ActionScript: Move object with smooth acceleration and deceleration with inertia

Hello,

Today a simple sample of code to create an inertia effect in ActionScript 3.0.

The character will start to move with smooth acceleration if you press an arrow key.

You can download the source code here : Flash_acceleration_benoifreslon.com.fla

// Copyrights
// Benoît Freslon
// http://www.benoitfreslon.com

import flash.events.KeyboardEvent;
import flash.events.MouseEvent;

// Constants (You can modify this values)
const acceleration:Number = 0.1;
const decceleration:Number = 0.9;
const speedMax:Number = 5;

// Variables
var speedX:Number = 0;
var speedY:Number = 0;

// Key states
var leftPressed:Boolean = false;
var rightPressed:Boolean = false;
var upPressed:Boolean = false;
var downPressed:Boolean = false;

// Keyboard events
stage.addEventListener(KeyboardEvent.KEY_DOWN, keyDown);
stage.addEventListener(KeyboardEvent.KEY_UP, keyUp);
function keyDown(e:KeyboardEvent):void {
	switch (e.keyCode) {
		case 37 :
			leftPressed = true;
			break;
		case 38 :
			upPressed = true;
			break;
		case 39 :
			rightPressed = true;
			break;
		case 40 :
			downPressed = true;
			break;
	}
}
function keyUp(e:KeyboardEvent):void {
	switch (e.keyCode) {
		case 37 :
			leftPressed = false;
			break;
		case 38 :
			upPressed = false;
			break;
		case 39 :
			rightPressed = false;
			break;
		case 40 :
			downPressed = false;
			break;
	}
}

// Evénement répété toutes les images
pig.addEventListener(Event.ENTER_FRAME, enterFrame);
function enterFrame(e:Event):void {

	// If a key is pressed I will increase the speed with the acceleration value

	if (upPressed) {
		speedY -=  acceleration;
	} else if (downPressed) {
		speedY +=  acceleration;
	} else {
		// If the up key and the down key are released I will decrease the speed with the decceleration value
		speedY *=  decceleration;
	}

	if (leftPressed) {
		speedX -=  acceleration;
	} else if (rightPressed) {
		speedX +=  acceleration;
	} else {
		speedX *=  decceleration;
	}

	// Ne pas oublier de limiter les vitesses
	if (speedX > speedMax) {
		speedX = speedMax;
	}
	if (speedX  <-speedMax) { 		                 speedX =  -  speedMax; 	         } 	         if (speedY > speedMax) {
		speedY = speedMax;
	}
	if (speedY <-speedMax) {
		speedY =  -  speedMax;
	}

	// Move the pig
	pig.x +=  speedX;
	pig.y +=  speedY;

	// Display values on HUD
	tValues.text = "speedX: " + speedX + "\nspeedY: " + speedY;

}

// Reset pig
stage.addEventListener(MouseEvent.CLICK , click);
function click(e:MouseEvent):void {
	pig.x = stage.stageWidth / 2;
	pig.y = stage.stageHeight / 2;
}