（译）开发第一个Html5游戏（三）

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这一章加入了图片滚动功能，已经实现了一个游戏对话框。

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JS代码：（完整代码请看文章结尾链接）

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// inner variables
var canvas, ctx;
var button;
var backgroundImage;
var spaceShip;
var iBgShiftX = 1024;
var bDrawDialog = true;
var iDialogPage = 1;
// -------------------------

// objects :
function Button(x, y, w, h, state, image) {
    this.x = x;
    this.y = y;
    this.w = w;
    this.h = h;
    this.state = state;
    this.imageShift = 0;
    this.image = image;
}
function SpaceShip(x, y, w, h, image) {
    this.x = x;
    this.y = y;
    this.w = w;
    this.h = h;
    this.image = image;
    this.bDrag = false;
}
// -------------------------

// draw functions :

function clear() { // clear canvas function
    ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
}

function drawDialog() { // draw dialog function
    if (bDrawDialog) {
        var bg_gradient = ctx.createLinearGradient(0, 200, 0, 400);
        bg_gradient.addColorStop(0.0, 'rgba(160, 160, 160, 0.8)');
        bg_gradient.addColorStop(1.0, 'rgba(250, 250, 250, 0.8)');

        ctx.beginPath(); // custom shape begin
        ctx.fillStyle = bg_gradient;
        ctx.moveTo(100, 100);
        ctx.lineTo(700, 100);
        ctx.lineTo(700, 500);
        ctx.lineTo(100, 500);
        ctx.lineTo(100, 100);
        ctx.closePath(); // custom shape end
        ctx.fill(); // fill custom shape

        ctx.lineWidth = 2;
        ctx.strokeStyle = 'rgba(128, 128, 128, 0.5)';
        ctx.stroke(); // draw border

        // draw the title text
        ctx.font = '42px DS-Digital';
        ctx.textAlign = 'center';
        ctx.textBaseline = 'top';
        ctx.shadowColor = '#000';
        ctx.shadowOffsetX = 2;
        ctx.shadowOffsetY = 2;
        ctx.shadowBlur = 2;
        ctx.fillStyle = '#fff';
        if (iDialogPage == 1) {
            ctx.fillText('Welcome to lesson #3', ctx.canvas.width/2, 150);
            ctx.font = '24px DS-Digital';
            ctx.fillText('After closing dialog you will able', ctx.canvas.width/2, 250);
            ctx.fillText('to handle with spaceship with your mouse', ctx.canvas.width/2, 280);
        } else if (iDialogPage == 2) {
            ctx.fillText('Second page of dialog', ctx.canvas.width/2, 150);
            ctx.font = '24px DS-Digital';
            ctx.fillText('Any another text', ctx.canvas.width/2, 250);
        }
    }
}

function drawScene() { // main drawScene function
    clear(); // clear canvas

    // draw background
    iBgShiftX -= 10;
    if (iBgShiftX <= 0) {
        iBgShiftX = 1024;
    }
    ctx.drawImage(backgroundImage, 0 + iBgShiftX, 0, 1024, 768, 0, 0, 800, 600);

    // draw space ship
    ctx.drawImage(spaceShip.image, 0, 0, spaceShip.w, spaceShip.h, spaceShip.x-128, spaceShip.y-128, spaceShip.w, spaceShip.h);

    // draw dialog
    drawDialog();

    // draw button
    ctx.drawImage(button.image, 0, button.imageShift, button.w, button.h, button.x, button.y, button.w, button.h);

    // draw button's text
    ctx.font = '22px DS-Digital';
    ctx.fillStyle = '#ffffff';
    ctx.fillText('next/hide/show', 400, 465);
    ctx.fillText('dialog', 400, 500);
}

// -------------------------

// initialization
$(function(){
    canvas = document.getElementById('scene');
    ctx = canvas.getContext('2d');

    var width = canvas.width;
    var height = canvas.height;

    // load background image
    backgroundImage = new Image();
    backgroundImage.src = 'images/stars.jpg';
    backgroundImage.