Coding Dynamic Events?

Question

I have no idea what the title of this question should be so bare with me.

My game has turns. On a turn a player does something and this can result in a random number of explosions that occur at different times. I know when the explosions are done. I need to know when ALL are done and then do some other action. Also, each explosion is the same amount of time, say 3 seconds..

Right now I'm thinking of using a counter to hold how many explosions are happening. Then once the explosion is finished decrement this counter. Once the counter is zero, do my action. This idea is inspired by objective-c memory management btw. Anyways, does this sound like a good approach or would there be another way.

An alternative might be to figure out the explosion who happened last and let it be responsible for calling this subsequent action.

I'm asking mostly, because I haven't done this before and am trying to figure out if there are bugs that may occur that I'm not foreseeing.

Answer 1

You know when each individual explosion is done. You know how many explosions there will be. Just make whatever is in control of your explosions (random event handler?) trigger an event saying the explosions are finished. If you don't have a separate object controlling your explosions, make one.

A global event handler will likely be very useful to you. Any object should be able to push events to the handler. And any object should be able to be extended to handle events.

See these questions for more details:

Best way to manage in-game events?

What role does developing with events have in games?

What should I be considering when designing an Event Manager System?

Answer 2

For situations like this, I prefer what I call the "action lists" approach. It's a generalization of a technique that's been around in various forms for a very long time.

The basic implementation is that you have a queue of Action objects. An Action is something that takes time to complete. Each Action has an Update() method that can be called every iteration of the game state update loop, to perform any portion of the remaining work, and which can mark itself as being finished when the work is complete.

You can then keep this queue of Action objects, and call the Update() method for the Action in the front of the queue each frame. If the Action is marked as finished after it is run, it is removed the queue.

In your case, your ExplosionAction might start the explosion animation the first time its Update() is called. Each subsequence call to Update() would check if the animation is complete (and/or some other timer is complete), and if so, mark itself as finished.

You can then either wait for the action list to be emptied or you can create a SignalEventAction that is pushed onto the back of the queue after all the ExplosionAction objects. The SignalEventAction Update() method would call some callback function (or raise an event, if you have a more complete event system, as Byte56 suggested) and mark itself as finished. The end result will be that the explosions all fire, then the callback gets invoked to signal the game logic that all explosions are done. Nice and simple, and requires no explicit state tracking explosion counters or any special logic for the last explosion; this in turn makes the system a fair bit more robust, in my experience.

It can be helpful to put a small bit of state into the base Action implementation, such as flags whether the Action has already had its Update() method called once or not. You can then let derived implementations override specific callback methods like OnStart(), OnUpdate(), OnFinished(), etc.

There's quite a bit more you can do with an action list with very little extra implementation work. For your needs listed above, you should be fine with the most basic implementation.

If you're interested in more on the technique and how it can be applied to more complicated scheduling scenarios, see the following link to some PDF slides of an informal talk I did over the summer on it (the video is online too if you care to search for it, but it doesn't add anything over the slides): Action! Lists Slides (PDF)