android-6-for-programmers-3rd

6.11 Cannon Class 233
38 // creates and fires Cannonball in the direction Cannon points
39 public void fireCannonball() {
40 // calculate the Cannonball velocity's x component
41 int velocityX = (int) (CannonView.CANNONBALL_SPEED_PERCENT *
42 view.getScreenWidth() * Math.sin(barrelAngle));
43
44 // calculate the Cannonball velocity's y component
45 int velocityY = (int) (CannonView.CANNONBALL_SPEED_PERCENT *
46 view.getScreenWidth() * -Math.cos(barrelAngle));
47
48 // calculate the Cannonball's radius
49 int radius = (int) (view.getScreenHeight() *
50 CannonView.CANNONBALL_RADIUS_PERCENT);
51
52 // construct Cannonball and position it in the Cannon
53 cannonball = new Cannonball(view, Color.BLACK,
54 CannonView.CANNON_SOUND_ID, -radius,
55 view.getScreenHeight() / 2 - radius, radius, velocityX,
56 velocityY);
57
58 cannonball.playSound(); // play fire Cannonball sound
59 }
60
Fig. 6.12 | Cannon method fireCannonball.
6.11.4 Method draw
The draw method (Fig. 6.13) draws the Cannon on the screen. We draw the Cannon in two
parts. First we draw the Cannon’s barrel, then the Cannon’s base.
61 // draws the Cannon on the Canvas
62 public void draw(Canvas canvas) {
63 // draw cannon barrel
64 canvas.drawLine(0, view.getScreenHeight() / 2, barrelEnd.x,
65 barrelEnd.y, paint);
66
67 // draw cannon base
68 canvas.drawCircle(0, (int) view.getScreenHeight() / 2,
69 (int) baseRadius, paint);
70 }
71
Fig. 6.13 | Cannon method draw.
Drawing the Cannon Barrel with Canvas Method drawLine
We use Canvas’s drawLine method to display the Cannon barrel (lines 64–65). This method
receives five parameters—the first four represent the x-y coordinates of the line’s start
and end, and the last is the Paint object specifying the line’s characteristics, such as its
thickness. Recall that paint was configured to draw the barrel with the thickness given in
the constructor (Fig. 6.10, line 25).