Building the Flak Cannon Game Loop - The Essential Guide to Flash Games - page 184

Handling mouse events and starting the crosshairs
The player in Flak Cannon will control crosshairs on the screen that they will use to target shots.
The crosshairs will be controlled by the mouse; the mouse button will be used to fire a Flak shell
at incoming Enemy planes.
The first thing we must do to support the crosshairs is to define a variable to hold the bitmap in
the properties definition section of the Game class. Note, we already created this in the properties,
this line of code is just for clarification.
private var crosshairs:Bitmap;
Now, in newLevel() , we will test to see if the crosshairs exists. If not, we will hide the mouse
pointer using the Mouse.hide() function and then define a new instance of the crosshairs Bitmap
that we can add to the screen. Notice that if you are using Flex, the call to create a new instance
is slightly different than in Flash. This is because Flex already considers the CrosshairsGif()
class to be a Bitmap , while in Flash, we need to create a new Bitmap instance by passing an
instance of CrosshairsGif() . The difference is subtle, but both sets of code are necessary to
support either technology.
if (crosshairs == null) {
Mouse.hide();
//***** Flex *****
//crosshairs = new CrosshairsGif();
//***** Flash *****
crosshairs = new Bitmap(new CrosshairsGif(0,0));
Now, we need to position the crosshairs on the screen. This is the first place where we use
gameWidth and gameHeight that we passed in from Main . We will then add crosshairs to the screen
using addChild() . Finally, we will add a new event handler to the stage to listen for the
MouseEvent.MOUSE_MOVE event, so we can move the crosshairs when the mouse is moved,
making it appear to move with the mouse.
crosshairs.x=gameWidth/2;
crosshairs.y=gameHeight/2;
addChild(crosshairs);
stage.addEventListener(MouseEvent.MOUSE_MOVE, mouseMoveListener, false, 0, true);
}
Now that we have our crosshairs on the screen, we need to do one more thing to support user
input. We need to listen for the mouse button click, so we know when to fire a flak shell for the
player. We do this by listening for the MouseEvent.MOUSE_DOWN event. This event is better for a
game like Flak Cannon than MouseEvent.MOUSE_CLICK , as that event will wait for the mouse button
to be pressed and released, while MouseEvent.MOUSE_DOWN will fire when the mouse is pressed
down. This will make the firing more responsive for the player.
stage.addEventListener(MouseEvent.MOUSE_DOWN, shootListener, false, 0, true);
Having now defined listeners for mouse event, we must also create functions for those listeners
to call. The first function mouseMoveListener() is quite simple. It is called whenever the mouse
moves to update the position of crosshairs.