Don't repeat checks on event methods (Bukkit/Spigot)

Question

I was reviewing my codebase for a major project of mine and i've noticed that there are over 150 event handlers that all run essentially the same check:

        if (!(event.getDamager() instanceof Player))
            return;
        if (!(event.getEntity() instanceof Player))
            return;

        Player player = (Player) event.getDamager();

        Player victim = (Player) event.getEntity();

        if(!AntigriefManager.canInjure(player, victim)) return;

        if (!EnchantChecks.mainhand(player, this)) return;

My question is, can would there be a good way of abstracting this away?

Is it possible to pass those player and victim values to multiple EntityDamageByEntityEvent handlers instead of writing them above each one?

Thanks!

Answer 1

Make use of EventHandler , specifically priority and ignoreCancelled .

Right now you have this:

@EventHandler
public void onMyEvent(SomeEvent event) {
    //a) A bunch of precheck code
    //b) The actual code you want to run, in several different code bodies
}

What you will end up doing is splitting these tasks into two different listeners; one will handle the checks, the other will run whatever it is responsible for. For this, our checks will run on EventPriority.LOWEST , because those are run before any other priority. On top of that, if the event is cancelled, we will use EventHandler#ignoreCancelled to indicate our code should not be run:

@EventHandler(priority = EventPriority.LOWEST)
public void onMyPrechecks(SomeEvent event) {
    //a) A bunch of precheck code, for example:
    if (!AntigriefManager.canInjure(player, victim)) {
        //notice we cancel, instead of returning from a method
        event.setCancelled(true);
    }
}

//Elsewheres, in another class probably:
//priority should never go lower than LOW
@EventHandler(priority = EventPriority.LOW, ignoreCancelled = true)
public void onMyEvent(SomeEvent event) {
    //b) The actual code you want to run, e.g.:
    event.getPlayer().sendMessage("running event!");
}