首先看几张效果图:
上面三张图分别对应游戏的三种状态 ready,play,pause。体验一下
先介绍一下canvas 画图的原理,在这个游戏中的背景,飞机,子弹以及飞机被击中爆炸的效果都是一张张的图片,通过canvas的 drawImage() 函数把这一帧需要的所有图片按其所在的位置(坐标)画到画布上,当然有时候也需要画些文本,比如左上角的得分;然后接着画下一帧,同时改变飞机和子弹的位置;画下一帧之前一定要清除画布(通过这个函数 clearRect(x, y, width, height)),不然就是下图的效果啦:
辣眼睛!!!
不过在本例中因为每帧都要重新画上背景图,背景图又是填满整个画布的,所以画背景图时就等于把上一帧全部覆盖了,也就相当于清除画布了。
下面我们开始聊实现的细节:
加载需要的图片
在让游戏跑起来之前要先把需要的图片加载进来,类似:
代码如下:
1 // 所以图片的链接,包括背景图、各种飞机和飞机爆炸图、子弹图等
2 var imgName = [‘background.png‘, ‘game_pause_nor.png‘, ‘m1.png‘, ‘start.png‘,
3 // 敌机1
4 [‘enemy1.png‘, ‘enemy1_down1.png‘, ‘enemy1_down2.png‘, ‘enemy1_down3.png‘, ‘enemy1_down4.png‘],
5 // 敌机2
6 [‘enemy2.png‘, ‘enemy2_down1.png‘, ‘enemy2_down2.png‘, ‘enemy2_down3.png‘, ‘enemy2_down4.png‘],
7 // 敌机3
8 [‘enemy3_n1.png‘, ‘enemy3_n2.png‘, ‘enemy3_hit.png‘, ‘enemy3_down1.png‘, ‘enemy3_down2.png‘, ‘enemy3_down3.png‘, ‘enemy3_down4.png‘, ‘enemy3_down5.png‘, ‘enemy3_down6.png‘, ],
9 // 游戏loading图
10 [‘game_loading1.png‘, ‘game_loading2.png‘, ‘game_loading3.png‘, ‘game_loading4.png‘],
11 // 玩家飞机图
12 [‘hero1.png‘, ‘hero2.png‘, ‘hero_blowup_n1.png‘, ‘hero_blowup_n2.png‘, ‘hero_blowup_n3.png‘, ‘hero_blowup_n4.png‘]
13 ];
14 // 存储不同类型的图片
15 var bg = null,
16 pause = null,
17 m = null,
18 startImg = null,
19 enemy1 = [],
20 enemy2 = [],
21 enemy3 = [],
22 gameLoad = [],
23 heroImg = [];
24 // 加载图片的进度
25 var progress = 1;
26 /*********加载图片*********/
27 function download() {
28 bg = nImg(imgName[0]);
29 pause = nImg(imgName[1]);
30 m = nImg(imgName[2]);
31 startImg = nImg(imgName[3]);
32 for (var i = 0; i < imgName[4].length; i++) {
33 enemy1[i] = nImg(imgName[4][i]);
34 }
35 for (var i = 0; i < imgName[5].length; i++) {
36 enemy2[i] = nImg(imgName[5][i]);
37 }
38 for (var i = 0; i < imgName[6].length; i++) {
39 enemy3[i] = nImg(imgName[6][i]);
40 }
41 for (var i = 0; i < imgName[7].length; i++) {
42 gameLoad[i] = nImg(imgName[7][i]);
43 }
44 for (var i = 0; i < imgName[8].length; i++) {
45 heroImg[i] = nImg(imgName[8][i]);
46 }
47
48 function nImg(src) {
49 var img = new Image();
50 img.src = ‘img/‘ + src;
51 img.onload = imgLoad;
52 return img;
53 }
54 // 绘制游戏加载进度画面
55 function imgLoad() {
56 progress += 3;
57 ctx.clearRect(0, 0, canvas.width, canvas.height);
58 var text = progress + ‘%‘;
59 var tw = ctx.measureText(text).width;
60 ctx.font = ‘60px arial‘;
61 ctx.fillStyle = ‘red‘;
62 ctx.lineWidth = ‘0‘;
63 ctx.strokeStyle = ‘#888‘;
64 //ctx.strokeText(text,(width-tw)/2,height/2);
65 ctx.fillText(text, (width - tw) / 2, height / 2);
66 if (progress >= 100) {
67 start();
68 }
69 }
70 }
71 download();
其中有处理图片分类和加载进度的问题,代码有些冗余。
让背景动起来
从上面的游戏ready状态图可以看出游戏背景在不停的往上移动;
实现原理:连续画两张背景图到画布上,一上一下,第一张画在坐标为(0,0) 的位置,第二张紧接着第一张,然后每画一帧往上移动一点(一到两个像素吧),当上面的那张图片移出画布之后,将Y轴的坐标重置为0;代码如下:
1 var y = 0;
2 function paintBg() {
3 ctx.drawImage(bg, 0, y); // bg是背景图元素
4 ctx.drawImage(bg, 0, y - 852);
5 y++ == 852 && (y = 0);
6 }
构造玩家飞机(hero)
1 /*********构造hero************/
2 var hero = null;
3
4 function Hero() {
5 this.x = (width - heroImg[0].width) / 2; // hero的坐标
6 this.y = height - heroImg[0].height;
7 this.index = 0; // 用于切换hero的图片
8 this.count = 0; // 用于控制hero图片切换的频率
9 this.hCount = 0; // 用于控制子弹发射的频率
10 this.eCount = 0; // 用于控制敌机出现的频率
11 this.n = 0;
12 this.draw = function() {
13 ctx.drawImage(heroImg[this.index], this.x, this.y);
14 ctx.fillText(‘SCORE:‘ + gameScore, 10, 30);
15 this.count++;
16 if (this.count % 3 == 0) { // 切换hero的图片
17 this.index = this.index == 0 ? 1 : 0;
18 this.count = 0;
19 }
20 this.hCount++;
21 if (this.hCount % 3 == 0) { // 同时生成三颗子弹
22 this.n == 32 && (this.n = 0);
23 hullet.push(new Hullet(this.n));
24 this.n == 0 && (this.n = -32);;
25 hullet.push(new Hullet(this.n));
26 this.n == -32 && (this.n = 32);;
27 hullet.push(new Hullet(this.n));
28 this.hCount = 0;
29 }
30 this.eCount++;
31 if (this.eCount % 8 == 0) { //生成敌机
32 liveEnemy.push(new Enemy());
33 this.eCount = 0;
34 }
35 }
36
37 function move(e) {
38 if (curPhase == PHASE_PLAY || curPhase == PHASE_PAUSE) {
39 curPhase = PHASE_PLAY;
40 var offsetX = e.offsetX || e.touches[0].pageX;
41 var offsetY = e.offsetY || e.touches[0].pageY;
42 var w = heroImg[0].width,
43 h = heroImg[0].height;
44 var nx = offsetX - w / 2,
45 ny = offsetY - h / 2;
46 nx < 20 - w / 2 ? nx = 20 - w / 2 : nx > (canvas.width - w / 2 - 20) ? nx = (canvas.width - w / 2 - 20) : 0;
47 ny < 0 ? ny = 0 : ny > (canvas.height - h / 2) ? ny = (canvas.height - h / 2) : 0;
48 hero.x = nx;
49 hero.y = ny;
50 hero.count = 2;
51 }
52 }
53 // 绑定鼠标移动和手指触摸事件,控制hero移动
54 canvas.addEventListener("mousemove", move, false);
55 canvas.addEventListener("touchmove", move, false);
56 // 鼠标移除时游戏暂停
57 canvas.onmouseout = function(e) {
58 if (curPhase == PHASE_PLAY) {
59 curPhase = PHASE_PAUSE;
60 }
61 }
62 }
本例中并没有设置hero的碰撞检测和生命值,所以英雄无敌!!!哈哈哈哈!!!
然并卵,我已经写不下去了;可是,坚持就是胜利呀;好吧,继续!
构造子弹
1 /**********构造子弹***********/
2 var hullet = []; // 存储画布中所以子弹的数组
3
4 function Hullet(n) {
5 this.n = n; // 用于确定是左中右哪一颗子弹
6 // 子弹的坐标
7 this.mx = hero.x + (heroImg[0].width - m.width) / 2 + this.n;
8 this.my = this.n == 0 ? hero.y - m.height : hero.y + m.height;
9 this.width = m.width; // 子弹的宽和高
10 this.height = m.height;
11 this.removable = false; // 标识子弹是否可移除了
12 }
13 Hullet.drawHullet = function() {
14 for (var i = 0; i < hullet.length; i++) { //在画布上画出所以子弹
15 hullet[i].draw();
16 if (hullet[i].removable) { // 如果为true就移除这颗子弹
17 hullet.splice(i, 1);
18 }
19 }
20 }
21 Hullet.prototype.draw = function() { // 在画布上画子弹
22 ctx.drawImage(m, this.mx, this.my);
23 this.my -= 20;
24 this.mx += this.n == 32 ? 3 : this.n == -32 ? -3 : 0;
25 if (this.my < -m.height) { // 如果子弹飞出画布,就标记为可移除
26 this.removable = true;
27 };
28 }
构造敌机
1 /***********构造敌机********/
2 var liveEnemy = []; // 用于存储画布上的所有敌机
3
4 function Enemy() {
5 this.n = Math.random() * 20;
6 this.enemy = null; // 保存敌机图片的数组
7 this.speed = 0; // 敌机的速度
8 this.lifes = 2; // 敌机的生命值
9 if (this.n < 1) { // 不同大小的敌机随机出现
10 this.enemy = enemy3[0];
11 this.speed = 2;
12 this.lifes = 50;
13 } else if (this.n < 6) {
14 this.enemy = enemy2[0];
15 this.speed = 4;
16 this.lifes = 10;
17 } else {
18 this.enemy = enemy1[0];
19 this.speed = 6;
20 }
21 this.x = parseInt(Math.random() * (canvas.width - this.enemy.width));
22 this.y = -this.enemy.height;
23 this.width = this.enemy.width;
24 this.height = this.enemy.height;
25 this.index = 0;
26 this.removable = false;
27 // 标识敌机是否狗带,若狗带就画它的爆炸图(也就是遗像啦)
28 this.die = false;
29 this.draw = function() {
30 // 处理不同敌机的爆炸图轮番上阵
31 if (this.speed == 2) {
32 if (this.die) {
33 if (this.index < 2) { this.index = 3; }
34 if (this.index < enemy3.length) {
35 this.enemy = enemy3[this.index++];
36 } else {
37 this.removable = true;
38 }
39 } else {
40 this.enemy = enemy3[this.index];
41 this.index == 0 ? this.index = 1 : this.index = 0;
42 }
43 } else if (this.die) {
44 if (this.index < enemy1.length) {
45 if (this.speed == 6) {
46 this.enemy = enemy1[this.index++];
47 } else {
48 this.enemy = enemy2[this.index++];
49 }
50 } else {
51 this.removable = true;
52 }
53 }
54 ctx.drawImage(this.enemy, this.x, this.y);
55 this.y += this.speed; // 移动敌机
56 this.hit(); //判断是否击中敌机
57 if (this.y > canvas.height) { // 若敌机飞出画布,就标识可移除(让你不长眼!)
58 this.removable = true;
59 }
60 }
61 this.hit = function() { //判断是否击中敌机
62 for (var i = 0; i < hullet.length; i++) {
63 var h = hullet[i];
64 // 敌机与子弹的碰撞检测,自己体会吧
65 if (this.x + this.width >= h.mx && h.mx + h.width >= this.x &&
66 h.my + h.height >= this.y && this.height + this.y >= h.my) {
67 if (--this.lifes == 0) { // 若生命值为零,标识为死亡
68 this.die = true;
69 // 计分
70 gameScore += this.speed == 6 ? 10 : this.speed == 4 ? 20 : 100;
71 }
72 h.removable = true; // 碰撞后的子弹标识为可移除
73 }
74 }
75 }
76 }
游戏的几种状态
1 /********定义游戏状态***********/
2 const PHASE_DOWNLOAD = 1;
3 const PHASE_READY = 2;
4 const PHASE_LOADING = 3;
5 const PHASE_PLAY = 4;
6 const PHASE_PAUSE = 5;
7 const PHASE_GAMEOVER = 6;
8 /**********游戏当前状态************/
9 var curPhase = PHASE_DOWNLOAD;
有了状态,我只需要起一个定时器,判断游戏的状态,绘制对应的帧就行;像这样:
/**********游戏主引擎*********/
function gameEngine() {
switch (curPhase) {
case PHASE_READY:
pBg();
paintLogo();
break;
case PHASE_LOADING:
pBg();
load();
break;
case PHASE_PLAY:
pBg();
drawEnemy();
Hullet.drawHullet();
hero.draw();
break;
case PHASE_PAUSE:
drawPause();
break;
}
//requestAnimationFrame(gameEngine);
}
setInterval(gameEngine, 50);
代码见:github