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Because copying over properties into an object is such a common need, Sprite also defines a merge
method that does just that. This method is used in the setup method.
Finally, the draw method, which is nearly identical in every sprite so far, can be defined once here and then
will be available in every other sprite.
Refactoring PlayerShip
Armed with the Sprite class, the PlayerShip object can be refactored to simplify setup. The new code is
marked in bold in Listing 2-5 :
Listing 2-5: A Refactored PlayerShip
var PlayerShip = function() {
this.setup('ship', { vx: 0, frame: 1, reloadTime: 0.25, maxVel: 200
this.reload = this.reloadTime;
this.x = Game.width/2 - this.w / 2;
this.y = Game.height - 10 - this.h;
this.step = function(dt) {
if(Game.keys['left']) { this.vx = -this.maxVel; }
else if(Game.keys['right']) { this.vx = this.maxVel; }
else { this.vx = 0; }
this.x += this.vx * dt;
if(this.x < 0) { this.x = 0; }
else if(this.x > Game.width - this.w) {
this.x = Game.width - this.w
if(Game.keys['fire'] && this.reload < 0) {
this.reload = this.reloadTime;
this.board.add(new PlayerMissile(this.x,this.y+this.h/2));
this.board.add(new PlayerMissile(this.x+this.w,this.y+this.h/2));
PlayerShip.prototype = new Sprite();
At the top of the constructor function, the setup method is called, wiping out some boilerplate code. A few
of the properties are set when setup is called, but a few are set afterward because they depend on the values
of the other properties such as the object's width and height, which isn't available until after setup is called.
Next, the draw method is removed because it is handled by Sprite .
Finally, the code to actually set up PlayerShip 's prototype comes after the PlayerShip constructor
function is defined.
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