.

app/src/main/java/org/mozilla/fenix/perf/HotStartPerformanceMonitor.kt

@@ -22,19 +22,22 @@ import android.util.Log
  * To see logs from this class, the user must enable VERBOSE logging for the appropriate tag:
  *   adb shell setprop log.tag.FenixPerf VERBOSE
  */
-class HotStartPerformanceMonitor {
+class HotStartPerformanceMonitor(
+    private val log: (String) -> Unit = { Log.v(Performance.TAG, it) }, // android log to minimize overhead.
+    private val getElapsedRealtime: () -> Long = { SystemClock.elapsedRealtime() }
+) {
 
     private var onRestartMillis: Long = -1
 
     fun onRestartFirstMethodCall() {
-        onRestartMillis = SystemClock.elapsedRealtime()
+        onRestartMillis = getElapsedRealtime()
     }
 
     fun onPostResumeFinalMethodCall() {
         // If onRestart was never called, this is not a hot start: ignore it.
         if (onRestartMillis >= 0) {
-            val elapsedMillis = SystemClock.elapsedRealtime() - onRestartMillis
-            Log.v(Performance.TAG, "hot start: $elapsedMillis") // android log to minimize overhead.
+            val elapsedMillis = getElapsedRealtime() - onRestartMillis
+            log("hot start: $elapsedMillis")
         }
     }
 }

app/src/test/java/org/mozilla/fenix/perf/HotStartPerformanceMonitorTest.kt

@@ -0,0 +1,43 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix.perf
+
+import io.mockk.Called
+import io.mockk.MockKAnnotations
+import io.mockk.impl.annotations.MockK
+import io.mockk.verify
+import org.junit.Before
+import org.junit.Test
+
+class HotStartPerformanceMonitorTest {
+
+    private lateinit var monitor: HotStartPerformanceMonitor
+
+    @MockK(relaxed = true) private lateinit var log: (String) -> Unit
+    private var elapsedRealtime = 0L
+
+    @Before
+    fun setUp() {
+        MockKAnnotations.init(this)
+        monitor = HotStartPerformanceMonitor(log, getElapsedRealtime = { elapsedRealtime })
+    }
+
+    @Test
+    fun `WHEN onRestart is not called but onPostResume is called THEN we do not log`() {
+        monitor.onPostResumeFinalMethodCall()
+        verify { log.invoke(any()) wasNot Called }
+    }
+
+    @Test
+    fun `WHEN onRestart then onPostResume is called THEN we log the elapsed time`() {
+        elapsedRealtime = 10
+        monitor.onRestartFirstMethodCall()
+
+        elapsedRealtime = 100
+        monitor.onPostResumeFinalMethodCall()
+
+        verify { log.invoke("hot start: 90") } // fragile but it's not worth the time to make robust.
+    }
+}