Introduce TestScope.collectLastValue(Flow)

This CL introduces a helper function to easily collect value from an
infinite Flow from a test. See ag/20562448 for an example.

Bug: 242040009
Test: atest FooterActionsViewModelTest
Change-Id: Ie87220a7382424dc9c25728fca03c601952da5c0

packages/SystemUI/tests/utils/src/com/android/systemui/coroutines/Flow.kt

@@ -0,0 +1,40 @@
+/*
+ * Copyright (C) 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.systemui.coroutines
+
+import kotlin.coroutines.CoroutineContext
+import kotlin.coroutines.EmptyCoroutineContext
+import kotlinx.coroutines.CoroutineStart
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.test.TestScope
+import kotlinx.coroutines.test.runCurrent
+
+/** Collect [flow] in a new [Job] and return a getter for the last collected value. */
+fun <T> TestScope.collectLastValue(
+    flow: Flow<T>,
+    context: CoroutineContext = EmptyCoroutineContext,
+    start: CoroutineStart = CoroutineStart.DEFAULT,
+): () -> T? {
+    var lastValue: T? = null
+    backgroundScope.launch(context, start) { flow.collect { lastValue = it } }
+    return {
+        runCurrent()
+        lastValue
+    }
+}