How To Build a Tower Defense Game for the iPhone – Part 6 – Tower Powers

Welcome back to the Tower Defense Tutorial Series! Once again I am assuming that you have read the previous 5 parts of this tutorial and are happy with the content covered in them.

In this Tutorial, we will be incorporating a new tower type into our game, this will give you the knowledge to then continue on and make/implement your own towers types. The tower type which we will be incorporating is a freeze tower, this tower will shoot Ice projectiles at a creep which will freeze it in place for a period of time, before the creep then continues on its journey.

Source code after the break!

The first thing to do is to download the new Turret and Projectile images that I created in about 2 minutes. They can be found here: Freeze Turret and Ice Projectile. Alternatively you can use your own images (however i recommend re-naming them in the same way to avoid confusion later). Then add them to your resources folder in Xcode. Alternatively if you want to just see everything in action, here is the source code: Tower Defense Part 6.

Now we have our images in our project, we want to add our new turret image to our HUD so that the player can select and place it within the level. Open GameHUD.m and look at the init section.

NSArray *images = [NSArray arrayWithObjects:@"MachineGunTurret.png", @"FreezeTurret.png", @"MachineGunTurret.png", @"MachineGunTurret.png", nil];
for(int i = 0; i < images.count; ++i) {
     NSString *image = [images objectAtIndex:i];
     CCSprite *sprite = [CCSprite spriteWithFile:image];
     float offsetFraction = ((float)(i+1))/(images.count+1);
     sprite.position = ccp(winSize.width*offsetFraction, 35);
     sprite.tag = i+1;//individual tag for each Turret.
     [self addChild:sprite];
     [movableSprites addObject:sprite];

Within the "NSArray *images" we have changed the second "MachineGunTurret.png" to "FreezeTurret.png". This will load the freeze turret image as the second from the left image. Then in the "for loop" we added an individual tag to each tower type, "sprite.tag = i+1;" I chose to +1 so that the first Turret type(MachineGunTurret) starts at 1 rather than 0. And the New FreezeTurret will have a tag of 2. For now, turret types 3 + 4 are still machine gun turrets, but once you have completed this tutorial, feel free to add your own turrets types in here.

While we are in GameHUD.m go down to ccTouchesBegan and add the following piece of code directly above the line "[self addChild:newSprite];"

selSprite.tag = sprite.tag;

then go down to ccTouchedEnded and adjust the if statement:

if (!CGRectContainsPoint(backgroundRect, touchLocation))

to say:

if (!CGRectContainsPoint(backgroundRect, touchLocation)) {
     CGPoint touchLocationInGameLayer = [m._gameLayer convertTouchToNodeSpace:touch];
     [m._gameLayer addTower: touchLocationInGameLayer: selSprite.tag];
}

What do these changes do? In effect they take the tag of the currently selected turret type, and pass it to the addTower function in TutorialScene.m, when the player lets go of the tower placing it within the level. To get rid of the pesky error messages which are now occurring we need to adjust the addTower function.
Go into TutorialScene.h replace the addTower function definition with the following:

- (void)addTower: (CGPoint)pos: (int)towerTag;

then in TutorialScene.m replace the addTower function header (not the contents of the function) with the following:

-(void)addTower: (CGPoint)pos: (int)towerTag{
     //Keep the same function content!
}

At this moment in time, if we run the game, we will see the FreezeTurret in the HUD and be able to select and move it, however when we place it on the map, it will transform into a MachineGunTurret.This is partly because we have not created the FreezeTower class, we shall do this now. Open Tower.m and copy and paste the following class at the very end of the file (after @end).

@implementation FreezeTower
+ (id)tower {
     FreezeTower *tower = nil;
     if ((tower = [[[super alloc] initWithFile:@"FreezeTurret.png"] autorelease])) {
          tower.range = 200;
          tower.target = nil;
          [tower schedule:@selector(towerLogic:) interval:2];
     }

     return tower;
}

-(id) init
{
     if ((self=[super init]) ) {
          //[[CCTouchDispatcher sharedDispatcher] addTargetedDelegate:self priority:0 swallowsTouches:YES];
     }

     return self;
}

-(void)setClosestTarget:(Creep *)closestTarget {
     self.target = closestTarget;
}

-(void)towerLogic:(ccTime)dt {
     self.target = [self getClosestTarget];

     if (self.target != nil) {
          //rotate the tower to face the nearest creep

          CGPoint shootVector = ccpSub(self.target.position, self.position);
          CGFloat shootAngle = ccpToAngle(shootVector);
          CGFloat cocosAngle = CC_RADIANS_TO_DEGREES(-1 * shootAngle);

          float rotateSpeed = 0.5 / M_PI; // 1/2 second to roate 180 degrees
          float rotateDuration = fabs(shootAngle * rotateSpeed);

          [self runAction:[CCSequence actions: [CCRotateTo actionWithDuration:rotateDuration angle:cocosAngle], [CCCallFunc actionWithTarget:self selector:@selector(finishFiring)], nil]];
     }
}

-(void)creepMoveFinished:(id)sender {
     DataModel * m = [DataModel getModel];

     CCSprite *sprite = (CCSprite *)sender; [self.parent removeChild:sprite cleanup:YES];

     [m._projectiles removeObject:sprite];
}

- (void)finishFiring {
     DataModel *m = [DataModel getModel];
     self.nextProjectile = [Projectile projectile];
     self.nextProjectile.position = self.position;
     [self.parent addChild:self.nextProjectile z:1];
     [m._projectiles addObject:self.nextProjectile];

     ccTime delta = 0.5;
     CGPoint shootVector = ccpSub(self.target.position, self.position);
     CGPoint normalizedShootVector = ccpNormalize(shootVector);
     CGPoint overshotVector = ccpMult(normalizedShootVector, 320);
     CGPoint offscreenPoint = ccpAdd(self.position, overshotVector);

     [self.nextProjectile runAction:[CCSequence actions: [CCMoveTo actionWithDuration:delta position:offscreenPoint], [CCCallFuncN actionWithTarget:self selector:@selector(creepMoveFinished:)], nil]];
     self.nextProjectile.tag = 2;!!
     self.nextProjectile = nil;
}
@end

then in Tower.H: copy and paste the following class definitions at the very end of the file (after @end)

@interface FreezeTower : Tower {

}

+ (id)tower;
- (void)setClosestTarget:(Creep *)closestTarget;
- (void)towerLogic:(ccTime)dt;
- (void)creepMoveFinished:(id)sender;
- (void)finishFiring;
@end

So what have we done here? Well if you look through the code you will see that we have done very little apart from copy the MachineGunTurret class and change a few parameters. Such as: the image, the fire rate of the turret (towerLogic schedule), the speed of the projectile (just made it a bit faster). So now we have our FreezeTurret class, we should go back into addTower and allow it to create a FreezeTurret if a player selects a freeze turret. Now open TutorialScene.m, and go to the addTower function. Under the check to see if the tile is build able add in the following:

NSLog(@"Buildable: %@", type);
if([type isEqualToString: @"1"]) {
     printf("tower tag %i", towerTag);

     switch (towerTag) {
          case 1:
               target = [MachineGunTower tower];
          break;
          case 2:
               target = [FreezeTower tower];
          break;
          case 3:
               target = [MachineGunTower tower];
          break;
          case 4:
               target = [MachineGunTower tower];
          break;
          default:
          break;
     }

Now our towerTag, passed from GameHUD, comes in handy as we check to see which tower the player has selected. When the player lets go we add the same tower type to the level. So now we should be able to select and add two different classes of Tower to our game world.

Run, Check, Debug.

Ok, so you might have noticed, at the moment the FreezeTurret behaves more like a slow MachineGunTurret than a FreezeTurret. In this next section we shall improve on this by adding the freeze logic to the update function, and then allowing the creeps to resume its path. The first thing that we should do is to go into Tower.M and make sure that the projectiles coming from MachineGunTurret have a tag of 1. Whilst the projectiles coming from FreezeTurret have a tag of 2. Next, open TutorialScene.m and go down to our Update function, and find the section where we check for collisions between projectiles and creeps and replace the entire if statement with the following.

if (CGRectIntersectsRect(projectileRect, targetRect)) {
     [projectilesToDelete addObject:projectile];
     Creep *creep = (Creep *)target;

     if (projectile.tag ==1) { //MachineGun Projectile
          creep.hp--;
          if (creep.hp <= 0) {
               [targetsToDelete addObject:target];
          }
          break;
     }
     else if (projectile.tag ==2) { //Freeze projectile
     id actionFreeze = [CCMoveTo actionWithDuration:1 position:creep.position];
     id actionMoveResume = [CCCallFuncN actionWithTarget:self selector:@selector(ResumePath:)];
     [creep stopAllActions];
     [creep runAction:[CCSequence actions:actionFreeze, actionMoveResume, nil]];
     break;
     }
     break;
}

When a collision between a projectile and a creep is detected, we then check to see what the projectile tag is. If the projectile.tag == 1 then we know this projectile come from a MachineGunTurret and we perform the same actions as we have done in the previous tutorials. However if the projectile.tag == 2 then we know this is a freeze projectile, in which case we run a series of actions. First we Freeze the creep for 1 second at its current location. Then we call a new function ResumePath:, in which we will allow the creep to continue along its path.

in tutorialscene.m add in the following function

-(void)ResumePath:(id)sender {
     Creep *creep = (Creep *)sender;
     WayPoint * cWaypoint = [creep getCurrentWaypoint];//destination
     WayPoint * lWaypoint = [creep getLastWaypoint];//startpoint
     float waypointDist = fabsf(cWaypoint.position.x -lWaypoint.position.x);
     float playerDist = fabsf(creep.position.x - cWaypoint.position.x);
     float distFraction = playerDist / waypointDist;
     float moveDuration = creep.moveDuration * (distFraction); //Time it takes to go from one way point to another * the fraction of how far is left to go (meaning it will move at the correct speed)

     id actionMove = [CCMoveTo actionWithDuration:moveDuration position:cWaypoint.position];
     id actionMoveDone = [CCCallFuncN actionWithTarget:self selector:@selector(FollowPath:)];
     [creep stopAllActions];
     [creep runAction:[CCSequence actions:actionMove, actionMoveDone, nil]];
}

then in creep.m add in the following function

- (WayPoint *)getLastWaypoint{
     DataModel *m = [DataModel getModel];
     //int lastWaypoint = m._waypoints.count;
     self.lastWaypoint = self.curWaypoint -1;
     WayPoint *waypoint = (WayPoint *) [m._waypoints objectAtIndex:self.lastWaypoint];
     return waypoint;
}

[Side Note: REMEMBER TO ADD IN THE HEADERS FOR THESE FUNCTIONS.]

What do we do in these functions? Well in essence we find the distance between the previous waypoint and the current waypoint. Then we find the distance between the creep and the current waypoint. We then represent the distance between the creep and the current waypoint as a fraction of the distance between the last waypoint and the current waypoint. Then we use this fraction to affect the creep.moveDuration so that when the creep continues towards the current waypoint, it moves at a speed relative to its fellow creeps. After it hits the current waypoint it will then continue along its path normally. One thing that I would like to do here is to correct a slight error which occurs within the creep.m code. Replace the getNextWaypoint function with the following:

[Update: found a small issue in the getNextWaypoint function that would cause the health bar to act weird - It's been fixed now in the below code]

- (WayPoint *)getNextWaypoint{
DataModel *m = [DataModel getModel];
self.curWaypoint++;

if (self.curWaypoint >= m._waypoints.count){
        self.curWaypoint--;
        gameHUD = [GameHUD sharedHUD];
        if (gameHUD.baseHpPercentage > 0) {
            [gameHUD updateBaseHp:-10];
        }

        Creep *target = (Creep *) self;
        NSMutableArray *endtargetsToDelete = [[NSMutableArray alloc] init];
        [endtargetsToDelete addObject:target];

        for (CCSprite *target in endtargetsToDelete) {
            [m._targets removeObject:target];
            [self.parent removeChild:target cleanup:YES];
        }
        return NULL;

    }

WayPoint *waypoint = (WayPoint *) [m._waypoints objectAtIndex:self.curWaypoint];
return waypoint;

}

If you have noticed an intermittent crash, it is because when the creeps gets to the last waypoint this function was not working correctly. However this update will recognize when a creep is at the last waypoint and delete the creep. So, run this and we should see that the FreezeTowers, freeze the creeps, and then the creeps (after one second) will continue along their path at a normal speed.

One final thing we can implement is to add in an IceProjectile that the FreezeTower fires, rather than the usual machine gun projectile. Once again this is fairly simple, open Projectile.m and add the following class at the end of the file (after @end):

@implementation IceProjectile
+ (id)projectile {
     IceProjectile *projectile = nil;
     if ((projectile = [[[super alloc] initWithFile:@"IceProjectile.png"] autorelease])) {

     }
     return projectile;
}

- (void) dealloc
{
     [super dealloc];
}

@end

then in Projectile.h add the following at the end of the file:

@interface IceProjectile : CCSprite {
}

+ (id)projectile;
@end

Simply this creates another class of projectile, the IceProjectile. Now go into Tower.h and go down to the FreezeTower class, and find the function finishedFiring. Adjust the call to [Projectile projectile]; to say the following:

self.nextProjectile = [IceProjectile projectile];

And there we have it! FreezeTowers, IceProjectiles, and FreezeCreep functionality. As always, comments / suggestions / found errors? All very welcome! Thanks for Reading!

Yours,
Aiden Fry
iPhone Game Tutorials Contributor

Aiden Fry is a recent graduate, working to get into the games industry. He is a games programmer with a special interest in audio programming. Please check out his website aidenfry.tk to see some of his work.

Aiden Fry is a recent graduate, working to get into the games industry. He is a games programmer with a special interest in audio programming. Please check out his website aidenfry.tk to see some of his work.

Aiden Fry

View all posts by Aiden Fry
Aidens website

Share this: