Building the Flak Cannon Game Loop - The Essential Guide to Flash Games - page 185

public function mouseMoveListener(e:MouseEvent):void {

crosshairs.x = e.stageX-(crosshairs.width/2);
crosshairs.y = e.stageY-(crosshairs.height/2);
}
There are a couple of interesting things to note about this function. The first is self-explanatory
but should still be noted. We can find the position of the mouse relative to the stage using the
stageX and stageY properties of the MouseEvent object. The second thing is more subtle. We
need to subtract half of the width and height from stageX and stageY to position the crosshairs
properly (see Figure 5-1).

Figure 5-1. Adjusting the crosshairs into the center

We need to do this because of the way bitmaps are loaded dynamically in AS3. Without any
adjustments, a bitmap image will load at its upper-left-hand registration point (see top of Figure 4-12).
If we stopped there, shots fired would not explode in the center of the crosshairs, but in the upper-left.
By performing the adjustment (see bottom of Figure 4-12), you can see that the center of the
crosshairs is now where the mouse pointer would be and is also the point at which shots will explode.

Next, we need to write a function that will fire shots when the player presses the mouse button.
That function is shootListener() :
public function shootListener(e:MouseEvent):void {
The first thing we to do in this function is to check and see if the player has any shots left to fire
from the cannon. This is quite easy to accomplish by simply creating an if:then statement to
check the value of the shots variable:
if (shots > 0) {
If shots are remaining, we drop into the if:then statement and create an instance of the Shot class,
add it to the stage, push it into shotArray() , and then decrement the shot variable. Notice that we
subtract (crosshairs.width/2) from mouseX and (crosshairs.height/2) from mouseY . We do this to
center the shot in the middle of the crosshairs when it explodes into a Flak explosion.
tempShot = new Shot(gameWidth/2,gameHeight,mouseX-

(crosshairs.width/2),mouseY-(crosshairs.height/2));

this.addChild(tempShot);
shotArray.push(tempShot);
shots--;