Use Android callback API to obtain cellular signal strength

net/android/cellular_signal_strength.cc

 // Copyright 2016 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 "net/android/cellular_signal_strength.h"
 
 #include "jni/AndroidCellularSignalStrength_jni.h"
 
 namespace net {
 
 namespace android {
 
-namespace cellular_signal_strength {
+namespace {
 
 // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.net
 enum CellularSignalStrengthError {
   // Value returned by CellularSignalStrength APIs when a valid value is
   // unavailable. This value is same as INT32_MIN, but the following code uses
   // the explicit value of INT32_MIN so that the auto-generated Java enums work
   // correctly.
   ERROR_NOT_SUPPORTED = -2147483648,
 };
 
 static_assert(
     INT32_MIN == ERROR_NOT_SUPPORTED,
     "CellularSignalStrengthError.ERROR_NOT_SUPPORTED has unexpected value");
 
-bool GetSignalStrengthDbm(int32_t* signal_strength_dbm) {
-  int32_t signal_strength_dbm_tmp =
-      Java_AndroidCellularSignalStrength_getSignalStrengthDbm(
-          base::android::AttachCurrentThread());
-  if (signal_strength_dbm_tmp == ERROR_NOT_SUPPORTED)
-    return false;
+}  // namespace
 
-  *signal_strength_dbm = signal_strength_dbm_tmp;
-  return true;
+CellularSignalStrength::CellularSignalStrength() {
+  j_cellular_signal_strength_.Reset(Java_AndroidCellularSignalStrength_create(
+      base::android::AttachCurrentThread()));
 }
 
-bool GetSignalStrengthLevel(int32_t* signal_strength_level) {
-  int32_t signal_strength_level_tmp =
-      Java_AndroidCellularSignalStrength_getSignalStrengthLevel(
-          base::android::AttachCurrentThread());
-  if (signal_strength_level_tmp == ERROR_NOT_SUPPORTED)
-    return false;
-
-  *signal_strength_level = signal_strength_level_tmp;
-  return true;
+CellularSignalStrength::~CellularSignalStrength() {
+  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
 }
 
-}  // namespace cellular_signal_strength
+base::Optional<int32_t> CellularSignalStrength::GetSignalStrengthLevel() const {
+  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
+
+  int32_t signal_strength_level =
+      Java_AndroidCellularSignalStrength_getSignalStrengthLevel(
+          base::android::AttachCurrentThread(), j_cellular_signal_strength_);
+  if (signal_strength_level == ERROR_NOT_SUPPORTED)
+    return base::Optional<int32_t>();
+
+  DCHECK_LE(0, signal_strength_level);
+  DCHECK_GE(4, signal_strength_level);
+
+  return signal_strength_level;
+}
 
 }  // namespace android
 
 }  // namespace net