Flash MX 2004 Games : Art to ActionScript

17 Platformers
Creating a platform-based game is quite a challenge, but by this stage in the book you should
have acquired all the necessary skills. In this chapter we are going to look at the problems of
controlling the behaviour of a sprite in a platform world. First we will look at an example in which
we move the sprite whilst the background remains stationary and then we will look at using a
multi-layered scrolling background. Platform games often use dynamically created sprites; we will
look at creating sprites on the fly and removing them when they are no longer in use.
The basics of platform games
Platform games involve two fundamental concepts. Firstly, user control of a sprite character that
can usually perform several actions: walk, run, jump fall etc. Secondly, collision detection to
ensure that the character remains anchored to the world that is presented to the player. Both parts
of creating a platform game need careful coding to ensure success. You may prefer to start with
the collision testing or with the user control, but either way they are two separate problems and
should be tested and debugged separately as far as possible. Here we will start with the problem of
user control.
Responding to user input
Usually the only control the user has over the gameplay will be via a few key presses or use of the
mouse. Often the control is context sensitive. For example, if a character is falling then the player
cannot set a new direction until the character has landed and the player regains control. For this
reason always use a control flag. If the character is under player control then set the flag to ‘true’
and if the character is under computer control then set the control flag to ‘false’. Because wherever
possible we want to keep the data pertaining to a movie clip within that movie clip, place this flag
as a variable inside the clip. The actions a sprite can perform are often limited both by its placement
on screen and the action that is currently being performed, so for this reason use a variable that
stores the current action. Responding to keyboard input is easier to handle than mouse input for a
platform game. Usually keyboard control will mean reading the arrow keys, space bar and possibly
the control and shift keys. Reading the keyboard is easy using the ‘Key.isDown( )’ construct. To
clarify the principles let’s look at an example. Open ‘Examples/Chapter17/FatCat01.fla’. Press
‘Ctrl + Enter’ to run the program. Using the arrow keys you can move the cat around the screen,
jumping and landing on the various platforms that you can see on the screen.
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