[vr] Add a frame rate counter for the vr UI

chrome/browser/android/vr_shell/fps_meter_unittest.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/browser/android/vr_shell/fps_meter.h"
+
+#include "base/macros.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace vr_shell {
+
+namespace {
+
+static constexpr double kTolerance = 0.01;
+
+base::TimeTicks usToTicks(uint64_t us) {
+  return base::TimeTicks::FromInternalValue(us);
+}
+
+base::TimeDelta usToDelta(uint64_t us) {
+  return base::TimeDelta::FromInternalValue(us);
+}
+
+}  // namespace
+
+TEST(FPSMeter, GetFPSWithTooFewFrames) {
+  FPSMeter meter;
+  EXPECT_FALSE(meter.CanComputeFPS());
+  EXPECT_FLOAT_EQ(0.0, meter.GetFPS());
+
+  meter.AddFrame(usToTicks(16000));
+  EXPECT_FALSE(meter.CanComputeFPS());
+  EXPECT_FLOAT_EQ(0.0, meter.GetFPS());
+
+  meter.AddFrame(usToTicks(32000));
+  EXPECT_TRUE(meter.CanComputeFPS());
+  EXPECT_LT(0.0, meter.GetFPS());
+}
+
+TEST(FPSMeter, AccurateFPSWithManyFrames) {
+  FPSMeter meter;
+  EXPECT_FALSE(meter.CanComputeFPS());
+  EXPECT_FLOAT_EQ(0.0, meter.GetFPS());
+
+  base::TimeTicks now = usToTicks(1);
+  base::TimeDelta frame_time = usToDelta(16666);
+
+  meter.AddFrame(now);
+  EXPECT_FALSE(meter.CanComputeFPS());
+  EXPECT_FLOAT_EQ(0.0, meter.GetFPS());
+
+  for (int i = 0; i < 5; ++i) {
+    now += frame_time;
+    meter.AddFrame(now);
+    EXPECT_TRUE(meter.CanComputeFPS());
+    EXPECT_NEAR(60.0, meter.GetFPS(), kTolerance);
+  }
+}
+
+TEST(FPSMeter, AccurateFPSWithHigherFramerate) {
+  FPSMeter meter;
+  EXPECT_FALSE(meter.CanComputeFPS());
+  EXPECT_FLOAT_EQ(0.0, meter.GetFPS());
+
+  base::TimeTicks now = usToTicks(1);
+  base::TimeDelta frame_time = base::TimeDelta::FromSecondsD(1.0 / 90.0);
+
+  meter.AddFrame(now);
+  EXPECT_FALSE(meter.CanComputeFPS());
+  EXPECT_FLOAT_EQ(0.0, meter.GetFPS());
+
+  for (int i = 0; i < 5; ++i) {
+    now += frame_time;
+    meter.AddFrame(now);
+    EXPECT_TRUE(meter.CanComputeFPS());
+    EXPECT_NEAR(1.0 / frame_time.InSecondsF(), meter.GetFPS(), kTolerance);
+  }
+}
+
+}  // namespace vr_shell