Basic Game Objects - Building JavaScript Games for Phones, Tablets, and Desktop - page 93

You can see in the body of this method that you use the return keyword to draw the ball only if it
isn't shooting. Inside the painterGameWorld object, you have to call the game-loop methods on
the ball. For example, this is the draw method in painterGameWorld , from which the ball.draw
method is called:
painterGameWorld.draw = function () {
Canvas2D.drawImage(sprites.background, { x : 0, y : 0 }, 0,
{ x : 0, y : 0 });
ball.draw();
cannon.draw();
};
Note the order in which the game objects are drawn: first the background image, then the ball, and
then the cannon.
Shooting the Ball
The player can click the left mouse button in the game screen to shoot a ball of paint. The speed
of the ball and the direction in which it's moving are determined by the position where the player
clicks. The ball should move in the direction of that position; and the further from the cannon a
player clicks, the higher the speed of the ball. This is an intuitive way for the user to control the
speed of the ball. Whenever you design a game, think carefully about how instructions from the user
are received and what the most natural or efficient way is to process them.
In order to handle input, you add a handleInput method to the ball object. Inside this method, you
can check whether the player clicks with the left button by using the Mouse object:
if (Mouse.leftPressed)
// do something...
However, because there can be only a single ball in the air at any moment, you want to do
something only if the ball isn't already in the air. This means you have to check the ball's shooting
status. If the ball has already been shot, you don't have to handle the mouse click. So, you extend
your if instruction with an extra condition that the ball currently isn't in the air:
if (Mouse.leftPressed && !ball.shooting)
// do something...
As you can see, you're using two logical operators ( && and ! ) in conjunction. Because of the logical
not ( ! ) operator, the entire condition in the if instruction will evaluate to true only if the shooting
variable has the value false : in other words, the ball is not currently shooting.
Inside the if instruction, you need to do a couple of things. You know the player has clicked
somewhere and that the ball has to be shot from the cannon. The first thing you need to do is set
the variable shooting to the correct value, because the status of the ball needs to be changed to
“currently shooting”:
ball.shooting = true;