java lang RuntimeException Can t create handler inside thread that has not called Looper prepare

Question

I have an Android app running a thread  I want a Toast message to show with a message   When I do this  I get the below exception   Logcat trace   FATAL EXCEPTION  Timer-0   java lang RuntimeException  Can t create handler inside thread that has not      called Looper prepare     at android os Handler  lt init gt  Handler java 121   at android widget Toast TN  lt init gt  Toast java 322   at android widget Toast  lt init gt  Toast java 91   at android widget Toast makeText Toast java 238     Is there a work around for pushing Toast messages from threads to the User Interface

User · Answer

You can simply use BeginInvokeOnMainThread(). It invokes an Action on the device main (UI) thread.

Device.BeginInvokeOnMainThread(() => { displayToast("text to display"); });

It is simple and works perfectly for me!

EDIT : Works if you're using C# Xamarin

User · Answer

runOnUiThread new Runnable       public void run          Toast makeText getApplicationContext     Status       message getBody     Toast LENGTH LONG  show                  this works for me

User · Answer

Android basically works on two thread types namely UI thread and background thread  According to android documentation -  Do not access the Android UI toolkit from outside the UI thread to fix this problem  Android offers several ways to access the UI thread from other threads  Here is a list of methods that can help   Activity runOnUiThread Runnable    View post Runnable    View postDelayed Runnable  long    Now there are various methods to solve this problem  I will explain it by code sample  runOnUiThread  new Thread         public void run                 myactivity this runOnUiThread new runnable                         public void run                                   Do your UI operations like dialog opening or Toast here                                   start      LOOPER  Class used to run a message loop for a thread  Threads by default do not have a message loop associated with them  to create one  call prepare   in the thread that is to run the loop  and then loop   to have it process messages until the loop is stopped   class LooperThread extends Thread       public Handler mHandler       public void run             Looper prepare             mHandler   new Handler                 public void handleMessage Message msg                       process incoming messages here                                   Looper loop            AsyncTask  AsyncTask allows you to perform asynchronous work on your user interface  It performs the blocking operations in a worker thread and then publishes the results on the UI thread  without requiring you to handle threads and or handlers yourself   public void onClick View v        new CustomTask   execute  Void   null       private class CustomTask extends AsyncTask lt Void  Void  Void gt         protected Void doInBackground Void    param              Do some work         return null             protected void onPostExecute Void param              Print Toast or open dialog           Handler  A Handler allows you to send and process Message and Runnable objects associated with a thread s MessageQueue   Message msg   new Message          new Thread                 public void run                         msg arg1 1              handler sendMessage msg                   start           Handler handler   new Handler new Handler Callback               Override         public boolean handleMessage Message msg                if msg arg1  1                                  Print Toast or open dialog                                   return false

User · Answer

Here s what I ve been doing     public void displayError final String errorText        Runnable doDisplayError   new Runnable             public void run                 Toast makeText getApplicationContext    errorText  Toast LENGTH LONG  show                         messageHandler post doDisplayError       That should allow the method to be called from either thread   Where messageHandler is declared in the activity as     Handler messageHandler   new Handler

User · Answer

I got this error in a JobService from the following code       BluetoothLeScanner bluetoothLeScanner   getBluetoothLeScanner        if  BluetoothAdapter STATE ON    getBluetoothAdapter   getState    amp  amp  null    bluetoothLeScanner                         else           Logger debug TAG   BluetoothAdapter isn t on so will attempting to turn on and will retry starting scanning in a few seconds            getBluetoothAdapter   enable             new Handler    postDelayed new Runnable                  Override             public void run                     startScanningBluetooth                             5000           The service crashed   2019-11-21 11 49 45 550 729-763   D BluetoothManagerService  MESSAGE ENABLE 0   mBluetooth   null      --------- beginning of crash 2019-11-21 11 49 45 556 8629-8856 com locuslabs android sdk E AndroidRuntime  FATAL EXCEPTION  Timer-1     Process  com locuslabs android sdk  PID  8629     java lang RuntimeException  Can t create handler inside thread that has not called Looper prepare           at android os Handler  lt init gt  Handler java 203          at android os Handler  lt init gt  Handler java 117          at com locuslabs sdk ibeacon BeaconScannerJobService startScanningBluetoothAndBroadcastAnyBeaconsFoundAndUpdatePersistentNotification BeaconScannerJobService java 120          at com locuslabs sdk ibeacon BeaconScannerJobService access 500 BeaconScannerJobService java 36          at com locuslabs sdk ibeacon BeaconScannerJobService 2 1 run BeaconScannerJobService java 96          at java util TimerThread mainLoop Timer java 555          at java util TimerThread run Timer java 505    So I changed from Handler to Timer as follows       new Timer    schedule new TimerTask                      Override                 public void run                         startScanningBluetooth                                     5000     Now the code doesn t throw the RuntimeException anymore

User · Answer

From http   developer android com guide components processes-and-threads html       Additionally  the Android UI toolkit is not thread-safe  So  you   must not manipulate your UI from a worker thread   you must do all   manipulation to your user interface from the UI thread  Thus  there   are simply two rules to Android s single thread model          Do not block the UI thread   Do not access the Android UI toolkit from outside the UI thread      You have to detect idleness in a worker thread and show a toast in the main thread   Please post some code  if you want a more detailed answer    After code publication     In strings xml   lt string name  idleness toast  gt  You are getting late do it fast  lt  string gt    In YourWorkerThread java  Toast makeText getApplicationContext    getString R string idleness toast        Toast LENGTH LONG  show      Don t use AlertDialog  make a choice  AlertDialog and Toast are two different things

User · Answer

I got this exception because I was trying to make a Toast popup from a background thread  Toast needs an Activity to push to the user interface and threads don t have that  So one workaround is to give the thread a link to the parent Activity and Toast to that   Put this code in the thread where you want to send a Toast message   parent runOnUiThread new Runnable         public void run             Toast makeText parent getBaseContext     Hello   Toast LENGTH LONG  show                Keep a link to the parent Activity in the background thread that created this thread  Use parent variable in your thread class   private static YourActivity parent    When you create the thread  pass the parent Activity as a parameter through the constructor like this   public YourBackgroundThread YourActivity parent        this parent   parent      Now the background thread can push Toast messages to the screen

[android] java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare();

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