Building the Flak Cannon Game Loop - The Essential Guide to Flash Games - page 198

We start the function by creating some variables to hold the lengths of the Array objects we will
be testing.
private function checkCollisions():void {
var enemyLength:int = enemyArray.length-1;
var flakLength:int = flakArray.length-1;
var shipLength:int = shipArray.length-1;
var bonusPlaneLength:int = bonusPlaneArray.length-1;
Next, we begin iterating backward through the enemyArray array. Recall that tempEnemy is a class
variable created one time so that we don't have to waste the processing cycles making a one
every time we go through this loop. Notice the enemy: that precedes the for loop. This is a label .
We will use this label to break out of a loop if the item we are testing has been removed from the
array (this will be explained a bit when we talk about the break statement).
enemy: for (var ctr2:int = enemyLength; ctr2 >= 0; ctr2--) {
The first collisions we are going to test are between the Flak explosions and the Enemy objects.
First, we need to create an instance of the Point class. We need a Point class for each object
that we are going to test for collisions. We will name this Point enemyPoint . This Point
represents the upper-left-hand corner of the Enemy object. bitmapData.hitTest() requires this
location as one of its parameters. Next, we start iterating through flakArray , setting tempFlak
in every iteration.
tempEnemy = enemyArray[ctr2];
var enemyPoint:Point = new Point(tempEnemy.x, tempEnemy.y);
for (var ctr:int = flakLength; ctr >= 0; ctr--) {
tempFlak = flakArray[ctr];
We also need a second Point that represents the upper left-hand corner of the Flak explosion.
This is required by the bitmapData.hitTest() function, so it will know where to start its pixel-level
collision detection.
var flakPoint:Point = new Point(tempFlak.x, tempFlak.y);
Next comes the big boy for this function. This is the gold , where everything really happens. This is
the actual bitmapData.hitTest() test (inside an if statement):
if (tempFlak.image.bitmapData.hitTest(flakPoint,255,
tempEnemy.image.bitmapData,enemyPoint)) {
Let's break down the actual test and parameters of the call to make it abundantly clear what is
actually happening here:
flackPoint : The upper-left hand corner of the Flak explosion we are testing.
255 : This is the first alpha channel threshold. It needs to be the highest opaque value of
the bitmap, which in our case is 255.
tempEnemy.image.bitmapData : This is the bitmapData of the Enemy we are testing
against.
ePoint : This is the upper-left hand corner of the Enemy we are testing.
If a collision is detected, we then move into this section of code. The first thing we do is call the
removeItemFromArray() function we created earlier passing tempEnemy and enemyArray array.
Notice that we don't remove the Flak explosion. That is because a Flak explosion can keep