为什么dialog.show（）导致我的应用崩溃？

Question

I've been trying to get a dialog message to work on android and the app always crash when it reaches "dialog.show();"

public class Logic extends BroadcastReceiver {
@Override
public void onReceive(Context context, Intent intent) {

    AlertDialog.Builder dialogBuilder = new AlertDialog.Builder(context);
    dialogBuilder.setTitle("Alarm");
    dialogBuilder.setMessage(messageActivity.getMes());
    dialogBuilder.setPositiveButton("OK",
            new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    dialog.dismiss();
                }
            });
    AlertDialog dialog = dialogBuilder.create();
    dialog.show();
}
}

Here is my logcat:

FATAL EXCEPTION: main

Process: it226.myapplicationit226androidapp, PID: 19598
java.lang.RuntimeException: Unable to start receiver it226.myapplicationit226androidapp.Logic: android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application
    at android.app.ActivityThread.handleReceiver(ActivityThread.java:2732)
    at android.app.ActivityThread.-wrap14(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1421)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5417)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
 Caused by: android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application
    at android.view.ViewRootImpl.setView(ViewRootImpl.java:571)
    at android.view.WindowManagerGlobal.addView(WindowManagerGlobal.java:310)
    at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:85)
    at android.app.Dialog.show(Dialog.java:319)
    at it226.myapplicationit226androidapp.Logic.onReceive(Logic.java:65)
    at android.app.ActivityThread.handleReceiver(ActivityThread.java:2725)
    at android.app.ActivityThread.-wrap14(ActivityThread.java) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1421) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:148) 
    at android.app.ActivityThread.main(ActivityThread.java:5417) 
    at java.lang.reflect.Method.invoke(Native Method) 

Answer 1

You can't create dialog from receiver. Creating dialogs is allowed only from UI components (which have looper).

You could start transparent activity with dialog, which would be the same for user.

Answer 2

You can't create a dialog using the context of a BroadcastReceiver , you has two options to solve this problem:

first one is to create a regular Activity which has only the size of the dialog as a shown part and the full remaining is transparent:

Intent newIntent = new Intent(context, Displayer.class);
newIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
context.startActivity(newIntent);

second one is to hold your Activity context and then use it in creating the dialog, but at this method you have to make sure what is the activity that is currently open:

// in your activity onCreate
ctx = YourActivity.this; // let's suppose ctx is static and general var
////////////////////////////////////////////////
// in the BroadcastReceiver
AlertDialog.Builder dialogBuilder = new AlertDialog.Builder(YourActivity.ctx);