JavaScript and AJAX For Dummies by Andy Harris

Author:Andy Harris
Language: eng
Format: epub, pdf
Publisher: Wiley
Published: 2010-03-08T16:00:00+00:00

I chose a simple but effective technique. I temporarily took out the checkbounds() call and just took a look at the output to see what the values of x and y were. I looked to see how large x should be before the sprite wraps and wrote the value down on paper. Likewise, I found largest and smallest values for y.

When I knew these values, I simply placed them in constants. I don’t really care that the maximum value for x is 365. I just want to know that when I’m messing around with x I don’t want it to go past the MAX_X value.

If the size of my playing surface changes, I can just change the constants and everything will work out fine.

If you’re interested, here are the other techniques you can use when a sprite is about to leave the visual area:

Bounce: The object bounces off the wall. This is done by inverting the dx or dy value (depending on whether it’s a vertical or horizontal wall).

Stop: The object simply stops moving when it hits the wall. Set dx and dy to 0 to achieve this effect.

Continue: The object keeps moving even though it is out of sight. This is sometimes used for air-traffic control simulations (where visualizing the location is part of the game) or orbital simulations (where presumably the object will return).

Combinations: Sometimes you’ll see combinations like the civilization games that simulate a cylindrical map by stopping on the top and bottom and scrolling on the sides.

Reading Input from the Keyboard

You can use JavaScript to read directly from the keyboard. Reading from the keyboard can be useful in a number of situations, but it’s especially handy in animation and simple gaming applications.

Figure 8-2 shows a program with another moving ball.

Figure 8-2: You can move the ball with the arrow keys.

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