Audio recording notification API

AudioService can register a recording callback with AudioSystem.
Callbacks are handled in a new class, RecordingActivityMonitor.
This class notifies registered AudioManager instances
  of updates, only if this AudioManager has client callbacks.
Each AudioManager dispatches updates to its registered
  callbacks.

Bug 22876530

Change-Id: I95c6e5ec0631e53af53a490bcecca96089490089

Android.mk

@@ -345,6 +345,7 @@ LOCAL_SRC_FILES += \
 	media/java/android/media/IMediaRouterService.aidl \
 	media/java/android/media/IMediaScannerListener.aidl \
 	media/java/android/media/IMediaScannerService.aidl \
+	media/java/android/media/IRecordingConfigDispatcher.aidl \
 	media/java/android/media/IRemoteDisplayCallback.aidl \
 	media/java/android/media/IRemoteDisplayProvider.aidl \
 	media/java/android/media/IRemoteVolumeController.aidl \

core/jni/android_media_AudioSystem.cpp

@@ -152,7 +152,8 @@ static struct {
 
 static struct {
     jmethodID postDynPolicyEventFromNative;
-} gDynPolicyEventHandlerMethods;
+    jmethodID postRecordConfigEventFromNative;
+} gAudioPolicyEventHandlerMethods;
 
 static Mutex gLock;
 
@@ -378,12 +379,26 @@ android_media_AudioSystem_dyn_policy_callback(int event, String8 regId, int val)
     const char* zechars = regId.string();
     jstring zestring = env->NewStringUTF(zechars);
 
-    env->CallStaticVoidMethod(clazz, gDynPolicyEventHandlerMethods.postDynPolicyEventFromNative,
+    env->CallStaticVoidMethod(clazz, gAudioPolicyEventHandlerMethods.postDynPolicyEventFromNative,
             event, zestring, val);
 
     env->ReleaseStringUTFChars(zestring, zechars);
     env->DeleteLocalRef(clazz);
+}
 
+static void
+android_media_AudioSystem_recording_callback(int event, int session, int source)
+{
+    JNIEnv *env = AndroidRuntime::getJNIEnv();
+    if (env == NULL) {
+        return;
+    }
+
+    jclass clazz = env->FindClass(kClassPathName);
+    env->CallStaticVoidMethod(clazz,
+            gAudioPolicyEventHandlerMethods.postRecordConfigEventFromNative,
+            event, session, source);
+    env->DeleteLocalRef(clazz);
 }
 
 static jint
@@ -1487,6 +1502,12 @@ android_media_AudioSystem_registerDynPolicyCallback(JNIEnv *env, jobject thiz)
     AudioSystem::setDynPolicyCallback(android_media_AudioSystem_dyn_policy_callback);
 }
 
+static void
+android_media_AudioSystem_registerRecordingCallback(JNIEnv *env, jobject thiz)
+{
+    AudioSystem::setRecordConfigCallback(android_media_AudioSystem_recording_callback);
+}
+
 
 static jint convertAudioMixToNative(JNIEnv *env,
                                     AudioMix *nAudioMix,
@@ -1659,6 +1680,8 @@ static const JNINativeMethod gMethods[] = {
                                             (void *)android_media_AudioSystem_registerPolicyMixes},
     {"native_register_dynamic_policy_callback", "()V",
                                     (void *)android_media_AudioSystem_registerDynPolicyCallback},
+    {"native_register_recording_callback", "()V",
+                                    (void *)android_media_AudioSystem_registerRecordingCallback},
     {"systemReady", "()I", (void *)android_media_AudioSystem_systemReady},
 };
 
@@ -1762,9 +1785,12 @@ int register_android_media_AudioSystem(JNIEnv *env)
     gEventHandlerFields.mJniCallback = GetFieldIDOrDie(env,
                                                     eventHandlerClass, "mJniCallback", "J");
 
-    gDynPolicyEventHandlerMethods.postDynPolicyEventFromNative =
+    gAudioPolicyEventHandlerMethods.postDynPolicyEventFromNative =
             GetStaticMethodIDOrDie(env, env->FindClass(kClassPathName),
                     "dynamicPolicyCallbackFromNative", "(ILjava/lang/String;I)V");
+    gAudioPolicyEventHandlerMethods.postRecordConfigEventFromNative =
+            GetStaticMethodIDOrDie(env, env->FindClass(kClassPathName),
+                    "recordingCallbackFromNative", "(III)V");
 
     jclass audioMixClass = FindClassOrDie(env, "android/media/audiopolicy/AudioMix");
     gAudioMixClass = MakeGlobalRefOrDie(env, audioMixClass);

media/java/android/media/AudioManager.java

@@ -51,6 +51,7 @@ import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.Iterator;
+import java.util.List;
 
 /**
  * AudioManager provides access to volume and ringer mode control.
@@ -2157,7 +2158,28 @@ public class AudioManager {
                                 }
                                 break;
                             case MSSG_RECORDING_CONFIG_CHANGE:
-                                // cool stuff happening here in a bit, patience
+                                // optimizing for the case of a single callback
+                                AudioRecordingCallback singleCallback = null;
+                                ArrayList<AudioRecordingCallback> multipleCallbacks = null;
+                                synchronized(mRecordCallbackLock) {
+                                    if ((mRecordCallbackList != null)
+                                            && (mRecordCallbackList.size() != 0)) {
+                                        if (mRecordCallbackList.size() == 1) {
+                                            singleCallback = mRecordCallbackList.get(0);
+                                        } else {
+                                            multipleCallbacks =
+                                                    new ArrayList<AudioRecordingCallback>(
+                                                            mRecordCallbackList);
+                                        }
+                                    }
+                                }
+                                if (singleCallback != null) {
+                                    singleCallback.onRecordConfigChanged();
+                                } else if (multipleCallbacks != null) {
+                                    for (int i=0 ; i < multipleCallbacks.size() ; i++) {
+                                        multipleCallbacks.get(i).onRecordConfigChanged();
+                                    }
+                                }
                                 break;
                             default:
                                 Log.e(TAG, "Unknown event " + msg.what);
@@ -2177,7 +2199,7 @@ public class AudioManager {
     private final IAudioFocusDispatcher mAudioFocusDispatcher = new IAudioFocusDispatcher.Stub() {
 
         public void dispatchAudioFocusChange(int focusChange, String id) {
-            Message m = mServiceEventHandlerDelegate.getHandler().obtainMessage(
+            final Message m = mServiceEventHandlerDelegate.getHandler().obtainMessage(
                     MSSG_FOCUS_CHANGE/*what*/, focusChange/*arg1*/, 0/*arg2 ignored*/, id/*obj*/);
             mServiceEventHandlerDelegate.getHandler().sendMessage(m);
         }
@@ -2676,6 +2698,8 @@ public class AudioManager {
     }
 
 
+    //====================================================================
+    // Audio policy
     /**
      * @hide
      * Register the given {@link AudioPolicy}.
@@ -2728,6 +2752,131 @@ public class AudioManager {
     }
 
 
+    //====================================================================
+    // Recording configuration
+    /**
+     * @hide
+     * candidate for public API
+     */
+    public static abstract class AudioRecordingCallback {
+        /**
+         * @hide
+         * candidate for public API
+         */
+        public void onRecordConfigChanged() {}
+    }
+
+    /**
+     * @hide
+     * candidate for public API
+     * @param non-null callback
+     */
+    public void registerAudioRecordingCallback(@NonNull AudioRecordingCallback cb) {
+        if (cb == null) {
+            throw new IllegalArgumentException("Illegal null AudioRecordingCallback argument");
+        }
+        synchronized(mRecordCallbackLock) {
+            // lazy initialization of the list of recording callbacks
+            if (mRecordCallbackList == null) {
+                mRecordCallbackList = new ArrayList<AudioRecordingCallback>();
+            }
+            final int oldCbCount = mRecordCallbackList.size();
+            if (!mRecordCallbackList.contains(cb)) {
+                mRecordCallbackList.add(cb);
+                final int newCbCount = mRecordCallbackList.size();
+                if ((oldCbCount == 0) && (newCbCount > 0)) {
+                    // register binder for callbacks
+                    final IAudioService service = getService();
+                    try {
+                        service.registerRecordingCallback(mRecCb);
+                    } catch (RemoteException e) {
+                        Log.e(TAG, "Dead object in registerRecordingCallback", e);
+                    }
+                }
+            } else {
+                Log.w(TAG, "attempt to call registerAudioRecordingCallback() on a previously"
+                        + "registered callback");
+            }
+        }
+    }
+
+    /**
+     * @hide
+     * candidate for public API
+     * @param non-null callback
+     */
+    public void unregisterAudioRecordingCallback(@NonNull AudioRecordingCallback cb) {
+        if (cb == null) {
+            throw new IllegalArgumentException("Illegal null AudioRecordingCallback argument");
+        }
+        synchronized(mRecordCallbackLock) {
+            if (mRecordCallbackList == null) {
+                return;
+            }
+            final int oldCbCount = mRecordCallbackList.size();
+            if (mRecordCallbackList.remove(cb)) {
+                final int newCbCount = mRecordCallbackList.size();
+                if ((oldCbCount > 0) && (newCbCount == 0)) {
+                    // unregister binder for callbacks
+                    final IAudioService service = getService();
+                    try {
+                        service.unregisterRecordingCallback(mRecCb);
+                    } catch (RemoteException e) {
+                        Log.e(TAG, "Dead object in unregisterRecordingCallback", e);
+                    }
+                }
+            } else {
+                Log.w(TAG, "attempt to call unregisterAudioRecordingCallback() on a callback"
+                        + " already unregistered or never registered");
+            }
+        }
+    }
+
+    /**
+     * @hide
+     * candidate for public API
+     * @return a non-null array of recording configurations. An array of length 0 indicates there is
+     *     no recording active when queried.
+     */
+    public @NonNull AudioRecordConfiguration[] getActiveRecordConfigurations() {
+        final IAudioService service = getService();
+        try {
+            return service.getActiveRecordConfigurations();
+        } catch (RemoteException e) {
+            Log.e(TAG, "Unable to retrieve active record configurations", e);
+            return null;
+        }
+    }
+
+    /**
+     * constants for the recording events, to keep in sync
+     * with frameworks/av/include/media/AudioPolicy.h
+     */
+    /** @hide */
+    public final static int RECORD_CONFIG_EVENT_START = 1;
+    /** @hide */
+    public final static int RECORD_CONFIG_EVENT_STOP = 0;
+
+    /**
+     * All operations on this list are sync'd on mRecordCallbackLock.
+     * List is lazy-initialized in {@link #registerAudioRecordingCallback(AudioRecordingCallback)}.
+     * List can be null.
+     */
+    private List<AudioRecordingCallback> mRecordCallbackList;
+    private final Object mRecordCallbackLock = new Object();
+
+    private final IRecordingConfigDispatcher mRecCb = new IRecordingConfigDispatcher.Stub() {
+
+        public void dispatchRecordingConfigChange() {
+            final Message m = mServiceEventHandlerDelegate.getHandler().obtainMessage(
+                    MSSG_RECORDING_CONFIG_CHANGE/*what*/);
+            mServiceEventHandlerDelegate.getHandler().sendMessage(m);
+        }
+
+    };
+
+    //=====================================================================
+
     /**
      *  @hide
      *  Reload audio settings. This method is called by Settings backup

media/java/android/media/AudioRecordConfiguration.aidl

@@ -0,0 +1,18 @@
+/* Copyright 2016, 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 android.media;
+
+parcelable AudioRecordConfiguration;

media/java/android/media/AudioRecordConfiguration.java

@@ -0,0 +1,102 @@
+/*
+ * Copyright (C) 2016 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 android.media;
+
+import android.os.Parcel;
+import android.os.Parcelable;
+
+import java.util.Objects;
+
+/**
+ * @hide
+ * Candidate for public API, see AudioManager.getActiveRecordConfiguration()
+ *
+ */
+public class AudioRecordConfiguration implements Parcelable {
+
+    private final int mSessionId;
+
+    private final int mClientSource;
+
+    /**
+     * @hide
+     */
+    public AudioRecordConfiguration(int session, int source) {
+        mSessionId = session;
+        mClientSource = source;
+    }
+
+    /**
+     * @return one of AudioSource.MIC, AudioSource.VOICE_UPLINK,
+     *       AudioSource.VOICE_DOWNLINK, AudioSource.VOICE_CALL,
+     *       AudioSource.CAMCORDER, AudioSource.VOICE_RECOGNITION,
+     *       AudioSource.VOICE_COMMUNICATION.
+     */
+    public int getClientAudioSource() { return mClientSource; }
+
+    /**
+     * @return the session number of the recorder.
+     */
+    public int getAudioSessionId() { return mSessionId; }
+
+
+    public static final Parcelable.Creator<AudioRecordConfiguration> CREATOR
+            = new Parcelable.Creator<AudioRecordConfiguration>() {
+        /**
+         * Rebuilds an AudioRecordConfiguration previously stored with writeToParcel().
+         * @param p Parcel object to read the AudioRecordConfiguration from
+         * @return a new AudioRecordConfiguration created from the data in the parcel
+         */
+        public AudioRecordConfiguration createFromParcel(Parcel p) {
+            return new AudioRecordConfiguration(p);
+        }
+        public AudioRecordConfiguration[] newArray(int size) {
+            return new AudioRecordConfiguration[size];
+        }
+    };
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(mSessionId, mClientSource);
+    }
+
+    @Override
+    public int describeContents() {
+        return 0;
+    }
+
+    @Override
+    public void writeToParcel(Parcel dest, int flags) {
+        dest.writeInt(mSessionId);
+        dest.writeInt(mClientSource);
+    }
+
+    private AudioRecordConfiguration(Parcel in) {
+        mSessionId = in.readInt();
+        mClientSource = in.readInt();
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || !(o instanceof AudioRecordConfiguration)) return false;
+
+        final AudioRecordConfiguration that = (AudioRecordConfiguration) o;
+         return ((mSessionId == that.mSessionId)
+                 && (mClientSource == that.mClientSource));
+    }
+}

media/java/android/media/AudioSystem.java

@@ -227,7 +227,7 @@ public class AudioSystem
     }
 
     /**
-     * Handles events for the audio policy manager about dynamic audio policies
+     * Handles events from the audio policy manager about dynamic audio policies
      * @see android.media.audiopolicy.AudioPolicy
      */
     public interface DynamicPolicyCallback
@@ -267,6 +267,33 @@ public class AudioSystem
         }
     }
 
+    /**
+     * Handles events from the audio policy manager about recording events
+     * @see android.media.AudioManager.AudioRecordingCallback
+     */
+    public interface AudioRecordingCallback
+    {
+        void onRecordingConfigurationChanged(int event, int session, int source);
+    }
+
+    private static AudioRecordingCallback sRecordingCallback;
+
+    public static void setRecordingCallback(AudioRecordingCallback cb) {
+        synchronized (AudioSystem.class) {
+            sRecordingCallback = cb;
+            native_register_recording_callback();
+        }
+    }
+
+    private static void recordingCallbackFromNative(int event, int session, int source) {
+        AudioRecordingCallback cb = null;
+        synchronized (AudioSystem.class) {
+            cb = sRecordingCallback;
+        }
+        if (cb != null) {
+            cb.onRecordingConfigurationChanged(event, session, source);
+        }
+    }
 
     /*
      * Error codes used by public APIs (AudioTrack, AudioRecord, AudioManager ...)
@@ -641,6 +668,8 @@ public class AudioSystem
 
     // declare this instance as having a dynamic policy callback handler
     private static native final void native_register_dynamic_policy_callback();
+    // declare this instance as having a recording configuration update callback handler
+    private static native final void native_register_recording_callback();
 
     // must be kept in sync with value in include/system/audio.h
     public static final int AUDIO_HW_SYNC_INVALID = 0;

media/java/android/media/IAudioService.aidl

@@ -20,9 +20,11 @@ import android.app.PendingIntent;
 import android.bluetooth.BluetoothDevice;
 import android.content.ComponentName;
 import android.media.AudioAttributes;
+import android.media.AudioRecordConfiguration;
 import android.media.AudioRoutesInfo;
 import android.media.IAudioFocusDispatcher;
 import android.media.IAudioRoutesObserver;
+import android.media.IRecordingConfigDispatcher;
 import android.media.IRingtonePlayer;
 import android.media.IVolumeController;
 import android.media.Rating;
@@ -157,4 +159,10 @@ interface IAudioService {
     int setFocusPropertiesForPolicy(int duckingBehavior, in IAudioPolicyCallback pcb);
 
     void setVolumePolicy(in VolumePolicy policy);
+
+    void registerRecordingCallback(in IRecordingConfigDispatcher rcdb);
+
+    oneway void unregisterRecordingCallback(in IRecordingConfigDispatcher rcdb);
+
+    AudioRecordConfiguration[] getActiveRecordConfigurations();
 }

media/java/android/media/IRecordingConfigDispatcher.aidl

@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2016 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 android.media;
+
+/**
+ * AIDL for the RecordingActivity monitor in AudioService to signal audio recording updates.
+ *
+ * {@hide}
+ */
+oneway interface IRecordingConfigDispatcher {
+
+    void dispatchRecordingConfigChange();
+
+}

services/core/java/com/android/server/audio/AudioService.java

@@ -58,10 +58,12 @@ import android.media.AudioFormat;
 import android.media.AudioManager;
 import android.media.AudioManagerInternal;
 import android.media.AudioPort;
+import android.media.AudioRecordConfiguration;
 import android.media.AudioRoutesInfo;
 import android.media.IAudioFocusDispatcher;
 import android.media.IAudioRoutesObserver;
 import android.media.IAudioService;
+import android.media.IRecordingConfigDispatcher;
 import android.media.IRingtonePlayer;
 import android.media.IVolumeController;
 import android.media.MediaPlayer;
@@ -705,6 +707,8 @@ public class AudioService extends IAudioService.Stub {
         LocalServices.addService(AudioManagerInternal.class, new AudioServiceInternal());
 
         mUserManagerInternal.addUserRestrictionsListener(mUserRestrictionsListener);
+
+        mRecordMonitor.initMonitor();
     }
 
     public void systemReady() {
@@ -6097,7 +6101,7 @@ public class AudioService extends IAudioService.Stub {
     }
 
     //======================
-    // Audio policy callback from AudioSystem
+    // Audio policy callbacks from AudioSystem for dynamic policies
     //======================
     private final AudioSystem.DynamicPolicyCallback mDynPolicyCallback =
             new AudioSystem.DynamicPolicyCallback() {
@@ -6126,7 +6130,23 @@ public class AudioService extends IAudioService.Stub {
                 }
             }
         }
+    }
 
+    //======================
+    // Audio policy callbacks from AudioSystem for recording configuration updates
+    //======================
+    private final RecordingActivityMonitor mRecordMonitor = new RecordingActivityMonitor();
+
+    public void registerRecordingCallback(IRecordingConfigDispatcher rcdb) {
+        mRecordMonitor.registerRecordingCallback(rcdb);
+    }
+
+    public void unregisterRecordingCallback(IRecordingConfigDispatcher rcdb) {
+        mRecordMonitor.unregisterRecordingCallback(rcdb);
+    }
+
+    public AudioRecordConfiguration[] getActiveRecordConfigurations() {
+        return mRecordMonitor.getActiveRecordConfigurations();
     }
 
     //======================

services/core/java/com/android/server/audio/RecordingActivityMonitor.java

@@ -0,0 +1,169 @@
+/*
+ * Copyright (C) 2016 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.server.audio;
+
+import android.media.AudioManager;
+import android.media.AudioRecordConfiguration;
+import android.media.AudioSystem;
+import android.media.IRecordingConfigDispatcher;
+import android.os.IBinder;
+import android.os.RemoteException;
+import android.util.Log;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.Iterator;
+
+/**
+ * Class to receive and dispatch updates from AudioSystem about recording configurations.
+ */
+public final class RecordingActivityMonitor implements AudioSystem.AudioRecordingCallback {
+
+    public final static String TAG = "AudioService.RecordingActivityMonitor";
+
+    private ArrayList<RecMonitorClient> mClients = new ArrayList<RecMonitorClient>();
+
+    private HashMap<Integer, AudioRecordConfiguration> mRecordConfigs =
+            new HashMap<Integer, AudioRecordConfiguration>();
+
+    RecordingActivityMonitor() {
+        RecMonitorClient.sMonitor = this;
+    }
+
+    /**
+     * Implementation of android.media.AudioSystem.AudioRecordingCallback
+     */
+    public void onRecordingConfigurationChanged(int event, int session, int source) {
+        if (updateSnapshot(event, session, source)) {
+            final Iterator<RecMonitorClient> clientIterator = mClients.iterator();
+            synchronized(mClients) {
+                while (clientIterator.hasNext()) {
+                    try {
+                        clientIterator.next().mDispatcherCb.dispatchRecordingConfigChange();
+                    } catch (RemoteException e) {
+                        Log.w(TAG, "Could not call dispatchRecordingConfigChange() on client", e);
+                    }
+                }
+            }
+        }
+    }
+
+    void initMonitor() {
+        AudioSystem.setRecordingCallback(this);
+    }
+
+    void registerRecordingCallback(IRecordingConfigDispatcher rcdb) {
+        if (rcdb == null) {
+            return;
+        }
+        synchronized(mClients) {
+            final RecMonitorClient rmc = new RecMonitorClient(rcdb);
+            if (rmc.init()) {
+                mClients.add(rmc);
+            }
+        }
+    }
+
+    void unregisterRecordingCallback(IRecordingConfigDispatcher rcdb) {
+        if (rcdb == null) {
+            return;
+        }
+        synchronized(mClients) {
+            final Iterator<RecMonitorClient> clientIterator = mClients.iterator();
+            while (clientIterator.hasNext()) {
+                RecMonitorClient rmc = clientIterator.next();
+                if (rcdb.equals(rmc.mDispatcherCb)) {
+                    rmc.release();
+                    clientIterator.remove();
+                    break;
+                }
+            }
+        }
+    }
+
+    AudioRecordConfiguration[] getActiveRecordConfigurations() {
+        synchronized(mRecordConfigs) {
+            return mRecordConfigs.values().toArray(new AudioRecordConfiguration[0]);
+        }
+    }
+
+    /**
+     * Update the internal "view" of the active recording sessions
+     * @param event
+     * @param session
+     * @param source
+     * @return true if the list of active recording sessions has been modified, false otherwise.
+     */
+    private boolean updateSnapshot(int event, int session, int source) {
+        synchronized(mRecordConfigs) {
+            switch (event) {
+            case AudioManager.RECORD_CONFIG_EVENT_STOP:
+                // return failure if an unknown recording session stopped
+                return (mRecordConfigs.remove(new Integer(session)) != null);
+            case AudioManager.RECORD_CONFIG_EVENT_START:
+                if (mRecordConfigs.containsKey(new Integer(session))) {
+                    // start of session that's already tracked, not worth an update
+                    // TO DO in the future when tracking record format: there might be a record
+                    //       format change during a recording that requires reporting
+                    return false;
+                } else {
+                    mRecordConfigs.put(new Integer(session),
+                            new AudioRecordConfiguration(session, source));
+                    return true;
+                }
+            default:
+                Log.e(TAG, String.format("Unknown event %d for session %d, source %d",
+                        event, session, source));
+                return false;
+            }
+        }
+    }
+
+    /**
+     * Inner class to track clients that want to be notified of recording updates
+     */
+    private final static class RecMonitorClient implements IBinder.DeathRecipient {
+
+        // can afford to be static because only one RecordingActivityMonitor ever instantiated
+        static RecordingActivityMonitor sMonitor;
+
+        final IRecordingConfigDispatcher mDispatcherCb;
+
+        RecMonitorClient(IRecordingConfigDispatcher rcdb) {
+            mDispatcherCb = rcdb;
+        }
+
+        public void binderDied() {
+            Log.w(TAG, "client died");
+            sMonitor.unregisterRecordingCallback(mDispatcherCb);
+        }
+
+        boolean init() {
+            try {
+                mDispatcherCb.asBinder().linkToDeath(this, 0);
+                return true;
+            } catch (RemoteException e) {
+                Log.w(TAG, "Could not link to client death", e);
+                return false;
+            }
+        }
+
+        void release() {
+            mDispatcherCb.asBinder().unlinkToDeath(this, 0);
+        }
+    }
+}