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Adding Behavior

Now that you've got a class that encapsulates data, you want to add behavior to it. Behavior in classes is added in the form of methods, which are functions that execute within the scope of an instance of a class.

Methods in Objective-C typically have their signature defined in the header (.h) file and the implementation defined in the implementation (.m) file. In C#, the method is defined directly on the class and there is no header used for exposing the method signature.

Some of the behavior that we want to add to our Combatant class might include attacking another combatant and moving. Listings 3.7 through 3.9 illustrate how we go about adding methods to the class to give our class some behavior. A good general rule is to think of members (or properties) as nouns on a model, whereas behaviors (or methods) should be considered verbs on a model.

Listing 3.7. Combatant.h with Behavior

@interface Combatant : NSObject {
}

- (void)attack:(Combatant *)target;

@property(nonatomic, assign) int maxHitPoints;
@property(nonatomic, assign) int currentHitPoints;
@property(nonatomic, assign) int armorClass;
@property(nonatomic, assign) int damageClass;
@property(nonatomic, retain) NSString *combatantName;

Listing 3.7 shows the header for the Combatant class, including the property declarations as well as a method signature for the Attack method.

Listing 3.8. Combatant.m with Behavior

#import "Combatant.h"

@implementation Combatant
    @synthesize maxHitPoints, currentHitPoints, armorClass, damageClass, combatantName;

- (void)attack:(Combatant *)target
{
    // obviously this should be more complicated...
    target.currentHitPoints -= rand() % 20;
}
@end

Listing 3.8 shows the Objective-C implementation of the Attack method. This method operates on a supplied instance of another combatant to allow it to do damage to the other target. It's important to keep in mind here that in Listing 3.8's Objective-C code and in Listing 3.9's C# code, both classes are using encapsulated accessors to manipulate the other objects; they are not modifying internal variables directly.

Listing 3.9. Combatant.cs with Behavior

using System;
using System.Net;
using System.Windows;
using System.Diagnostics;

namespace Chapter3
{
    public class Combatant
    {
        public int MaxHitpoints { get; set; }
        public int CurrentHitPoints { get; internal set; }
        public int ArmorClass { get; set; }
        public int DamageClass { get; set; }
        public string Name { get; set; }
        public Point Location { get; private set; }

        public int HitPointPercent
        {
            get
            {
                double pct =
                 (double)CurrentHitPoints / (double)MaxHitpoints;
                return (int)Math.Floor(pct * 100);
            }
        }

        public void MoveTo(Point newLocation)
        {
            this.Location = newLocation;
            Debug.WriteLine("Combatant {0} just moved to ({1},{2})",
                this.Name,
                this.Location.X,
                this.Location.Y);
        }

        public void Attack(Combatant target)
        {
            Random r = new Random();
            // obviously oversimplified algorithm...
            int damage =
             (this.DamageClass - target.ArmorClass) * r.Next(20);
            target.CurrentHitPoints -= damage;
        }
    }
}

In Listing 3.9 there are a couple of minor changes to the access modifiers for some of the class members, and we have the implementation of the Attack and MoveTo methods. What you might notice is that the CurrentHitPoints property now has an access modifier of internal for the set accessor. This means that only classes within the same Assembly as Combatant are able to modify that property. This allows your "game engine" to freely tweak combatant health but does not allow code outside the core engine to modify that data directly. This forces all changes to hit points to go through only authorized routes.

Additionally, the Location property now has a private access modifier. This means that only the Combatant class itself can modify its own location. This forces changes to the Combatant's location to go through the MoveTo method, which is the only acceptable means for moving a Combatant.

The reason I mention these here is because C# has much finer-grained control over access modifiers for methods and members than Objective-C, allowing you to place much more firm control over which code can and cannot affect certain pieces of data. Although this might seem like an unimportant detail, it becomes incredibly important when you are writing code that other developers will consume. An entire type of "accidental side effect" bugs can be eliminated by preventing unwanted changes to your member variables and properties.

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