Spearmen, Javelin Throwers, and the State Pattern. Oh My!15 Aug 2009
Last week one of Ayende/Oren's posts caught my eye: Let the fighting commence! In it he discussed a blog post seen on Jamie Farser's blog, where Jamie discusses some design difficulties he has run into while designing a turn-based game similar to Axis & Allies named Everland. Since then I've been following Jamie's progress (first try, further exploration, second try, third try, even further exploration) as he tries to take some of Ayende's advice and implement the state pattern. Like Jamie, I am fairly new to the state pattern - sure I've read about it plenty (starting in Head First Design Patterns - a must have!), but I honestly have just never implemented it "for reals." In my back & forth with Jamie I realized that it was time to put up or shut up - if I'm going to offer criticism I should also offer up an alternative solution.
Let's say we have a game where the board is made up of six-side polygons ("hexes", if you will) and each hex can contain a unit like a spearman or a javelin thrower. Each unit on the board belongs to a player who can take turns moving his units, telling his units to attack other player's units, etc. Some units have different capabilities than other units - a spearman must be within 1 hex of another unit to attack that unit, whereas a javelin thrower can be 2 units away. How do we design the application in such a way that:
- The game engine doesn't need to know specifics about each unit (such as range of attack).
- A unit's capabilities may change during the course of gameplay (such as a spearman being upgraded to a javelin thrower).
Before we look at the code, lets run through what is going to happen in-game.
- The user selects one of their units, then selects an opposing unit somewhere else on the gameboard, then clicks an "Attack" button.
- The game engine creates a context (deriving from ITurnContext) that describes what the user has asked to do. In our case, an AttackContext is created containing references to the attacking unit and the defending unit.
- That context is then passed into the ExecuteTurn method of the attacking Unit, which then passes it along into the Handle method of its CurrentState property (which is an object deriving from UnitState). Our attacker's CurrentState property is currently Spearman, but could just as easily be JavelinThrower. Changing a unit from melee to ranged is as simple as changing the state of that unit.
- Our Spearman state derives from UnitState, and the UnitState base class' Handle method uses double dispatch to forward the Handle request on to the appropriate method of our subclass. In our case that would be the Handle (AttackContext) method of the Spearman class.
- Our Spearman class' Handle method then checks to see if the unit is within range to attack the defending unit - if it is, an AttackCommand is returned denoting who the attacker and the defender are; if it isn't, a MovementCommand is returned denoting which hex to move to.
- The Unit class then calls Execute on the returned command, causing the movement or attack operation to be, well, executed.
Let's go through each of the above steps and take a look at the code involved one by one. Skipping the first step (the UI), we'll move on to where the game engine is creating an AttackContext and passing it into the attacking Unit:
Okay, easy enough. Moving on we can see the Unit class' ExecuteTurn method pass the context to its CurrentState property's Handle method:
Our attacking unit's CurrentState property is currently set to an instance of Spearman, but before we take a look at that class we'll take a look at its base class (UnitState - which is doing double dispatch to the Spearman's Handle method):
The Spearman class' Handle method is then invoked:
Notice that the only concern Spearman currently has when handling an attack context is ensuring that it gets within range and then attacks the target. We'll see an example of a JavelinThrower later. Finally we'll peek at what the MovementCommand and AttackCommand actually do:
Before we move on, here is the promised JavelinThrower class:
Obviously JavelinThrower & Spearman currently only differ in the range at which they can attack (<=2 and <= 1, respectively), so the code in their Handle methods could be abstracted out using the strategy pattern.
So now that we've had a chance to see the code involved the question comes down to: does it actually work? Here are our unit tests:
And the results...
So that is the road I went down, but what I really want to know is how would *you *solve this problem? If you'd like to take a deeper look at my code, download it and give it a try yourself: