Tutorial: Create a Tower Defense Game in AS2 – Part 4

Written By MrSun at 8:04 am - Saturday, April 25th, 2009
Categories: Advanced Tutorials, All Tutorials, AS2, Flash, Game Development

Step 4: Making Turrets Attack Enemies

Well, it’s now time to let the enemies we just created be destroyed. Let’s dig in, shall we?

First, we’ll define a single health variable for the Enemy. Find this code in the makeEnemies() function (~ Line 236):

//add a few variables to the enemy
enemyHolder['enemy'+currentEnemy].maxSpeed = 3;//how fast it can possibly go
enemyHolder['enemy'+currentEnemy].xSpeed = 0;
enemyHolder['enemy'+currentEnemy].ySpeed = 0;

Just add this code:

enemyHolder['enemy'+currentEnemy].health = 5;

Now, we’re going to have to add some variables to the turrets. In the makeRoad() function, find the onRelease function of the Empty Block (~ Line 77). Add this code to the bottom of that function (~ Line 116).

_root['t'+this._name].angle = 0; //the angle that the turret is currently rotated at
_root['t'+this._name].radiansToDegrees = 180/Math.PI;//this is needed for the rotation
_root['t'+this._name].damage = 3;//how much damage this little baby can inflict
_root['t'+this._name].range = 100;//how far away (in pixels) it can hit a target
_root['t'+this._name].enTarget = null;//the current target that it's rotating towards
_root['t'+this._name].cTime = 0;//how much time since a shot was fired by this turret
_root['t'+this._name].reloadTime = 12;//how long it takes to fire another shot
_root['t'+this._name].loaded = true;//whether or not this turret can shoot

Those are a lot of variables, aren’t they? Well, this is a pretty complex task, so we’re going to need all of them. So, brace yourself, for we are now going to jump into some code. Add the following after the code you just added above:

_root['t'+this._name].onEnterFrame = function(){
	//FINDING THE NEAREST ENEMY WITHIN RANGE
	this.distance = this.range;//let's define a variable which will be how far the nearest enemy is
	this.enTarget = null;//right now, we don't have a target to shoot at
	for(var i=_root.currentEnemy-1;i>=0;i--){//loop through the children in enemyHolder
		var cEnemy = _root.enemyHolder['enemy'+i];//define a movieclip that will hold the current child
		//this simple formula with get us the distance of the current enemy
		if(Math.sqrt(Math.pow(cEnemy._y - this._y, 2) + Math.pow(cEnemy._x - this._x, 2)) < this.distance){
			//if the selected enemy is close enough, then set it as the target
			this.enTarget = cEnemy;
		}
	}
	//ROTATING TOWARDS TARGET
	if(this.enTarget != null){//if we have a defined target
		//turn this baby towards it
		this.gun._rotation = Math.atan2((this.enTarget._y-this._y), this.enTarget._x-this._x)/Math.PI*180-90;
		if(this.loaded){//if the turret is able to shoot
			//subtract the enemy's health
			this.enTarget.health -= this.damage;
			this.loaded = false;//then make in unable to do it for a bit
			//create a bullet
			_root.createEmptyMovieClip('b'+this._name,_root.getNextHighestDepth());
			//draw the bullet
			_root['b'+this._name].beginFill(0xFFFFFF);
			_root['b'+this._name].lineTo(0,0);
			_root['b'+this._name].lineTo(0,3);
			_root['b'+this._name].lineTo(3,3);
			_root['b'+this._name].lineTo(3,0);
			_root['b'+this._name].endFill();
			//set the bullet's coordinates
			_root['b'+this._name]._x = this._x+12.5;
			_root['b'+this._name]._y = this._y+12.5;
			//set the bullet's target and damage
			_root['b'+this._name].target = this.enTarget;
 
			//add some functions to this bullet
			_root['b'+this._name].onEnterFrame = function(){
				this.maxSpeed=4;
				this.yDist=this.target._y+12.5 - this._y;//how far this guy is from the enemy (x)
				this.xDist=this.target._x+12.5 - this._x;//how far it is from the enemy (y)
				this.angle=Math.atan2(this.yDist,this.xDist);//the angle that it must move
				this.ySpeed=Math.sin(this.angle) * this.maxSpeed;//calculate how much it should move the enemy vertically
				this.xSpeed=Math.cos(this.angle) * this.maxSpeed;//calculate how much it should move the enemy horizontally
				//move the bullet towards the enemy
				this._x+= this.xSpeed;
				this._y+= this.ySpeed;
				//check if it is close to the enemy
				if(this._x+this.maxSpeed*2>=this.target._x && this._x-this.maxSpeed*2<=this.target._x
				&& this._y+this.maxSpeed*2>=this.target._y && this._y-this.maxSpeed*2<=this.target._y){
					this.target.health -= this.damage;//make the enemy lose health
					this.removeMovieClip();//remove this sucker
				}
			}
		}
	}
	//LOADING THE TURRET
	if(!this.loaded){//if it isn't loaded
		this.cTime ++;//then continue the time
		if(this.cTime == this.reloadTime){//if time has elapsed for long enough
			this.loaded = true;//load the turret
			this.cTime = 0;//and reset the time
		}
	}
}

Now, if you test out the game and create some turrets, they should start shooting at those darn enemies! Next, we have to make it so the enemy dies when shot. This will be pretty easy to do. Find the code where we added the onEnterFrame() code to the enemy and made it move (~line 342). Just add this little tidbit of code:

if(this.health <= 0){
	this.removeMovieClip();
}

Pretty hot stuff, ain’t it? Well, this concludes the fourth installment of this tutorial. Join us next time when we make levels and have winning and losing scenarios!