Use Android callback API to obtain cellular signal strength

net/android/java/src/org/chromium/net/AndroidCellularSignalStrength.java

 // 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.
 
 package org.chromium.net;
 
-import android.Manifest;
 import android.annotation.TargetApi;
 import android.content.Context;
-import android.content.pm.PackageManager;
 import android.os.Build;
-import android.os.Process;
-import android.telephony.CellInfo;
-import android.telephony.CellInfoCdma;
-import android.telephony.CellInfoGsm;
-import android.telephony.CellInfoLte;
-import android.telephony.CellInfoWcdma;
+import android.os.Handler;
+import android.os.HandlerThread;
+import android.telephony.PhoneStateListener;
+import android.telephony.SignalStrength;
 import android.telephony.TelephonyManager;
 
+import org.chromium.base.ApplicationState;
+import org.chromium.base.ApplicationStatus;
 import org.chromium.base.ContextUtils;
+import org.chromium.base.NonThreadSafe;
+import org.chromium.base.ThreadUtils;
 import org.chromium.base.annotations.CalledByNative;
 import org.chromium.base.annotations.JNINamespace;
-
-import java.util.Iterator;
-import java.util.List;
+import org.chromium.base.annotations.SuppressFBWarnings;
 
 /**
- * This class interacts with the CellInfo API provided by Android. This class is thread safe.
+ * This class provides the cellular signal strength using the APIs provided by Android. This class
+ * is thread safe.
  */
-@JNINamespace("net::android::cellular_signal_strength")
+@JNINamespace("net::android")
 public class AndroidCellularSignalStrength {
+    // {@link mSignalLevel} is set to volatile since may be accessed across threads.
+    private volatile int mSignalLevel = CellularSignalStrengthError.ERROR_NOT_SUPPORTED;
+
+    @SuppressFBWarnings("URF_UNREAD_FIELD")
+    private CellStateListener mCellStateListener;
+
+    // {@link mHandlerThread} is the thread on which {@link mCellStateListener} listens to the
+    // changes in the cellular signal strength.
+    private HandlerThread mHandlerThread;

[pauljensen, 2017/06/22 13:32:14]

mHandlerThread should be a local not a member variable.

[tbansal1, 2017/06/22 23:17:52]

Done.

+
+    private static AndroidCellularSignalStrength sInstance;
+
+    private static volatile NonThreadSafe sThreadCheck;
+
     /**
-     * @return Signal strength (in dbM) for the currently registered cellular network. Returns
-     * {@link CellularSignalStrengthError#ERROR_NOT_SUPPORTED} if the signal strength is
-     * unavailable or if there are multiple cellular radios on the device.
+     * This class listens to the changes in the cellular signal strength level and updates {@link
+     * mSignalLevel}. {@link CellStateListener} registers as a signal strength observer only if the
+     * application has running activities.
      */
-    @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR2)
-    @CalledByNative
-    public static int getSignalStrengthDbm() {
-        List<CellInfo> cellInfos = getRegisteredCellInfo();
-        return cellInfos == null || cellInfos.size() != 1
-                ? CellularSignalStrengthError.ERROR_NOT_SUPPORTED
-                : getSignalStrengthDbm(cellInfos.get(0));
+    private class CellStateListener
+            extends PhoneStateListener implements ApplicationStatus.ApplicationStateListener {
+        private final TelephonyManager mTelephonyManager;
+
+        CellStateListener() {
+            ThreadUtils.assertOnBackgroundThread();
+
+            mTelephonyManager =
+                    (TelephonyManager) ContextUtils.getApplicationContext().getSystemService(
+                            Context.TELEPHONY_SERVICE);
+
+            if (mTelephonyManager.getSimState() != TelephonyManager.SIM_STATE_READY) return;
+
+            ApplicationStatus.registerApplicationStateListener(this);
+            onApplicationStateChange(ApplicationStatus.getStateForApplication());
+        }
+
+        private void register() {

[pauljensen, 2017/06/22 13:32:14]

I think you want to initialize mSignalLevel to ERROR_NOT_SUPPORTED here,
otherwise you'll get a prior value that may be incorrect.

[tbansal1, 2017/06/22 23:17:52]

Done.

+            mTelephonyManager.listen(this, PhoneStateListener.LISTEN_SIGNAL_STRENGTHS);
+        }
+
+        private void unregister() {
+            mTelephonyManager.listen(this, PhoneStateListener.LISTEN_NONE);
+        }
+
+        @Override
+        @TargetApi(Build.VERSION_CODES.M)
+        public void onSignalStrengthsChanged(SignalStrength signalStrength) {
+            mSignalLevel = signalStrength.getLevel();
+        }
+
+        // ApplicationStatus.ApplicationStateListener
+        @Override
+        public void onApplicationStateChange(int newState) {
+            if (newState == ApplicationState.HAS_RUNNING_ACTIVITIES) {
+                register();
+            } else if (newState == ApplicationState.HAS_PAUSED_ACTIVITIES) {
+                unregister();
+            }
+        }
+    }
+
+    // Suppress FindBugs warning about unsynchronized initializing of the static field. The warning
+    // does not apply since {@link sInstance} is accessed on a single thread, which is enforced
+    // using {@link sThreadCheck}.
+    @SuppressFBWarnings("LI_LAZY_INIT_STATIC")
+    private static AndroidCellularSignalStrength getInstance() {
+        if (sInstance == null) sInstance = new AndroidCellularSignalStrength();
+        return sInstance;
+    }
+
+    private AndroidCellularSignalStrength() {
+        if (!isAPIAvailable(ContextUtils.getApplicationContext())) return;
+
+        mHandlerThread = new HandlerThread("AndroidCellularSignalStrength");
+        mHandlerThread.start();
+
+        new Handler(mHandlerThread.getLooper()).post(new Runnable() {
+            @Override
+            public void run() {
+                mCellStateListener = new CellStateListener();
+            }
+        });
     }
 
     /**
      * @return the signal strength level (between 0 and 4, both inclusive) for the currently
      * registered cellular network with lower value indicating lower signal strength. Returns
      * {@link CellularSignalStrengthError#ERROR_NOT_SUPPORTED} if the signal strength level is
-     * unavailable or if there are multiple cellular radios on the device.
+     * unavailable. Must be called on the same thread.
      */
-    @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR2)
+    @TargetApi(Build.VERSION_CODES.M)
     @CalledByNative
-    public static int getSignalStrengthLevel() {
-        List<CellInfo> cellInfos = getRegisteredCellInfo();
-        return cellInfos == null || cellInfos.size() != 1
-                ? CellularSignalStrengthError.ERROR_NOT_SUPPORTED
-                : getSignalStrengthLevel(cellInfos.get(0));
+    private static int getSignalStrengthLevel() {
+        if (sThreadCheck == null) {
+            sThreadCheck = new NonThreadSafe();
+        }

[pauljensen, 2017/06/22 13:32:14]

This is racy and also I don't see any reason to restrict to single-threaded
access.  Why not just make sInstance a "static final"?  Then the race is gone,
sThreadCheck can be removed, getInstance can be removed.

[tbansal1, 2017/06/22 23:17:52]

Done.

+        assert sThreadCheck.calledOnValidThread();
+        return getInstance().mSignalLevel;
     }
 
     /**
      * Returns true if the API for quering the signal strength is available.
-     * {@link android.telephony#CellInfoWcdma} is only available on API Level
-     * {@link Build.VERSION_CODES#JELLY_BEAN_MR2} and higher. Also verifies that appropriate
-     * permissions are already available. This ensures that on Android M and higher, Chromium will
-     * not request run-time permission from the user when querying for cellular signal strength.
-     * TODO(tbansal): Consider using {@link TelephonyManager#getNeighboringCellInfo}
-     * for earlier versions of Android.
-    */
-    private static boolean isAPIAvailable() {
-        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR2) return false;
-
-        try {
-            return ContextUtils.getApplicationContext().checkPermission(
-                           Manifest.permission.ACCESS_COARSE_LOCATION, Process.myPid(),
-                           Process.myUid())
-                    == PackageManager.PERMISSION_GRANTED;
-        } catch (Exception ignored) {
-            // Work around certain platforms where this method sometimes throws a runtime exception.
-            // See crbug.com/663360.
-        }
-        return false;
-    }
-
-    /**
-     * Returns all observed cell information from all radios on the device including the primary
-     * and neighboring cells. Returns only the information of cells that are registered to a
-     * mobile network. May return {@code null}.
+     * {@link android.telephony.SignalStrength#getLevel} is only available on API Level
+     * {@link Build.VERSION_CODES#M} and higher.
      */
-    @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR2)
-    private static List<CellInfo> getRegisteredCellInfo() {
-        if (!isAPIAvailable()) {
-            return null;
-        }
-
-        TelephonyManager telephonyManager =
-                (TelephonyManager) ContextUtils.getApplicationContext().getSystemService(
-                        Context.TELEPHONY_SERVICE);
-        if (telephonyManager == null) {
-            return null;
-        }
-
-        List<CellInfo> cellInfos = telephonyManager.getAllCellInfo();
-        if (cellInfos == null) {
-            return null;
-        }
-
-        Iterator<CellInfo> iter = cellInfos.iterator();
-        while (iter.hasNext()) {
-            if (!iter.next().isRegistered()) {
-                iter.remove();
-            }
-        }
-        return cellInfos;
-    }
-
-    /**
-     * @return Signal strength (in dbM) from {@link cellInfo}. Returns {@link
-     * CellularSignalStrengthError#ERROR_NOT_SUPPORTED} if the signal strength is unavailable.
-     */
-    @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR2)
-    private static int getSignalStrengthDbm(CellInfo cellInfo) {
-        if (cellInfo instanceof CellInfoCdma) {
-            return ((CellInfoCdma) cellInfo).getCellSignalStrength().getDbm();
-        }
-        if (cellInfo instanceof CellInfoGsm) {
-            return ((CellInfoGsm) cellInfo).getCellSignalStrength().getDbm();
-        }
-        if (cellInfo instanceof CellInfoLte) {
-            return ((CellInfoLte) cellInfo).getCellSignalStrength().getDbm();
-        }
-        if (cellInfo instanceof CellInfoWcdma) {
-            return ((CellInfoWcdma) cellInfo).getCellSignalStrength().getDbm();
-        }
-        return CellularSignalStrengthError.ERROR_NOT_SUPPORTED;
-    }
-
-    /**
-     * @return the signal level from {@link cellInfo}. Returns {@link
-     * CellularSignalStrengthError#ERROR_NOT_SUPPORTED} if the signal
-     * level is unavailable with lower value indicating lower signal strength.
-     */
-    @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR2)
-    private static int getSignalStrengthLevel(CellInfo cellInfo) {
-        if (cellInfo instanceof CellInfoCdma) {
-            return ((CellInfoCdma) cellInfo).getCellSignalStrength().getLevel();
-        }
-        if (cellInfo instanceof CellInfoGsm) {
-            return ((CellInfoGsm) cellInfo).getCellSignalStrength().getLevel();
-        }
-        if (cellInfo instanceof CellInfoLte) {
-            return ((CellInfoLte) cellInfo).getCellSignalStrength().getLevel();
-        }
-        if (cellInfo instanceof CellInfoWcdma) {
-            return ((CellInfoWcdma) cellInfo).getCellSignalStrength().getLevel();
-        }
-        return CellularSignalStrengthError.ERROR_NOT_SUPPORTED;
+    private static boolean isAPIAvailable(Context context) {
+        return Build.VERSION.SDK_INT >= Build.VERSION_CODES.M;
     }
 }