This code is an iOS port of this easy to follow original tutorial & web example. I’m using the phone’s accelerometer to move my ship, instead of the arrow keys on a keyboard, so the main difference is in the Update() function of playerScript.js. There’s also a Boo port on YouTube and a video series based on the original. Don’t forget – Javascript is statically typed in Unity for iOS to improve performance.

playerScript.js

#pragma strict

static var LEFT_BOUND:float = -2.9;
static var RIGHT_BOUND:float = 2.9;
static var playerScore:int;
static var playerLives:int;

var playerSpeed:int = 0.1;
var bullet:Rigidbody; //ref to bulletPrefab in level1 scene
var explosion:Transform; //ref to explosionPrefab in level1 scene
var angl:int = 0;
var timeDiff:float;
var delay1:float = 1.5; //two second delay.

function Start () {

    playerScore = 0;
    playerLives = 3;

    timeDiff = Time.time + delay1;
}

function Update () {

    //set up accelerometer-based movement:
    var dir: Vector3 = Vector3.zero;
    var quat: Quaternion = Quaternion.AngleAxis(180, Vector3.forward);

    // we assume that device is held parallel to the ground
    // and Home button is in the right hand    
    // remap device acceleration axis to game coordinates:
    //  1) X plane of the device is mapped onto X plane
    dir.x = Input.acceleration.x;  

    // clamp acceleration vector to unit sphere, since we only care about direction, not magnitude
    if (dir.sqrMagnitude > 1)
        dir.Normalize();
   
    // Move object
    transform.Translate ( (dir * playerSpeed) * Time.deltaTime); 

    //detect single touch to fire: 
    for (var touch : Touch in Input.touches) {
        if (touch.phase == TouchPhase.Began) {
            var tempBullet:Rigidbody;
            tempBullet = Instantiate( bullet, transform.position, transform.rotation );             
        }
    }

    if (Time.time > timeDiff) //delay by 2 seconds before switching scene
    {
        timeDiff = Time.time + delay1;    

        //you won
        if(playerScore >= 500){
            Application.LoadLevel(3);
        }
        
        //you lost    
        if( (playerLives <= 0) ){ 
            Application.LoadLevel(2);
        }
    } 
}

function OnGUI() {
    
    GUI.Label(Rect(10,50,200,50), "Score: " + playerScore);
    GUI.Label(Rect(10,80,200,50), "Lives: " + playerLives);
}

function OnTriggerEnter(otherObject:Collider) {
    
    var o:GameObject = otherObject.gameObject;

    if(o.tag == 'enemy'){ //Enemy's "is Trigger" is checked

        o.transform.position.y = 7;
        o.transform.position.x = Random.Range( playerScript.LEFT_BOUND, playerScript.RIGHT_BOUND );

        var tempExplosion:Transform;
        tempExplosion = Instantiate(explosion, transform.position, transform.rotation);

        playerLives--;
    }
}

bulletScript.js*

#pragma strict

var bulletSpeed:int;
var explosion:Transform;

function Start () {

}

function Update () {

	var amtToMove = bulletSpeed * Time.deltaTime;

	transform.Translate(Vector3.up * amtToMove);

	if(transform.position.y >= 6.5) {
		Destroy(gameObject);
	}

}

function OnTriggerEnter(otherObject:Collider) {
	
	var o = otherObject.gameObject;

	if(o.tag == 'enemy'){ //Enemy's "is Trigger" is checked
		playerScript.playerScore += 100;

		o.transform.position.y = 7; //tell the enemy to reset it's Y off screen
		o.transform.position.x = Random.Range( playerScript.LEFT_BOUND, playerScript.RIGHT_BOUND );

		var tempExplosion:Transform;
		tempExplosion = Instantiate(explosion, transform.position, transform.rotation);

		Destroy(gameObject); //destroy self, aka this bullet
	}
} 

* NOTE: in more complex games consider using a Ray cast from the gun/ship/object that fires the bullet for a more realistic bullet collision effect.

enemyScript.js

#pragma strict

var enemySpeed:int;

function Start () {

}

function Update () {

	var amtToMove = enemySpeed * Time.deltaTime;

	transform.Translate( Vector3.down * amtToMove );

	if(transform.position.y <= -5) {

		var rnd = Random.Range( playerScript.LEFT_BOUND, playerScript.RIGHT_BOUND );
		transform.position.x = rnd;	
		transform.position.y = 7;
	} 

}

mainMenuScript.js

#pragma strict

function OnGUI () {

	var instructionText = 'Instructions\nTilt your phone left or right to move the ship\nTouch screen to fire';

	GUI.Label(Rect(10, 50, 270, 200), instructionText);

	if(GUI.Button(Rect(10, 110, 200, 60), 'START GAME')) {

		Application.LoadLevel(1); //1 comes from Build Settings list of items in Scenes in Build
					
	} 
}

loseScript.js

#pragma strict

function OnGUI () {

	if( GUI.Button(Rect(10, 50, 300, 50), 'You Lost. Press to play again.')){
		Application.LoadLevel(0);
	}
}

winScript.js

#pragma strict

function OnGUI () {

	if( GUI.Button(Rect(10, 50, 300, 50), 'You Won! Press to play again.')){
		Application.LoadLevel(0);
	}
}