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

private void refreshContent(Spot atPosition) {
	if(atPosition == null & parentActivity.isLocationFixed())
		setActionButton(R.id.menu_plant, R.drawable.ic_menu_plant);
	else
		setActionButton(R.id.menu_plant, R.drawable.ic_menu_plant_disabled);
}

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