Running code in main thread from another thread

Question

In an android service I have created thread s  for doing some background task  I have a situation where a thread needs to post certain task on main thread s message queue  for example a Runnable  Is there a way to get Handler of the main thread and post Message Runnable to it from my other thread

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I know this is an old question  but I came across a main thread one-liner that I use in both Kotlin and Java  This may not be the best solution for a service  but for calling something that will change the UI inside of a fragment this is extremely simple and obvious   Java  8     getActivity   runOnUiThread   - gt           your main thread code        Kotlin   this runOnUiThread          your main thread code

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As a commenter below pointed correctly  this is not a general solution for services  only for threads launched from your activity  a service can be such a thread  but not all of those are   On the complicated topic of service-activity communication please read the whole Services section of the official doc - it is complex  so it would pay to understand the basics  http   developer android com guide components services html Notifications The method below may work in the simplest cases  If I understand you correctly you need some code to be executed in the GUI thread of the application  cannot think about anything else called  quot main quot  thread   For this there is a method on Activity  someActivity runOnUiThread new Runnable              Override         public void run                  Your code to run in GUI thread here            public void run          Doc  http   developer android com reference android app Activity html runOnUiThread 28java lang Runnable 29 Hope this is what you are looking for

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for Kotlin  you can use Anko corountines   update  doAsync              deprecated   async UI           Code run on UI thread        Use ref   instead of this MyActivity

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Follow this method  Using this way you can simply update the UI from a background thread  runOnUiThread work on the main UI  thread   I think this code snippet is less complex and easy  especially for beginners   AsyncTask execute new Runnable                  Override             public void run                    code you want to run on the background             someCode                  the code you want to run on main thread  MainActivity this runOnUiThread new Runnable                          public void run        the code you want to run after the background operation otherwise they will executed earlier and give you an error                           executeAfterOperation                                                                          in the case of a service  create a handler in the oncreate   handler   new Handler      then use it like this   private void runOnUiThread Runnable runnable            handler post runnable

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More precise Kotlin code using handler    Handler Looper getMainLooper    post         your codes here run on main Thread

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One method I can think of is this   1  Let the UI bind to the service  2  Expose a method like the one below by the Binder that registers your Handler   public void registerHandler Handler handler        mHandler   handler      3  In the UI thread  call the above method after binding to the service   mBinder registerHandler new Handler       4  Use the handler in the Service s thread to post your task   mHandler post runnable

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The simplest way especially if you don t have a context  if you re using RxAndroid you can do   AndroidSchedulers mainThread   scheduleDirect       runCodeHere

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If you run code in a thread  e g  do delaying some action  then you need to invoke runOnUiThread from the context  For example  if your code is inside MainActivity class then use this   MainActivity this runOnUiThread new Runnable          Override     public void run             myAction                If your method can be invoked either from main  UI thread  or from other threads you need a check like   public void myMethod        if  Looper myLooper      Looper getMainLooper              myAction            else

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Kotlin versions When you are on an activity  then use runOnUiThread         code that runs in main    When you have activity context  mContext then use mContext runOnUiThread         code that runs in main    When you are in somewhere where no context available  then use Handler Looper getMainLooper    post           code that runs in main

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There is another simple way  if you don t have an access to the Context   1   Create a handler from the main looper   Handler uiHandler   new Handler Looper getMainLooper       2   Implement a Runnable interface   Runnable runnable   new Runnable        your code here     3   Post your Runnable to the uiHandler   uiHandler post runnable     That s all  -  Have fun with threads  but don t forget to synchronize them

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With Kotlin  it is just like this inside any function   runOnUiThread         Do work

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A condensed code block is as follows      new Handler Looper getMainLooper    post new Runnable             Override        public void run                   things to do on the main thread                   This does not involve passing down the Activity reference or the Application reference   Kotlin Equivalent       Handler Looper getMainLooper    post Runnable              things to do on the main thread

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HandlerThread is better option to normal java Threads in Android     Create a HandlerThread and start it Create a Handler with Looper from HandlerThread  requestHandler post a Runnable task on requestHandler   Communication with UI Thread from HandlerThread   Create a Handler with Looper for main thread     responseHandler and override handleMessage method Inside Runnable task of other Thread   HandlerThread in this case   call sendMessage on responseHandler This sendMessage result invocation of handleMessage in responseHandler  Get attributes from the Message and process it  update UI   Example  Update TextView with data received from a web service  Since web service should be invoked on non-UI thread  created HandlerThread for Network Operation  Once you get the content from the web service  send message to your main thread  UI Thread  handler and that Handler will handle the message and update UI   Sample code   HandlerThread handlerThread   new HandlerThread  NetworkOperation    handlerThread start    Handler requestHandler   new Handler handlerThread getLooper      final Handler responseHandler   new Handler Looper getMainLooper           Override     public void handleMessage Message msg            txtView setText  String  msg obj             Runnable myRunnable   new Runnable          Override     public void run             try               Log d  Runnable    Before IO call                URL page   new URL  http   www your web site com fetchData jsp                StringBuffer text   new StringBuffer                HttpURLConnection conn    HttpURLConnection  page openConnection                conn connect                InputStreamReader in   new InputStreamReader  InputStream  conn getContent                 BufferedReader buff   new BufferedReader in               String line              while   line   buff readLine       null                    text append line     n                              Log d  Runnable    After IO call    text toString                 Message msg   new Message                msg obj   text toString                responseHandler sendMessage msg               catch  Exception err                err printStackTrace                       requestHandler post myRunnable     Useful articles   handlerthreads-and-why-you-should-be-using-them-in-your-android-apps  android-looper-handler-handlerthread-i

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So most handy is to do sort of   import android os AsyncTask import android os Handler import android os Looper  object Dispatch       fun asyncOnBackground call    - gt Unit            AsyncTask execute               call                        fun asyncOnMain call    - gt Unit            Handler Looper getMainLooper    post               call                       And after   Dispatch asyncOnBackground       val value         super processing     Dispatch asyncOnMain   completion value

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public void mainWork         new Handler Looper getMainLooper    post new Runnable              Override         public void run                   Add Your Code Here                       This can also work in a service class with no issue

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NOTE  This answer has gotten so much attention  that I need to update it  Since the original answer was posted  the comment from  dzeikei has gotten almost as much attention as the original answer  So here are 2 possible solutions   1  If your background thread has a reference to a Context object   Make sure that your background worker threads have access to a Context object  can be the Application context or the Service context   Then just do this in the background worker thread      Get a handler that can be used to post to the main thread Handler mainHandler   new Handler context getMainLooper      Runnable myRunnable   new Runnable          Override      public void run             This is your code    mainHandler post myRunnable     2  If your background thread does not have  or need  a Context object   suggested by  dzeikei       Get a handler that can be used to post to the main thread Handler mainHandler   new Handler Looper getMainLooper      Runnable myRunnable   new Runnable          Override      public void run             This is your code    mainHandler post myRunnable

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