android work thread and ui thread

onCreate(Bundle savedInstanceState){
    // show dialog A if something is not correct
    new Thread(){
         public void run(){
               if(something is wrong) {
                   runOnUIThread(new Runnable(){
                       public void run(){
                           showDialog(A);
                       }
                   });
               }
         }
    }.start();
  开发者_开发百科  // show dialog B
    showDialog(B);
}

I want to know

1. which dialog will be shown first, and is the order indeterminate? why?
2. if the order is indeterminate, how can i reproduce the case that A is shown before B?

Thanks!

1. Which dialog will be shown first is not defined and you should not rely on one occurring before the other as above. The thread scheduler is not identically deterministic in all situations.
2. You need to lock on a mutex (or any other locking device) to make sure one is shown before the other.

Your question about which dialog will show first is indeterminate. There are cases where the order will flip flop. But generally B would be shown first since 9/10 it will get to place it's event on the UI thread before your thread could detect there was a problem.

I'd suggest using AsyncTask to perform whatever mechanisms are needed to startup, then in the onPostExecute() allow your program to resume starting up so it can showDialog(B) for whatever it needs. That way if dialog A is showing you can stop the startup process there and not show b.

public class MyAsyncStartup extends AsyncTask<Integer,Integer,MyResult> {
     MyActivity activity;

     public MyResult handleBackground() {
         if( somethingWentWrong ) return null;
     }

     public onPostExecute( MyResult result ) {
          if( result == null ) {
             showDialog(B);
          } else {
             activity.resumeStartupAndShowA();
          }
     }
}

1. I don't think it is possible that A is shown before B... this is because runOnUIThread adds the event TO THE END of the event queue. The code in that event (showing dialog A) is not going to get executed until after the onCreate() finishes (which means that dialog B gets shown first).

What cannot be guaranteed is the order between showing dialog B and calling runOnUIThread, but that doesn't matter. Here is a fragment from the official docs:

[runOnUIThread] Runs the specified action on the UI thread. If the current thread is the UI thread, then the action is executed immediately. If the current thread is not the UI thread, the action is posted to the event queue of the UI thread.

1. N/A

You can't show B until you know whether or not A will be shown. So you have to wait for the worker thread no matter what. Would it be possible to put showDialog(B) in your other thread like this?

onCreate(Bundle savedInstanceState){
    // show dialog A if something is not correct
    new Thread(){
         public void run(){
               runOnUiThread(new Runnable(){
                   public void run(){
                       if(something is wrong) {
                           showDialog(A);
                       }
                       showDialog(B);
                   }
               });
               }
         }
    }.start();
}