<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:id="@+id/menu_gather"
        android:title="@string/gather"
        android:icon="@drawable/ic_menu_drop_disabled"
        android:enabled="true"
        android:orderInCategory="0"
        android:showAsAction="always" />

private void refreshActionButtonsState() {
	if(isInRange() & atPosition.isHasWater())
		setActionButton(R.id.menu_gather, R.drawable.ic_menu_drop);
	else
		setActionButton(R.id.menu_gather, R.drawable.ic_menu_drop_disabled);
}

public void onTaskFinished(final Object result) {
	// type checks . . .
	parentActivity.runOnUiThread(new Runnable() {
		@Override
		public void run() {
			refreshActionButtonsState();
		}
	});
}