android - What's the reason of activity self duplication after some action in handler? -

i'm developing game uses bluetooth connect other devices. it's standard client-server scheme application.

when player, whom device acting server, starts game, activity started. in oncreate method set , show progressdialog. dialog appearing until other player connects. in activity there handler receives messages background thread manages bluetooth connections. when first player joining game, handler receive proper message, , started initializing game, means dismiss progressdialog , load layout in setcontendview(view v). , there problem because works fine, new instance of activity created (while original 1 still exists) , takes on game management. , wouldn't major problem, when game ends, , new activity finished, original 1 gets on top , when try close it, app crashing.

i thought it's because of usage of setcontendview(view v) in wrong place, happened if player joining during game's term (then setcontendview(view v) not called again) in extreme case 3 instances of activity existing @ same time, causing mess , crash of players.

code sample:

public void oncreate(bundle savedinstancestate) {     pdialog = new progressdialog(this);      pdialog.setmessage("...");     pdialog.setcancelable(false);     pdialog.setcanceledontouchoutside(false);     pdialog.setbutton(dialoginterface.button_negative,          context.getstring(r.string.alert_button_cancel),          new dialoginterface.onclicklistener() {             @override             public void onclick(dialoginterface dialog, int which) {                 pdialog.dismiss();                 finishactivity();             }         });     pdialog.show(); } 

and handler:

public void handlemessage(message message) {     if(message.what == constants.message_new_player) {         newplayermessage m = (newplayermessage) message.obj;         if(pdialog != null && pdialog.isshowing()) {             pdialog.dismiss();         }         bluetoothservice.broadcast(new playersupdatemessage(game.getallplayersfields()));         bluetoothservice.broadcast(game.addmessage(new chatmessage(m.getaddress(), chatmessage.player_join)));         if (game.getdrawingplayeraddr().equals("")){             game.setdrawingplayeraddr(bluetoothservice.my_address);             //...             setgamemode(true);             setgametimer(game.getroundtime());         }     }     //... } 

where setgamemode() contains mentioned setcontendview(view v) , initialization of buttons , other layout elements.

i have no idea what's reason of strange behaviour. seems it's totally random. noticed it's more probable on less powerfull devices.

to evade creating duplicate activities should set property launchmode

   android:launchmode="singletask" 

here example:

  <activity             android:name=".mainactivity"             android:label="@string/app_name"             android:launchmode="singletask" /> 

this create activity @ root of new task , route intent it. however, if instance of activity exists, system routes intent existing instance through call onnewintent() method, rather creating new one

detailed information each of available options in:

http://developer.android.com/guide/topics/manifest/activity-element.html