在 Canvas 中使用带有 requestAnimationFrame 的 JS 倒数计时器

Question

我正在制作一个简单的 canvas- 游戏，我正在为游戏循环使用 requestAnimationFrame（Paul Irish 的版本）。我在游戏中使用 javascript 倒计时（显示秒数和第 100 秒），但它没有以正确的速度倒计时。我知道这与游戏的刷新率有关，因此计数器每帧更新一次，而不是每 100 秒更新一次。我该如何解决？

这是计时器对象：

/**
 * The timer as an object
*/
function Timer(position, time) {
  this.position = position || new Vector(150,210);
  this.time = time || 6000;
}

Timer.prototype = {
  start: function() {
    console.log('start running');
    this.initial = this.time; //time in 100'ths of seconds
    this.count = this.initial;
    this.counter; //10 will  run it every 100th of a second
    clearInterval(this.counter);
    //this.counter = setInterval(this.timer, 10);
    this.timer();
  },

  timer: function() {
    //console.log(this.count);
    this.count--;
      var res = this.count / 100;
    //Show counter in canvas
    return 'Tid kvar: ' + res.toPrecision(this.count.toString().length) + ' sekunder';
  },

  draw: function(ct) {
    if(this.initial === undefined){this.start();} //Start the timer
    ct.save();
    if(this.count <=0){ //Remove timer if timer has reached 0
      ct.clearRect(this.position.x, this.position.y, Racetrack.innerTrackWidth, Racetrack.innerTrackHeight);
      return false;
    } else { //draw timer
      ct.save();
      ct.font = 'bold 3em arial';
      ct.fillStyle = 'orange';
      ct.fillText(this.timer(), this.position.x, this.position.y);
    }
    ct.restore();
  }, 
}

以及游戏循环和调用计时器的部分：

  var init = function(canvas) {
    timer = new Timer(new Vector(160,210), 3000);
  }

  var render = function() {
    ct.clearRect(0,0,width,height);
    ship.draw(ct);
    racetrack.draw(ct);
    //draw timer or results if timer reached 0
    timer.draw(ct) !=false ? timer.draw(ct) : endFrame.draw(ct);
  };

  var gameLoop = function() {
    var now = Date.now();
    td = (now - (lastGameTick || now)) / 1000; // Timediff since last frame / gametick
    lastGameTick = now;
    if(timer.draw(ct) == false) { //stop the game if timer has reached 0.
      cancelRequestAnimFrame(gameLoop);
      console.log('Time\'s up!');
    } else { //Otherwise, keep the game going.
      requestAnimFrame(gameLoop);
    }
    update(td);
    render();
  };

我也尝试过将其作为计时器对象，但调试 this.count 显示第一个循环的数字，未定义的第二个循环和之后的每个循环的 NaN（而且我不确定这是否会修复无论哪种方式的时间问题？）：

function Timer(position, time) {
  this.position = position || new Vector(150,210);
  this.time = time || 6000;
}

Timer.prototype = {
  start: function() {
    console.log('start running');
    this.initial = this.time; //time in 100'ths of seconds
    this.count = this.initial;
    this.counter; //10 will  run it every 100th of a second
    clearInterval(this.counter);
    this.counter = setInterval(this.timer, 10);
    this.timer();
  },


  timer: function() {
    console.log(this.count);
    this.count--;
  },

  getTime: function() {
    var res = this.count / 100;
    return 'Tid kvar: ' + res.toPrecision(this.count.toString().length) + ' sekunder';
  },

  draw: function(ct) {
    if(this.initial === undefined){this.start();} //Start the timer
    ct.save();
    if(this.count <=0){ //Remove timer if timer has reached 0
      ct.clearRect(this.position.x, this.position.y, Racetrack.innerTrackWidth, Racetrack.innerTrackHeight);
      return false;
    } else { //draw timer
      ct.save();
      ct.font = 'bold 3em arial';
      ct.fillStyle = 'orange';
      ct.fillText(this.getTime(), this.position.x, this.position.y);
    }
    ct.restore();
  },
}

Answer 1

不确定您是否要求以 1/100 间隔显示时间，或者使用 setInterval 时时间是否不准确。

答：setInterval 不应该用于计时，因为它远非准确，而且间隔越小越差，如果有动画则更差运行。

B：浏览器每 1/60 秒刷新一次。您可以强制显示器以 1/100 秒的速度呈现和呈现，但随后根据图形硬件，它可能永远不会显示，因为显示器当时正在扫描屏幕上其他地方的像素。或者在将图形写入显示器时覆盖显示器时会发生剪切。

使用 requestAnimationFrame 获得倒计时的最佳方式

var start = true;     // flags that you want the countdown to start
var stopIn = 3000;    // how long the timer should run
var stopTime = 0;     // used to hold the stop time
var stop = false;     // flag to indicate that stop time has been reached
var timeTillStop = 0; // holds the display time

// main update function
function update(timer){

    if(start){  // do we need to start the timer
        stopTime = timer + stopIn; // yes the set the stoptime
        start = false;             // clear the start flag
    }else{                         // waiting for stop
        if(timer >= stopTime){     // has stop time been reached?
            stop = true;           // yes the flag to stop
        }
    }

    timeTillStop = stopTime - timer;      // for display of time till stop
    // log() should be whatever you use to display the time.
    log(Math.floor(timeTillStop / 10) );  // to display in 1/100th seconds

    if(!stop){
        requestAnimationFrame(update); // continue animation until stop 
    }
}
requestAnimationFrame(update);  // start the animation

忘记补充了。

您永远无法通过该方法获得百分之一秒的精度。您将平均延迟 7-8 毫秒。但是除非你让它变得更复杂，否则这是你能做的最好的。 7-8ms 的平均值是恒定的，并且在 2 小时 1 秒内是相同的，并且仅由大约 16 ms 的动画刷新时间决定。

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