Develop Reference

Can I write a system property listener in android?

Having set a system property in android using the setprop command (through adb) is there a way to listen to this change in my own service?
I tried with SystemProperties.addChangeCallback and was not notified. Was there something that I missed?

You can create a method in your service which should fetch any Systemproperty and that method should call Looper.loop(); so that that loop will poll for SystemProperty time to time
This implementation may not be optimized way of doing this but it is used in Android 4.4.2, you can see here http://androidxref.com/4.4.2_r2/xref/frameworks/base/services/java/com/android/server/SystemServer.java
you can see at above link:
boolean disableStorage = SystemProperties.getBoolean("config.disable_storage", false);
boolean disableMedia = SystemProperties.getBoolean("config.disable_media", false);
boolean disableBluetooth = SystemProperties.getBoolean("config.disable_bluetooth", false);
boolean disableTelephony = SystemProperties.getBoolean("config.disable_telephony", false);
boolean disableLocation = SystemProperties.getBoolean("config.disable_location", false);
boolean disableSystemUI = SystemProperties.getBoolean("config.disable_systemui", false);
boolean disableNonCoreServices = SystemProperties.getBoolean("config.disable_noncore", false);
boolean disableNetwork = SystemProperties.getBoolean("config.disable_network", false);
These boolean variables are being checked in initAndLoop() method with the help of Looper.loop(); here you can notify your other components on any change in even a single SystemProperty.
Another way is to create static callback and get call for any change in any of SystemProperty, see the master branch's code for SystemService here: https://android.googlesource.com/platform/frameworks/base/+/refs/heads/master/core/java/android/os/SystemService.java
you can see in above link what following code is doing:
private static Object sPropertyLock = new Object();
static {
SystemProperties.addChangeCallback(new Runnable() {
#Override
public void run() {
synchronized (sPropertyLock) {
sPropertyLock.notifyAll();
}
}
});
}
/**
* Wait until given service has entered specific state.
*/
public static void waitForState(String service, State state, long timeoutMillis)
throws TimeoutException {
final long endMillis = SystemClock.elapsedRealtime() + timeoutMillis;
while (true) {
synchronized (sPropertyLock) {
final State currentState = getState(service);
if (state.equals(currentState)) {
return;
}
if (SystemClock.elapsedRealtime() >= endMillis) {
throw new TimeoutException("Service " + service + " currently " + currentState
+ "; waited " + timeoutMillis + "ms for " + state);
}
try {
sPropertyLock.wait(timeoutMillis);
} catch (InterruptedException e) {
}
}
}
}
/**
* Wait until any of given services enters {#link State#STOPPED}.
*/
public static void waitForAnyStopped(String... services) {
while (true) {
synchronized (sPropertyLock) {
for (String service : services) {
if (State.STOPPED.equals(getState(service))) {
return;
}
}
try {
sPropertyLock.wait();
} catch (InterruptedException e) {
}
}
}
}
This information originates from Shridutt Kothari. Check this google post about listening to single SystemProperty changes

Too long for comments, so adding as an answer:
The setprop tool does not appear to fire change callbacks. In my read of the OS source, it simply sets a property value in a hashmap (see: https://cs.android.com/android/platform/superproject/+/master:system/libbase/properties.cpp;bpv=1;bpt=1;l=133?q=setprop).
For the callback to happen, someone needs to call do_report_sysprop_change (https://cs.android.com/android/platform/superproject/+/master:system/core/libutils/misc.cpp;bpv=1;bpt=1;l=102?q=syspropchange&ss=android%2Fplatform%2Fsuperproject&gsn=do_report_sysprop_change&gs=kythe%3A%2F%2Fandroid.googlesource.com%2Fplatform%2Fsuperproject%3Flang%3Dc%252B%252B%3Fpath%3Dsystem%2Fcore%2Flibutils%2Fmisc.cpp%23tC5_BHx-Z-jKUw_rRXWlL0wtVyVh15Oh60E0YnrdfSg&gs=kythe%3A%2F%2Fandroid.googlesource.com%2Fplatform%2Fsuperproject%3Flang%3Dc%252B%252B%3Fpath%3Dsystem%2Fcore%2Flibutils%2Fmisc.cpp%234FfZ9jgPIUEg7IKCGYGLCaQ4-cj6enFs8AeI7SIRLBs) and this is not done via setprop. I do see it invoked in a variety of places in the OS since invocation of the set prop callback is a method in the IBase interface implemented by a variety of services in Android.

Codenameone when I read the qrCode uzing Zxing on Android the application goes back to 'init' and 'start'

I am using Codenameone and ZXing to read a QRCode. When I call the Scanner, my mobile opens the QRCode reader application and I get to read the QRCode except that when android takes me back to my app it goes through init then start statuses. Which moves me back to the login form of my application instead of continuing filling the form that I was in.
Any help on what to do to stay in the same form? Is there something I'm doing wrong? Thanks in advance.
EverproX.addMessage("Before Scan\n");
CodeScanner.getInstance().scanQRCode(new ScanResult() {
public void scanCompleted(String contents, String formatName, byte[] rawBytes) {
EverproX.addMessage("Scan Completed "+contents);
}
public void scanCanceled() {
EverproX.addMessage("Scan Cancelled");
}
public void scanError(int errorCode, String message) {
EverproX.addMessage("Scan Error "+errorCode+" "+message);
}
});
EverproX can be seen as a log class.
By analyzing our log we can say that as soon as we call the CodeScanner.getInstance().scanQRCode() the application is called for 'Destroy'. Then after the scanning is done it goes again through the init and start. It never goes into the scanComplete scanCanceled or scanError Callbacks.
Is it normal that the App is destroyed upon call of CodeScanner? Many thanks.

Inside your codenameone project, you should find a class named (for example MyApp.java) based on your app's name, modify the code to read something like similar to this:
public class MyApp {
private Form current;
public void init(Object context) {
// Pro users - uncomment this code to get crash reports sent to you automatically
Display.getInstance().addEdtErrorHandler(new ActionListener() {
public void actionPerformed(ActionEvent evt) {
evt.consume();
Log.p("Exception in AppName version " + Display.getInstance().getProperty("AppVersion", "Unknown"));
Log.p("OS " + Display.getInstance().getPlatformName());
Log.p("Error " + evt.getSource());
Log.p("Current Form " + Display.getInstance().getCurrent().getName());
Log.e((Throwable) evt.getSource());
Log.sendLog();
}
});
}
public void start() {
if (current != null) {
current.show();
return;
}
new StateMachine("/theme");
}
public void stop() {
current = Display.getInstance().getCurrent();
}
public void destroy() {
current = null;
}
}

Why does Android AppOps (privacy manager) send duplicate events when user change his/her privacy settings?

When a user changes his/her privacy settings through AppOps (e.g. denying an application access to phone contacts), AppOpsManager sends to anyone who listens what the users have changed (i.e. the package name and the operation (e.g. Read contacts)).
So I wrote a listener to do so. However, we the user make only one change, I receive too many duplicate events (e.g. 10 events that the user decided to deny Angry Bird access to his/her location) and then the app crashes.
Here is my code to register listners for each pair of package & operation:
public void startWatchingOperations(AppOpsManager appOps, List<AppOpsManager.PackageOps> opsforapps) {
SharedPreferences myAppListnerPreferences = getSharedPreferences(APP_OPS_PREFERENCES, Activity.MODE_PRIVATE);
for (AppOpsManager.PackageOps o:opsforapps) {
List<OpEntry> opEntry = o.getOps();
//if I already assigned a listener to this pari of package & operation, then skip
if (myAppListnerPreferences.getBoolean(o.getPackageName(), false)==false) {
for (OpEntry entry:opEntry) {
//for each pair of package & operation, assign a new listener
ChangePrivacySettingsListener opsListner = new ChangePrivacySettingsListener(getApplicationContext());
appOps.startWatchingMode(entry.getOp(),o.getPackageName(),opsListner);
}
myAppListnerPreferences.edit().putBoolean(o.getPackageName(), true).apply();
}
}
}
Here is a snippet of the listener
public class ChangePrivacySettingsListener implements AppOpsManager.Callback {
public void opChanged(int op, String packageName) {
AppOpsManager appOps= (AppOpsManager)context.getSystemService(Context.APP_OPS_SERVICE);
PackageManager pkg = context.getPackageManager();
try {
//this is an object to store the event: package name,
// the operation that has been changed, & time stamp
PrivacySetting privacySetting = new PrivacySetting();
privacySetting.setPackageName(packageName);
privacySetting.setOperation(OPERATIONS_STRINGS[op]);
privacySetting.setDecisionTime(Calendar.getInstance(TimeZone.getDefault()).getTimeInMillis());
privacySetting.setUserId(userId);
} catch (NameNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
Her is the part of AppOpsManager.java that allows me to listen to user's changes.
public class AppOpsManager {
final HashMap<Callback, IAppOpsCallback> mModeWatchers
= new HashMap<Callback, IAppOpsCallback>();
public void startWatchingMode(int op, String packageName, final Callback callback) {
synchronized (mModeWatchers) {
IAppOpsCallback cb = mModeWatchers.get(callback);
if (cb == null) {
cb = new IAppOpsCallback.Stub() {
public void opChanged(int op, String packageName) {
callback.opChanged(op, packageName);
}
};
mModeWatchers.put(callback, cb);
}
try {
mService.startWatchingMode(op, packageName, cb);
} catch (RemoteException e) {
}
}
}
I double checked to ensure that I've never assigned more than one listener to each pair of package & operation.
I would appreciate hints about potential causes.
Here is a link to AppOpsManager.java

Try moving the deceleration of ChangePrivacySettingsListener opsListner to be out side of the for block:
public void startWatchingOperations(AppOpsManager appOps, List<AppOpsManager.PackageOps> opsforapps) {
ChangePrivacySettingsListener opsListner;
SharedPreferences myAppListnerPreferences = getSharedPreferences(APP_OPS_PREFERENCES, Activity.MODE_PRIVATE);
for (AppOpsManager.PackageOps o:opsforapps) {
List<OpEntry> opEntry = o.getOps();
//if I already assigned a listener to this pari of package & operation, then skip
if (myAppListnerPreferences.getBoolean(o.getPackageName(), false)==false) {
for (OpEntry entry:opEntry) {
//for each pair of package & operation, assign a new listener
opsListner = new ChangePrivacySettingsListener(getApplicationContext());
appOps.startWatchingMode(entry.getOp(),o.getPackageName(),opsListner);
}
myAppListnerPreferences.edit().putBoolean(o.getPackageName(), true).apply();
}
}
}
And please let me know what happened?

Just in case this is helpful to someone, up to at least Android Oreo, calling AppOpsManager.startWatchingMode(op, packageName, callback) will cause callback to be invoked when the setting is changed (1) for the op with any package, AND (2) for any AppOps setting changes with packageName. This can be seen from the AppOpsService.java source, particularly AppOpsService.startWatchingMode() which registers the callback, AppOpsService.setMode() which calls the callback when the AppOps setting is changed.
For example, if you register a callback with startWatchingMode(appOps1, package1, callback) and startWatchingMode(appOps2, package1, callback),
when there is a change in the setting for appOps3 for package1, the callback will be called twice since you have registered for package1 two times. If there is a change in appOps1 for package1, the callback will be invoked 3 times, because you have registered once for appOps1, and twice for package1.
The solution is to register either for the set of AppOps you are interested in (without duplications), with the packageName parameter set to null, or register for the set of packages you are interested in, with op parameter set to AppOpsManager.OP_NONE.
Also you need to ensure that all listeners are unregistered (e.g. in onDestroy of your activity) using stopWatchingMode. Otherwise, the callback entries will accumulate across Activity lifecycles (until the app is terminated) and you will start getting duplicates. This also means that you should keep references to all the listeners created.

What's the difference between "getApplicationLabel" and "loadLabel"?

Background
I'm working on my app that is an alternative to the app manager (link here), and wish to optimize it a bit.
As it turns out, one of the slowest things on the app is its bootup, and the main reason for this is getting the app name . I intend on caching it, but I also wish to optimize the way it's being queried, if possible.
The problem
Android has two ways to get the app name: PackageManager.getApplicationLabel and ApplicationInfo.loadLabel .
both have about the same description, but I'm not sure which one should be used.
Not only that, but looking at the code of "ApplicationInfo.loadLabel" , it looks something like this:
public CharSequence loadLabel(PackageManager pm) {
if (nonLocalizedLabel != null) {
return nonLocalizedLabel;
}
if (labelRes != 0) {
CharSequence label = pm.getText(packageName, labelRes, getApplicationInfo());
if (label != null) {
return label.toString().trim();
}
}
if (name != null) {
return name;
}
return packageName;
}
I can't find the code of "PackageManager.getApplicationLabel", as it's abstract.
The question
Is there any difference between the two?
If there is no difference, why do we have 2 very similar methods to get the same app name? I mean, I can use either of them only if I have both applicationInfo object and the PackageManager object, but that's enough to use any of the methods...
If there is difference, which of them is better in terms of speed?

The source of 'PackageManager.getApplicationLabel' is available in 'ApplicationPackageManager.java'. It is as follows;
#Override
public CharSequence getApplicationLabel(ApplicationInfo info) {
return info.loadLabel(this);
}
ApplicationPackageManager.java
I see in AppUtils.java the same wrapping is done as follows;
/** Returns the label for a given package. */
public static CharSequence getApplicationLabel(
PackageManager packageManager, String packageName) {
try {
final ApplicationInfo appInfo =
packageManager.getApplicationInfo(
packageName,
PackageManager.MATCH_DISABLED_COMPONENTS
| PackageManager.MATCH_ANY_USER);
return appInfo.loadLabel(packageManager);
} catch (PackageManager.NameNotFoundException e) {
Log.w(TAG, "Unable to find info for package: " + packageName);
}
return null;
}

Google Game Services onPeersDisconnected/onP2PDisconnected not being called

As in the title I've been able to connect to Google Game Services, exchange data between two devices and everything is running fine, except one thing: disconnection callbacks.
I tried to intercept both onPeersDisconnected and onP2PDisconnected without any success. The onP2PDisconnected method is being called in the device that get disconnected from Internet but not into device that is still online (so there is no way to tell the player that the other one got disconnected).
After the match is started it seems that the second device is never notified of the accidental disconnection. If the user close the game properly the onPeersLeft method is being called thought.
Is a ping between the two devices really necessary to overcome this "bug"? Am I doing something wrong?
Here is the code I use:
void startQuickGame() {
// quick-start a game with 1 randomly selected opponent
final int MIN_OPPONENTS = 1, MAX_OPPONENTS = 1;
Bundle autoMatchCriteria = RoomConfig.createAutoMatchCriteria(MIN_OPPONENTS,
MAX_OPPONENTS, 0);
RoomConfig.Builder rtmConfigBuilder = RoomConfig.builder(this);
rtmConfigBuilder.setMessageReceivedListener(this);
rtmConfigBuilder.setRoomStatusUpdateListener(this);
rtmConfigBuilder.setAutoMatchCriteria(autoMatchCriteria);
mListener.switchToScreen(R.id.screen_wait);
keepScreenOn();
resetGameVars();
getGamesClient().createRoom(rtmConfigBuilder.build());
}
And here the simple listeners:
#Override
public void onPeersDisconnected(Room room, List<String> peers) {
Log.d(TAG, "onPeersDisconnected");
updateRoom(room);
}
void updateRoom(Room room) {
Log.d(TAG, "UpdateRoom: "+room.getParticipants().size());
mParticipants = room.getParticipants();
}
#Override
public void onP2PDisconnected(String participantId) {
Log.d(TAG, "onP2PDisconnected");
}
public int getPartecipantsInRooom(){
if(mRoom != null)
return mRoom.getParticipants().size();
else
return -123456;
}
Note that calling getPartecipantsInRooom() after one of the two devices disconnects always return 2, and updateRoom never get called.

Just to be sure this might not work for you, for my applications I use this to let me know when another Participant has left the Room, and it is called immediately :
#Override
public void onPeerLeft(Room room, final List<String> participantIds) {
this.mRoomCurrent = room;
this.mRoomId = this.mRoomCurrent.getRoomId();
this.mParticipants = this.mRoomCurrent.getParticipants();
int connected = 0;
for (Participant p : room.getParticipants()) {
if(p.getStatus() == Participant.STATUS_JOINED) {
connected += 1;
}
}
final int fconnected = connected;
for (String s : listIgnoreTheseIDs) {
//checkint to see if we care anymore about this ID.. if out of game already.. nope
if(s.equals(participantIds.get(0))){
return;
}
}
Gdx.app.postRunnable(new Runnable() {
#Override
public void run() {
mGHInterface.onPeerLeft(fconnected, participantIds.size());
}
});
}
No idea why there are two items, but like you, I realized the onPeersDisconnected() isn't that reliable, but onPeerLeft() normally gets back to the other devices in under 1 second.

onPeerDisconnected() handles disconnects. So if somebody is still in the application but the network connection is lost, this is called for him.
onPeerLeft() handles participants who leave a room. This is called when somebody explizit leaves the room in the application or the application is minimized, and the room is left on the androids onStop() or onDestroy() callback.

I'm making two player game. So I use this approach
#Override
public void onPeerLeft(Room room, List<String> peersWhoLeft) {
updateRoom(room);
Toast.makeText(MyLauncherActivity.this, "Other player left the game", Toast.LENGTH_LONG).show();
quitGame();
}