JavaScript利用Canvas实现粒子动画倒计时

<canvas id="canvas-number"></canvas>
<canvas id="canvas-dots"></canvas>

body {
  background-color: #24282f;
  margin: 0;
  padding: 0;
}

canvas {
  position: absolute;
  top: 0;
  left: 0;
}

#canvas-number {
  width: 680px;
  height: 420px;
}

var numberStage,
  numberStageCtx,
  numberStageWidth = 680,
  numberStageHeight = 420,
  numberOffsetX,
  numberOffsetY,

  stage,
  stageCtx,
  stageWidth = window.innerWidth,
  stageHeight = window.innerHeight,
  stageCenterX = stageWidth / 2,
  stageCenterY = stageHeight / 2,

  countdownFrom = 3,
  countdownTimer = 2800,
  countdownRunning = true,

  number,
  dots = [],
  numberPixelCoordinates,
  circleRadius = 2,
  colors = ['61, 207, 236', '255, 244, 174', '255, 211, 218', '151, 211, 226'];

function init() {

  // 初始化canvas-number
  numberStage = document.getElementById("canvas-number");
  numberStageCtx = numberStage.getContext('2d');
  
  // 设置文字文本的窗口大小
  numberStage.width = numberStageWidth;
  numberStage.height = numberStageHeight;

  // 初始化canvas-dots和窗口大小
  stage = document.getElementById("canvas-dots");
  stageCtx = stage.getContext('2d');
  stage.width = stageWidth;
  stage.height = stageHeight;

  // 设置一定的偏移量，让文字居中
  numberOffsetX = (stageWidth - numberStageWidth) / 2;
  numberOffsetY = (stageHeight - numberStageHeight) / 2;
}

init();

for (var i = 0; i < 2240; i++) {
    var dot = new Dot(randomNumber(0, stageWidth), randomNumber(0, stageHeight), colors[randomNumber(1, colors.length)], .3);
    dots.push(dot);
    tweenDots(dot, '', 'space');
}

function Dot(x, y, color, alpha) {

  var _this = this;

  _this.x = x;
  _this.y = y;
  _this.color = color;
  _this.alpha = alpha;

  this.draw = function () {
    stageCtx.beginPath();
    stageCtx.arc(_this.x, _this.y, circleRadius, 0, 2 * Math.PI, false);
    stageCtx.fillStyle = 'rgba(' + _this.color + ', ' + _this.alpha + ')';
    stageCtx.fill();
  }

}

function countdown() {

  // 发送倒计时数字
  drawNumber(countdownFrom.toString());

  // 倒计时为 0 时停止
  if (countdownFrom === 0) {
    countdownRunning = false;
    drawNumber('蜡笔小心');
  }

  countdownFrom--;
}

countdown();

function drawNumber(num) {
  numberStageCtx.clearRect(0, 0, numberStageWidth, numberStageHeight);
  numberStageCtx.fillStyle = "#24282f";
  numberStageCtx.textAlign = 'center';
  numberStageCtx.font = "bold 118px Lato";
  numberStageCtx.fillText(num, 250, 300);

  var ctx = document.getElementById('canvas-number').getContext('2d');
  var imageData = ctx.getImageData(0, 0, numberStageWidth, numberStageHeight).data;
  numberPixelCoordinates = [];
  for (var i = imageData.length; i >= 0; i -= 4) {
    if (imageData[i] !== 0) {
      var x = (i / 4) % numberStageWidth;
      var y = Math.floor(Math.floor(i / numberStageWidth) / 4);
      if ((x && x % (circleRadius * 2 + 3) == 0) && (y && y % (circleRadius * 2 + 3) == 0)) {
        numberPixelCoordinates.push({
          x: x,
          y: y
        });
      }
    }
  }
  fORMNumber();
}

function formNumber() {
  for (var i = 0; i < numberPixelCoordinates.length; i++) {
    tweenDots(dots[i], numberPixelCoordinates[i], '');
  }

  if (countdownRunning && countdownFrom > 0) {
    setTimeout(function () {
      breakNumber();
    }, countdownTimer);
  }
}

function breakNumber() {
  for (var i = 0; i < numberPixelCoordinates.length; i++) {
    tweenDots(dots[i], '', 'space');
  }

  if (countdownRunning) {
    setTimeout(function () {
      countdown();
    }, countdownTimer);
  }

}

function loop() {
  stageCtx.clearRect(0, 0, stageWidth, stageHeight);
  for (var i = 0; i < dots.length; i++) {
    dots[i].draw(stageCtx);
  }
  requestAnimationFrame(loop);
}
loop();

function tweenDots(dot, pos, type) {
  if (type === 'space') {
    TweenMax.to(dot, (3 + Math.round(Math.random() * 100) / 100), {
      x: randomNumber(0, stageWidth),
      y: randomNumber(0, stageHeight),
      alpha: 0.3,
      ease: Cubic.easeInOut,
      onComplete: function () {
        tweenDots(dot, '', 'space');
      }
    });
  } else {
    TweenMax.to(dot, (1.5 + Math.round(Math.random() * 100) / 100), {
      x: (pos.x + numberOffsetX),
      y: (pos.y + numberOffsetY),
      delay: 0,
      alpha: 1,
      ease: Cubic.easeInOut,
      onComplete: function () {}
    });
  }
}

function randomNumber(min, max) {
  return Math.floor(Math.random() * (max - min) + min);
}