@Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if ((keyCode == KeyEvent.KEYCODE_BACK)) {
            if (this.isDirty()){

                AlertDialog.Builder builder = new AlertDialog.Builder(this);
                builder.setMessage(this.getString(R.string.desea_descartar_cambios))
                       .setCancelable(false)
                       .setPositiveButton(this.getResources().getString(R.string.si), new DialogInterface.OnClickListener() {
                           public void onClick(DialogInterface dialog, int id) {
                               AbstractDirtyActivity.this.setDirty(Boolean.FALSE);
                               AbstractDirtyActivity.this.finish();
                           }
                       })
                       .setNegativeButton(this.getResources().getString(R.string.no), new DialogInterface.OnClickListener() {
                           public void onClick(DialogInterface dialog, int id) {
                                dialog.cancel();
                           }
                       });
                AlertDialog alert = builder.create();

                alert.show();
            }