How to create transparent activity in android

Sometimes, we need an activity to some processing but we need not necessarily have a UI.
For this, you can do just not do a setContentView() call. This will show up an activity with the default theme applied.
 
** UPDATE: Android Complete tutorial now available here.
 
But, what if you need a transparent activity? It should be showing what is behind it and still do some stuff. Just follow this post to get it done.

Add the following style In your res/values/styles.xml file (if you don’t have one, create it.) Here’€™s a complete file:

(the value @color/transparent is the color value #00000000 which I put in res/values/color.xml file. You can also use @android:color/transparent in later Android versions)

Then apply the style to your activity, for example:

...

Sometimes, you may need to display a dialog on this activity. Say, you are processing something and want to show the user the progress. For this, you can create a ProgressDialog and show it. But, in case of a transparent activity, it displays an outer box for the dialog.

To get rid of this, in your onCreateDialog or the place where you are initializing the dialog do the following:

@Override
protected Dialog onCreateDialog(int id) {

switch(id) {
case 100:
dialog = new ProgressDialog(this, ProgressDialog.STYLE_SPINNER);
dialog.requestWindowFeature(Window.FEATURE_NO_TITLE);
dialog.setMessage("Message");
//The below line does the trick!!
dialog.getWindow().setBackgroundDrawableResource(android.R.color.transparent);
return dialog;
}
return super.onCreateDialog(id);
}