For #15279: add LazyMonitored and tests.

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

@@ -0,0 +1,44 @@
+/* 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 mozilla.components.support.base.log.logger.Logger
+import java.util.concurrent.atomic.AtomicInteger
+
+private val logger = Logger("LazyMonitored")
+
+/**
+ * A container for the number of components initialized.
+ */
+object ComponentInitCount {
+    val count = AtomicInteger(0)
+}
+
+/**
+ * A convenience function for setting the [LazyMonitored] property delegate, which wraps
+ * [lazy] to add performance monitoring.
+ */
+fun <T> lazyMonitored(initializer: () -> T) = LazyMonitored(initializer)
+
+/**
+ * A wrapper around the [lazy] property delegate to monitor for performance related issues.
+ * For example, we can count the number of components initialized to see how the number of
+ * components initialized on start up impacts start up time.
+ */
+class LazyMonitored<T>(initializer: () -> T) {
+    // Lazy is thread safe.
+    private val lazyValue = lazy {
+        // We're unlikely to have 4 billion components so we don't handle overflow.
+        val componentInitCount = ComponentInitCount.count.incrementAndGet()
+
+        initializer().also {
+            @Suppress("UNNECESSARY_NOT_NULL_ASSERTION") // the compiler fails with !! but warns with !!.
+            val className = if (it == null) "null" else it!!::class.java.canonicalName
+            logger.debug("Init component #$componentInitCount: $className")
+        }
+    }
+
+    operator fun getValue(thisRef: Any?, property: KProperty<*>): T = lazyValue.value
+}

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

@@ -0,0 +1,42 @@
+/* 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 org.junit.Assert.assertEquals
+import org.junit.Before
+import org.junit.Test
+
+class LazyMonitoredTest {
+
+    private val componentInitCount get() = ComponentInitCount.count.get()
+
+    @Before
+    fun setUp() {
+        ComponentInitCount.count.set(0)
+    }
+
+    @Test
+    fun `WHEN using the convenience function THEN it returns a lazy monitored`() {
+        val actual = lazyMonitored { }
+        assertEquals(LazyMonitored::class, actual::class)
+    }
+
+    @Test
+    fun `WHEN accessing a lazy monitored THEN it returns the initializer value`() {
+        val actual by lazyMonitored { 4 }
+        assertEquals(4, actual)
+    }
+
+    @Test
+    fun `WHEN accessing a lazy monitored THEN the component init count is incremented`() {
+        assertEquals(0, componentInitCount)
+
+        val monitored by lazyMonitored { }
+        // We must access the value to trigger init.
+        @Suppress("UNUSED_EXPRESSION") monitored
+
+        assertEquals(1, componentInitCount)
+    }
+}