Keeping Object in Stage-Flash - actionscript-3

Question

I am making a game for my class which is similar to Space Invaders/Galaga. I'm new to ActionScript and coding really, I have created an enemy class for which I have it spawn and then move in the x direction. I'm just wondering how I would go about having my enemy move down after it has reached the end of my stage which is 700x500 and then proceed to go to other side, I'm assuming planting an if statement in my enemy class, just unsure on how to go about it, any help will do, much appreciated guys.
Enemy Class
package
{
import flash.display.MovieClip;
public class Enemy extends MovieClip
{
public function Enemy()
{
x = 60;
y = 30;
}
public function moveDownABit():void
{
}
public function moveRight():void
{
x = x + 2;
}
public function moveDown():void
{
}
public function moveLeft():void
{
}
public function moveUp():void
{
}
}
}
Game
package
{
import flash.display.MovieClip;
import flash.events.Event;
import flash.utils.Timer;
import flash.events.TimerEvent;
import flash.events.KeyboardEvent;
import flash.ui.Keyboard;
public class SpaceVigilanteGame extends MovieClip
{
public var enemy:Enemy;
public var avatar:Avatar;
public var gameTimer:Timer;
var gameWidth:int = 0;
var gameHeight:int = 0;
public function SpaceVigilanteGame()
{
enemy = new Enemy();
addChild( enemy );
avatar = new Avatar();
addChild( avatar );
gameWidth = stage.stageWidth;
gameHeight = stage.stageHeight;
gameTimer = new Timer( 25 );
gameTimer.addEventListener( TimerEvent.TIMER, moveEnemy );
gameTimer.start();
}
public function moveEnemy( timerEvent:TimerEvent ):void
{
//enemy.moveDownABit();
if(enemy.x+enemy.width+2<=gameWidth)
{
enemy.moveRight();
}
else if(enemy.y+enemy.height+2<=gameHeight)
{
enemy.moveDown();
}
else if(enemy.x-2>=0)
{
enemy.moveLeft();
}
else if(enemy.y-2>=0)
{
enemy.moveUp();
}
}
}
}

Answer 1

This is pretty basic stuff.
Lets assume you have 4 functions in your Enemy class, moveDown, moveUp, moveRight and moveLeft.
We need to know the width and height of the area the Enemy will move around in, either by hardcoding (will bite you in the butt later on, possibly) or by doing it more dynamic like this:
var gameWidth:int = stage.stageWidth;
var gameHeight:int = stage.stageHeight;
stage.stageWidth or its Height counterpart will give you the size of your SWF movie in pixels, with scaling and so on.
You already have a Timer set up in your Document Class, every 80ms the function moveEnemy will get executed. An alternative approach is to use Event.ENTER_FRAME and execute a function every frame.
Now think about it, every time your function gets called, the enemy will move down a bit. As you said, there should be some kind of...checking mechanism. But we shouldn't put it in the Enemy class, the Enemy doesn't need to know when to stop, that's the problem of the Document class. Ideally, you should be able to use that Enemy in every sort of Class, right? So giving the Enemy boundaries will just be a lot of work when the Document Class could do it instead.
The solution is very easy, you need to check the position of your enemy against the dimensions of your stage every time you want to move your Enemy.
Essentially:
private var gameWidth:int = 0;
private var gameHeight:int = 0;
public function SpaceVigilanteGame(){
//...your init code for enemy
gameWidth = stage.stageWidth;
gameHeight = stage.stageHeight;
//...your timer code
}
public function moveEnemy(timerEvent:TimerEvent):void{
//lets first check if we can go to the right
if(enemy.x+enemy.width+2<=gameWidth){
//so, we take the right outermost border of your enemy by taking its x-position and adding its width. To check if a step right would be out of bounds, we add the amount it WOULD move
//as an example, your enemy has the x-position 100, and is 200 pixel wide, is 100+200+2 smaller or equal to 700 (your stage width)? Yes it is, thus you can move it
enemy.moveRight();
}
else if(enemy.y+enemy.height+2<=gameHeight){
//but what happens if we can't go right? we jump to this additional if condition that will check if the enemy can move down
//same thing as above, but instead we use y-position and the heights
enemy.moveDown();
}
else if(enemy.x-2>=0){
//if we can't move right or down, try left instead
//this time we only need the x-position of the enemy, because we need to look at the leftmost border
enemy.moveLeft();
}
else if(enemy.y-2>=0){
//same deal with going up
enemy.moveUp();
}
}
This is untested code, but I hope you're getting the general gist of this example. The problem here is though that the Enemy will always attempt to move right first. When you start at x:0,y:0 he will move right until he hits the end, then move down until he hits an end, then move left and right alternatively forever.
Movement 101:
If you want to move to the right x = x + 1
If you want to move to the left x = x - 1
If you want to move up y = y - 1
If you want to move down y = y + 1