Building a Dialogue System in Unity - Triggers

First order of business in creating a dialog system is having a methodology to trigger it. For our Tactical RPG, there will often be dialog popping up prior to the fight.

There are some other potential "triggers" too -

• When an enemy is attacked the first time
• At the start of the [nth] turn (player or enemy)
• When a character is defeated
• When a character moves into a certain cell on the game board
• Others?? Possibly!

It is clear that we need to be relatively dynamic and expressive here. If you read my previous article on game states you know that we've designed a state machine with one major holder of state - GameState. As the turns progress, different states are activated - a way of determining which "mode" the game is in at a given time, combined with GameState which keeps track of the board pieces, who's turn it is (and how many turns in we are).

Dialog triggers should be glue working inside this overall concept - the task is to keep an eye on GameState and once a condition is met, halt the normal progression of events in favor of the dialog snippet.

Here is the code that spins up periodically to check and start up the dialog scene:

public void CheckTriggers(Action preempted)
{
    // run through current pending triggers (if not in "triggered" state!)
    // find any that should fire
    // found? fire it.

    if (triggered)
    {
        return;
    }

    var matches = levelChapter.DialogScenes
        .Where(s => s.Trigger(gameState) && s.TriggerState == TriggerState.Pending).ToList();

    if (matches.Count > 0)
    {
        var candidate = matches.First();
        triggered = true;
        liveScene = candidate;
        Preempted = preempted;

        gameState.stateMachine.visualNovelState.Activate(
            new VisualNovelProps {DialogScene = candidate}
        );
    }
    else
    {
        preempted();
    }
}

The key here is the Action parameter. This is a handle to the next method that should kick off the next step in the overall game loop. Passing it in as a method means we can hold on to it - either run immediately if no triggers are fired, or starting up the visual novel state and holding on to it to run after the visual novel state is over.

I waffled on whether to run the checking in an update loop or during the few scenarios we know that could trigger a dialog event. I was leery of the performance hit of running it in Update() so I decided on the latter. Here is an example invocation:

void StartTurn()
{
    visualNovelEngine.CheckTriggers(() =>
        stateMachine.flourishState.Activate(new PlayerFlourishProps {Player = CurrentTurn.p})
    );
}

Finally - here's an example trigger. This methodology will have to grow a tad once we have multiple levels, but this strategy should generally work.

public override DialogScene[] DialogScenes => new DialogScene[]
{
    new()
    {
        YamlFilename = "R0/intro.yml",
        Trigger = gameState => gameState.CurrentTurn.p == Player.Human && gameState.CurrentTurn.turn == 0
    }
};

The trigger can look at the GameState and make decisions based on the state of that object. The only other piece is a pointer to the .yml file that contains the "Playbook" containing the instructions for the dialog scene. That'll be one of the upcoming articles!

Now that this is in place, we have a flexible way of interjecting into the game loop and presenting adhoc dialog scenes driven from yml files.