开发第一个Html5游戏（二）

今天我们继续学习Html5系列游戏开发，我们会在第一章的基础上做一些扩展，加入颜色渐变、

绘制文本、自定义文本样式、以及动画，我们还将学习UI中一个非常重要的元素，按钮。

我们的上一篇文章: （译）开发第一个Html5游戏（一） .

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JS代码：（完整代码请看文章结尾链接）

var canvas, ctx;
var circles = [];
var selectedCircle;
var hoveredCircle;
var button;
var moving = false;
var speed = 2.0;

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

// objects :

function Circle(x, y, radius){
    this.x = x;
    this.y = y;
    this.radius = radius;
}

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;
}
// -------------------------

// draw functions :

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

function drawCircle(ctx, x, y, radius) { // draw circle function
    ctx.fillStyle = 'rgba(255, 35, 55, 1.0)';

    ctx.beginPath();
    ctx.arc(x, y, radius, 0, Math.PI*2, true);
    ctx.closePath();

    ctx.fill();

    ctx.lineWidth = 1;
    ctx.strokeStyle = 'rgba(0, 0, 0, 1.0)';
    ctx.stroke(); // draw border
}

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

    // 绘制文本
    ctx.font = '42px DS-Digital';
    ctx.textAlign = 'center';
    ctx.fillStyle = '#ffffff';
    ctx.fillText('Welcome to lesson #2', ctx.canvas.width/2, 50);

    //这里实现颜色渐变
    var bg_gradient = ctx.createLinearGradient(0, 200, 0, 400);
    bg_gradient.addColorStop(0.0, 'rgba(255, 0, 0, 0.8)');
    bg_gradient.addColorStop(0.5, 'rgba(0, 255, 0, 0.8)');
    bg_gradient.addColorStop(1.0, 'rgba(0, 0, 255, 0.8)');

    ctx.beginPath(); // custom shape begin
    ctx.fillStyle = bg_gradient;
    ctx.moveTo(circles[0].x, circles[0].y);
    for (var i=0; i<circles.length; i++) {
        ctx.lineTo(circles[i].x, circles[i].y);
    }
    ctx.closePath(); // custom shape end
    ctx.fill(); // fill custom shape

    ctx.lineWidth = 2;
    ctx.strokeStyle = 'rgba(0, 0, 255, 0.5)';
    ctx.stroke(); // draw border

    // reverting direction
    if (circles[0].x <= 300 || circles[0].x >= 385) {
        speed = -speed;
    }

    // 实现图像抖动
    if (moving) {
        circles[0].x -= speed;
        circles[0].y -= speed;
        circles[1].x += speed;
        circles[1].y -= speed;
        circles[2].x += speed;
        circles[2].y += speed;
        circles[3].x -= speed;
        circles[3].y += speed;
    }

    drawCircle(ctx, circles[0].x, circles[0].y, (hoveredCircle == 0) ? 25 : 15);
    drawCircle(ctx, circles[1].x, circles[1].y, (hoveredCircle == 1) ? 25 : 15);
    drawCircle(ctx, circles[2].x, circles[2].y, (hoveredCircle == 2) ? 25 : 15);
    drawCircle(ctx, circles[3].x, circles[3].y, (hoveredCircle == 3) ? 25 : 15);

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

    // draw text
    ctx.font = '30px DS-Digital';
    ctx.fillStyle = '#ffffff';
    ctx.fillText('Play/Pause', 135, 480);
    ctx.fillText(button.state, 135, 515);
}

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

// initialization

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

    var circleRadius = 15;
    var width = canvas.width;
    var height = canvas.height;

    // lets add 4 circles manually
    circles.push(new Circle(width / 2 - 20, height / 2 - 20, circleRadius));
    circles.push(new Circle(width / 2 + 20, height / 2 - 20, circleRadius));
    circles.push(new Circle(width / 2 + 20, height /