Foundation Game Design with ActionScript 3.0, Second Edition (85 page)

This feature is very easy to implement by keeping track of which of the four arrow keys the player is pressing, and then changing the star's vx and vy properties based on that direction.

First,
LevelTwo
uses a new String variable called
_starDirection
to track which direction the star should move in. Note that
_starDirection
is changed to “left”, “right”, “up”, or “down” depending on which key is pressed and it's only changed if the star hasn't already been launched. This prevents the star from changing direction in mid-flight if the player presses the arrow keys after the star has been launched.

The
enterFrameHandler
then sets the star's velocity to move it in any of those four directions depending on what
_starDirection
is set to.

Those are the only changes that need to be made.

In Monster Mayhem the character can't fire another star while the first one is still on the stage. In many games, however, characters can fire limitless numbers of projectiles, no matter how many are on the stage. To implement this, you need to know about
arrays
and
loops
. You'll learn about arrays and loops in
Chapter 9
, and you'll learn how to use them to fire multiple projectiles in
Chapter 10
.

Level two ends in the same way as level one. The code checks to see whether the monsters or the character has won and displays the appropriate message.

Monster Mayhem is a great model for a game project, but if you're making a similar type of game, there are probably a few more advanced things you'll want it to do.

• A scrolling background
  : You'll want to make a big environment larger than the borders of the stage and have the character and monsters move through it.
  
• Intelligent monsters
  : The monsters in Monster Mayhem move randomly around the stage. Wouldn't it be fun if they could figure out where the character is and move towards it?
  

You can implement both these features. Let's find out how.

In
Chapter 7
I showed you how to make a game with a large scrolling background. The trick to making this work is to calculate the
scroll velocity
and apply it to game objects. This keeps their positions synchronized with the scrolling background. You're going to use exactly the same technique to make the objects in Monster Mayhem scroll so make sure that you fully understand the code from
Chapter 7
before you work through this next section.

In Monster Mayhem, the monsters and the star weapon are moving across the stage. This means that they have to adjust their velocities to account for the moving background. This is, fortunately, very easy to do but requires a bit of thought and planning, so I'm going to walk you through the details of this code so you'll be more easily able to implement scrolling in your own games.

You'll find a scrolling version of Monster Mayhem in the ScrollingMonsters project folder. It only has one level with four monsters, and the collision detection has been disabled so that it's easier to test how the scrolling works. Run the SWF file and you'll see that the monsters can now meander freely all over the game world (
Figure 8-22
).

Figure 8-22.
Moving objects in a large scrolling game world

I've structured this example in exactly the same way as the original Monster Mayhem so that you can see another, simplified version your game structure system at work. The application class,
ScrollingMonsters,
creates an object called
_scrollingLevel
and adds it to the stage. All of the game logic is in the
ScrollingLevel
class. Here's the
ScrollingMonsters
application class that loads the game level:

So that you can see all the code in context, here's the entire
ScrollingLevel
class that contains all the game logic. Most of it is identical to the code in the
LevelOne
and
LevelTwo
classes that you looked at earlier in the chapter. I've highlighted all the new code that's relevant to scrolling, and I'll explain all in detail ahead.