Creating a Flash platform game with Flixel and Flex - Jet pack

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All movement in Flixel can be described in terms of velocity and acceleration. Moving left and right is a matter of setting the acceleration in the x axis to some none zero value. Falling is done by setting the acceleration in y axis to a positive number. Jumping is a little different: it is implemented by subtracting a value (because to move up on the screen means a decrease in the y coordinates) from the velocity in the y axis, giving the player an instant push upward (this is referred to as an Impulse in Newtonian physics).

Adding a jet pack is really just like falling upwards, and so to implement it using Flixels physics we need to set a negative value for the acceleration in the y axis. While the player is pressing the jump key the acceleration in the y axis will be negative. Once the key is released gravity will take over, meaning the acceleration in the y axis will be positive.

First we specify some constants in the Player class that define how the jet pack works.

protected static const JETPACK_ACCELERATION:Number = 50;
protected static const JETPACK_COUNTDOWN:Number = 0.5;

The JETPACK_ACCELERATION constant defines the acceleration of the player in the y axis while the jump key in being pressed.

The JETPACK_COUNTDOWN constant defines how long the emitter that shows the jet pack smoke will remain on the screen once the jump key has been released.

[Embed(source="../media/jetpack.mp3")] 
protected var SndJetpack:Class;

[Embed(source="../media/jet.png")] 
protected var JetImage:Class;

We need to embed some additional graphics and sound effects for the jet pack.

protected var jets:FlxEmitter;
protected var jetpackEmitterCountdown:Number = -1;

The jet pack emitter is referenced by a variable called jets, and the jetpackEmitterCountdown variable is defined to keep track of how long the jet pack emitter should remain on the screen.

this.jets = FlxG.state.add(new FlxEmitter(0,0,0,0,null,0.02,0,0,0,0,0,0,0,0,JetImage,15)) as FlxEmitter;

The jet pack emitter is then initialised in the constructor.

The logic for the jet pack is implemented in the Player update function.

if(FlxG.keys.pressed("X"))
{
 acceleration.y = -JUMP_ACCELERATION;
 
 if(!this.jets.active)
  this.jets.restart();
 this.jets.x = this.x + (this.width>>1);
 this.jets.y = this.y + this.height; 
 jetpackEmitterCountdown = -1;
 
 FlxG.play(SndJetpack);
}

Here we replace the code that made the player jump with code that will make it fly. While the X key is pressed the vertical acceleration is set to the JETPACK_ACCELERATION constant. If the jet pack emitter is not active we start it up, and then position the emitter at the players current position. The jetpackEmitterCountdown variable is set to -1 to indicate that the player is flying. Finally a sound effect is played.

else
{
 acceleration.y = GRAVITY_ACCELERATION;
}

If the jump key is not being pressed, the players vertical acceleration is set to make it fall with gravity.

if (FlxG.keys.justReleased("X"))
{
 jetpackEmitterCountdown = JETPACK_COUNTDOWN;
}

If the player just released the X key we need to start a timer that will remove the jetpack emitter when it reaches 0.

if (jetpackEmitterCountdown != -1)
{
 jetpackEmitterCountdown -= FlxG.elapsed;
 if (jetpackEmitterCountdown <= 0)
 {
  jetpackEmitterCountdown = -1;
  this.jets.kill();
 }
}

If jetpackEmitterCountdown is not equal to -1 (meaning the player is not flying), we count down jetpackEmitterCountdown until it reaches 0, at which point we remove the emitter from the screen.