Step by Step 使用HTML5开发一个星际大战游戏（2）

浏览数：18 / 时间：2015年06月09日

HTML5 Canvas Game: 玩家飞船

本系列博文翻译自以下文章

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();
}