Step by Step 使用HTML5开发一个星际大战游戏（2）
HTML5 Canvas Game: 玩家飞船
http://blog.sklambert.com/html5-canvas-game-the-player-ship/
Languages: HTML5, JavaScript
Code: https://github.com/straker/galaxian-canvas-game/tree/master/part2
2.玩家飞船
上节，我们讲述了背景滚动的实现，本节我们来实现一下玩家飞船的移动和子弹的发射。
首先让我们看一下本次教程的效果，如果界面没有反应，确认您鼠标点击了下面的界面。
控制: 移动 – 上下左右箭头
射击 – 空格键
The HTML Page
再次，我们先看html页面代码：
<!DOCTYPE html>
<html>
<head>
<title>Space Shooter Demo</title>
<style>
canvas {
position: absolute;
top: 0px;
left: 0px;
background: transparent;
}
#background {
z-index: -2;
}
#main {
z-index: -1;
}
#ship {
z-index: 0;
}
</style>
</head>
<body>
<!-- The canvas for the panning background -->
<canvas id="background" width="600" height="360">
Your browser does not support canvas. Please try again with a different browser.
</canvas>
<!-- The canvas for all enemy ships and bullets -->
<canvas id="main" width="600" height="360">
</canvas>
<!-- The canvas the ship uses (can only move up
one forth the screen.) -->
<canvas id="ship" width="600" height="360">
</canvas>
<script src="space_shooter_part_two.js"></script>
</body>
</html>
与上一节的html文件相比，一些地方放生了变化，这里我们使用了3个画布（canvas），一个用来画背景，一个用来画飞船，一个用来画子弹和敌机，这样做的好处是我们不用每次都重绘游戏的每一帧。通过css定义了这三个图层的z序。
Drawable
首先，Drawable对象的init方法需要改变，代码如下：
function Drawable() {
this.init = function(x, y, width, height) {
// Defualt variables
this.x = x;
this.y = y;
this.width = width;
this.height = height;
}
}
因为我们的图像不再充满整个画布，所以我们补充了width, height 2个属性。
The imageRepository
图像仓库对象，也有改动，代码如下：
/**
* Define an object to hold all our images for the game so images
* are only ever created once. This type of object is known as a
* singleton.
*/
var imageRepository = new function() {
// Define images
this.background = new Image();
this.spaceship = new Image();
this.bullet = new Image();
// Ensure all images have loaded before starting the game
var numImages = 3;
var numLoaded = 0;
function imageLoaded() {
numLoaded++;
if (numLoaded === numImages) {
window.init();
}
}
this.background.onload = function() {
imageLoaded();
}
this.spaceship.onload = function() {
imageLoaded();
}
this.bullet.onload = function() {
imageLoaded();
}
// Set images src
this.background.src = "imgs/bg.png";
this.spaceship.src = "imgs/ship.png";
this.bullet.src = "imgs/bullet.png";
}
加载三张图片，背景，飞船，子弹。三张图片都加载完成后，调用window.init();开始游戏。玩家飞船
接下来我们将讨论飞船的移动和子弹的发射。
首先，我们先了解一个新的数据结构，对象池。
对于一些可能需要被频繁创建和销毁的对象，我们可以将其缓存起来，重复使用，以减少系统的开销，提高游戏运行的速度，这里我们的子弹就属于这种对象。
/**
* Custom Pool object. Holds Bullet objects to be managed to prevent
* garbage collection.
*/
function Pool(maxSize) {
var size = maxSize; // Max bullets allowed in the pool
var pool = [];
/*
* Populates the pool array with Bullet objects
*/
this.init = function() {
for (var i = 0; i < size; i++) {
// Initalize the bullet object
var bullet = new Bullet();
bullet.init(0,0, imageRepository.bullet.width,
imageRepository.bullet.height);
pool[i] = bullet;
}
};
/*
* Grabs the last item in the list and initializes it and
* pushes it to the front of the array.
*/
this.get = function(x, y, speed) {
if(!pool[size - 1].alive) {
pool[size - 1].spawn(x, y, speed);
pool.unshift(pool.pop());
}
};
/*
* Used for the ship to be able to get two bullets at once. If
* only the get() function is used twice, the ship is able to
* fire and only have 1 bullet spawn instead of 2.
*/
this.getTwo = function(x1, y1, speed1, x2, y2, speed2) {
if(!pool[size - 1].alive &&
!pool[size - 2].alive) {
this.get(x1, y1, speed1);
this.get(x2, y2, speed2);
}
};
/*
* Draws any in use Bullets. If a bullet goes off the screen,
* clears it and pushes it to the front of the array.
*/
this.animate = function() {
for (var i = 0; i < size; i++) {
// Only draw until we find a bullet that is not alive
if (pool[i].alive) {
if (pool[i].draw()) {
pool[i].clear();
pool.push((pool.splice(i,1))[0]);
}
}
else
break;
}
};
}
当对象池被初始化时，建立了一个给定个数的子弹对象数组，每当需要一个新的子弹的时候，对象池查看数组的最后一个元素，看该元素是否当前被占用。如果被占用，说明当前对象池已经满了，无法产生新子弹；如果没被占用，
取出最后一个子弹元素，放到数组的最前面。这样使得对象池中总是后面部分存放着待使用的子弹，而前面存放着正在使用的子弹（屏幕上看到的）。
当对象池animates子弹的时候，判断当前子弹是否还在屏幕中（draw方法返回false），如果draw方法返回true，说明该子弹已经脱离屏幕区域，可以被回收供再利用）。
现在对象池对象已经准备好，我们开始创建子弹对象，如下面代码：
/**
* Creates the Bullet object which the ship fires. The bullets are
* drawn on the "main" canvas.
*/
function Bullet() {
this.alive = false; // Is true if the bullet is currently in use
/*
* Sets the bullet values
*/
this.spawn = function(x, y, speed) {
this.x = x;
this.y = y;
this.speed = speed;
this.alive = true;
};
/*
* Uses a "drity rectangle" to erase the bullet and moves it.
* Returns true if the bullet moved off the screen, indicating that
* the bullet is ready to be cleared by the pool, otherwise draws
* the bullet.
*/
this.draw = function() {
this.context.clearRect(this.x, this.y, this.width, this.height);
this.y -= this.speed;
if (this.y <= 0 - this.height) {
return true;
}
else {
this.context.drawImage(imageRepository.bullet, this.x, this.y);
}
};
/*
* Resets the bullet values
*/
this.clear = function() {
this.x = 0;
this.y = 0;
this.speed = 0;
this.alive = false;
};
}
Bullet.prototype = new Drawable();
子弹对象初始化状态设置alive为false，子弹对象包含三个方法spawn，draw和clear，仔细看draw方法可以看到if (this.y <= 0 - this.height)，代表子弹已经在屏幕中消失，这里返回的true。
脏矩形
在刚才的draw方法中，我们使用了一个被称作脏矩形的技术，我们仅仅清除（clearRect）子弹前一瞬间的矩形区域，而不是整个屏幕。如果子弹离开了屏幕，draw方法返回true，指示该子弹对象可以被再回收；否则我们重绘它。
脏矩形的使用是我们开始创建3个画布的原因。如果我们只有一个画布，我们不得不在每一帧中，重绘所有的对象，屏幕背景，子弹，飞船。如果我们使用2个画布，背景一个画布，子弹和飞船共用一个，那么子弹每一帧将被重绘，而飞船只有移动的时候才被重绘，如果子弹和飞船发生重叠，子弹的区域的清除将使得飞船不再完整，直到我们移动飞船，这将大大地影响可视效果。所以这里我们采用3个画布。
飞船对象
我们最后要创建的是飞船对象，代码如下：
/**
* Create the Ship object that the player controls. The ship is
* drawn on the "ship" canvas and uses dirty rectangles to move
* around the screen.
*/
function Ship() {
this.speed = 3;
this.bulletPool = new Pool(30);
this.bulletPool.init();
var fireRate = 15;
var counter = 0;
this.draw = function() {
this.context.drawImage(imageRepository.spaceship, this.x, this.y);
};
this.move = function() {
counter++;
// Determine if the action is move action
if (KEY_STATUS.left || KEY_STATUS.right ||
KEY_STATUS.down || KEY_STATUS.up) {
// The ship moved, so erase it's current image so it can
// be redrawn in it's new location
this.context.clearRect(this.x, this.y, this.width, this.height);
// Update x and y according to the direction to move and
// redraw the ship. Change the else if's to if statements
// to have diagonal movement.
if (KEY_STATUS.left) {
this.x -= this.speed
if (this.x <= 0) // Keep player within the screen
this.x = 0;
} else if (KEY_STATUS.right) {
this.x += this.speed
if (this.x >= this.canvasWidth - this.width)
this.x = this.canvasWidth - this.width;
} else if (KEY_STATUS.up) {
this.y -= this.speed
if (this.y <= this.canvasHeight/4*3)
this.y = this.canvasHeight/4*3;
} else if (KEY_STATUS.down) {
this.y += this.speed
if (this.y >= this.canvasHeight - this.height)
this.y = this.canvasHeight - this.height;
}
// Finish by redrawing the ship
this.draw();
}
if (KEY_STATUS.space && counter >= fireRate) {
this.fire();
counter = 0;
}
};
/*
* Fires two bullets
*/
this.fire = function() {
this.bulletPool.getTwo(this.x+6, this.y, 3,
this.x+33, this.y, 3);
};
}
Ship.prototype = new Drawable();
飞船对象设置自己的移动速度为3， 子弹对象池大小为30，开火帧率为15。
KEY_STATUS对象的代码如下：
// The keycodes that will be mapped when a user presses a button.
// Original code by Doug McInnes
KEY_CODES = {
32: 'space',
37: 'left',
38: 'up',
39: 'right',
40: 'down',
}
// Creates the array to hold the KEY_CODES and sets all their values
// to false. Checking true/flase is the quickest way to check status
// of a key press and which one was pressed when determining
// when to move and which direction.
KEY_STATUS = {};
for (code in KEY_CODES) {
KEY_STATUS[ KEY_CODES[ code ]] = false;
}
/**
* Sets up the document to listen to onkeydown events (fired when
* any key on the keyboard is pressed down). When a key is pressed,
* it sets the appropriate direction to true to let us know which
* key it was.
*/
document.onkeydown = function(e) {
// Firefox and opera use charCode instead of keyCode to
// return which key was pressed.
var keyCode = (e.keyCode) ? e.keyCode : e.charCode;
if (KEY_CODES[keyCode]) {
e.preventDefault();
KEY_STATUS[KEY_CODES[keyCode]] = true;
}
}
/**
* Sets up the document to listen to ownkeyup events (fired when
* any key on the keyboard is released). When a key is released,
* it sets teh appropriate direction to false to let us know which
* key it was.
*/
document.onkeyup = function(e) {
var keyCode = (e.keyCode) ? e.keyCode : e.charCode;
if (KEY_CODES[keyCode]) {
e.preventDefault();
KEY_STATUS[KEY_CODES[keyCode]] = false;
}
}
最后的步骤
最后我们需要更新游戏对象以及动画功能，代码如下：
/**
* Creates the Game object which will hold all objects and data for
* the game.
*/
function Game() {
/*
* Gets canvas information and context and sets up all game
* objects.
* Returns true if the canvas is supported and false if it
* is not. This is to stop the animation script from constantly
* running on browsers that do not support the canvas.
*/
this.init = function() {
// Get the canvas elements
this.bgCanvas = document.getElementById('background');
this.shipCanvas = document.getElementById('ship');
this.mainCanvas = document.getElementById('main');
// Test to see if canvas is supported. Only need to
// check one canvas
if (this.bgCanvas.getContext) {
this.bgContext = this.bgCanvas.getContext('2d');
this.shipContext = this.shipCanvas.getContext('2d');
this.mainContext = this.mainCanvas.getContext('2d');
// Initialize objects to contain their context and canvas
// information
Background.prototype.context = this.bgContext;
Background.prototype.canvasWidth = this.bgCanvas.width;
Background.prototype.canvasHeight = this.bgCanvas.height;
Ship.prototype.context = this.shipContext;
Ship.prototype.canvasWidth = this.shipCanvas.width;
Ship.prototype.canvasHeight = this.shipCanvas.height;
Bullet.prototype.context = this.mainContext;
Bullet.prototype.canvasWidth = this.mainCanvas.width;
Bullet.prototype.canvasHeight = this.mainCanvas.height;
// Initialize the background object
this.background = new Background();
this.background.init(0,0); // Set draw point to 0,0
// Initialize the ship object
this.ship = new Ship();
// Set the ship to start near the bottom middle of the canvas
var shipStartX = this.shipCanvas.width/2 - imageRepository.spaceship.width;
var shipStartY = this.shipCanvas.height/4*3 + imageRepository.spaceship.height*2;
this.ship.init(shipStartX, shipStartY, imageRepository.spaceship.width,
imageRepository.spaceship.height);
return true;
} else {
return false;
}
};
// Start the animation loop
this.start = function() {
this.ship.draw();
animate();
};
}
/**
* The animation loop. Calls the requestAnimationFrame shim to
* optimize the game loop and draws all game objects. This
* function must be a gobal function and cannot be within an
* object.
*/
function animate() {
requestAnimFrame( animate );
game.background.draw();
game.ship.move();
game.ship.bulletPool.animate();
}
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