Add test utilities for C++ and Java

Summary:
@public

Test utility on top of the new event system that maintains a counter of instantiated nodes. Meant to replace the global node counter.

Reviewed By: SidharthGuglani

Differential Revision: D15174855

fbshipit-source-id: 6998472f95a09b8da652257a26596164bdcf43d6

testutil/testutil.cpp

@@ -0,0 +1,65 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the LICENSE
+ * file in the root directory of this source tree.
+ */
+#include "testutil.h"
+
+#include <yoga/YGNode.h>
+#include <yoga/events.h>
+
+#include <utility>
+
+namespace facebook {
+namespace yoga {
+namespace test {
+
+int nodeInstanceCount = 0;
+
+namespace {
+
+void yogaEventSubscriber(
+    const YGNode& node,
+    Event::Type eventType,
+    const Event::Data& eventData) {
+
+  switch (eventType) {
+    case Event::NodeAllocation:
+      nodeInstanceCount++;
+      break;
+    case Event::NodeDeallocation:
+      nodeInstanceCount--;
+      break;
+    default:
+      break;
+  }
+}
+} // namespace
+
+void TestUtil::startCountingNodes() {
+  nodeInstanceCount = 0;
+  Event::subscribe(yogaEventSubscriber);
+}
+
+int TestUtil::nodeCount() {
+  return nodeInstanceCount;
+}
+
+int TestUtil::stopCountingNodes() {
+  Event::reset();
+  return std::exchange(nodeInstanceCount, 0);
+}
+
+ScopedEventSubscription::ScopedEventSubscription(
+    std::function<Event::Subscriber>&& s) {
+  Event::subscribe(std::move(s));
+}
+
+ScopedEventSubscription::~ScopedEventSubscription() {
+  Event::reset();
+}
+
+} // namespace test
+} // namespace yoga
+} // namespace facebook