findViewById() returns null for custom component in layout XML, not for other components

Question

I have a res/layout/main.xml including these elements and others:

Answer 1

I ran into the same issue a while back when I added a custom View via the layout XML and then tried to attached a callback elsewhere in the application ...

I created a custom view and added it to my "layout_main.xml"

public class MUIComponent extends SurfaceView implements SurfaceHolder.Callback {
    public MUIComponent (Context context, AttributeSet attrs ) {
        super ( context, attrs );
    }
    // ..
}

And in the main Activity I wanted to attach some callbacks and get references to the UI elements from the XML.

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // ...

        MUIInitializer muiInit = new MUIInitializer();
        muiInit.setupCallbacks(this);
        muiInit.intializeFields(this);
    }       
}

The initilizer wasn't doing anything fancy but any changes it tried to make to the custom view (MUIComponent) or other non-custom UI elements simply were not appearing in the application.

public class MUIInitializer {

    // ...

    public void setupCallbacks ( Activity mainAct ) {


        // This does NOT work properly
        // - The object instance returned is technically an instance of my "MUICompnent" view
        //   but it is a *different* instance than the instance created and shown in the UI screen
        // - Callbacks never get triggered, changes don't appear on UI, etc.
        MUIComponent badInst = (MUIComponent) mainAct.findViewById(R.id.MUIComponent_TESTSURF);


        // ...
        // This works properly

        LayoutInflater inflater = (LayoutInflater) mainAct.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View inflatedLayout = inflater.inflate ( R.layout.activity_main, null );

        MUIComponent goodInst = (MUIComponent) inflatedLayout.findViewById(R.id.MUIComponent_TESTSURF);


        // Add callbacks
        // ...
    }

}

The difference between the "badInst" and "goodInst" is:

• badInst uses the Activity's findViewByID
• goodInst inflates the layout and uses the inflated layout to do the lookup