javascript - 对象方法内的 cancelAnimationFrame 不起作用

Question

cancelAnimationFrame() 在对象的方法内部调用时似乎不起作用。我已尝试将 this 值绑定(bind)到回调函数(as demonstrated on MDN 和 setTimeout)，但在使用 cancelAnimationFrame() 时收到 TypeError。然后，我尝试将 this 值设置为名为 _this 的局部变量，并再次调用 cancelAnimationFrame()。那一次，我没有收到错误，但动画本身仍在播放。如何取消动画？

我在下面重现了我遇到的问题。如果打开控制台窗口，您会看到动画仍在运行。

function WhyWontItCancel() {
  this.canvas = document.createElement("canvas");
  this.canvas.width = 200;
  this.canvas.height = 10;
  document.body.appendChild(this.canvas);
  this.draw = this.canvas.getContext("2d");
  this.draw.fillStyle = "#f00";
  this.position = 0;
};

WhyWontItCancel.prototype.play = function() {
  if (this.position <= 190) {
    this.draw.clearRect(0, 0, 400, 10);
    this.draw.fillRect(this.position, 0, 10, 10);
    this.position += 2;
  } else {
    //window.cancelAnimationFrame(this.animation.bind(this));
    var _this = this;
    window.cancelAnimationFrame(_this.animation);
    console.log("still running");
  }

  this.animation = window.requestAnimationFrame(this.play.bind(this));
};

var animation = new WhyWontItCancel();
animation.play();

Answer 1

似乎您在这里错过了两件事。首先，this.animation = window.requestAnimationFrame(this.play.bind(this)); 行在 play() 时被调用叫。与您的想法相反，cancelAnimationFrame 仅删除先前请求的 RAF 调用。严格来说，这里甚至没有必要。其次，您不必绑定(bind)每个 RAF 调用；你可能只做一次:

function AnimatedCanvas() {
  this.canvas = document.createElement("canvas");
  this.canvas.width = 200;
  this.canvas.height = 10;
  document.body.appendChild(this.canvas);
  this.draw = this.canvas.getContext("2d");
  this.draw.fillStyle = "#f00";
  this.position = 0;

  this.play = this.play.bind(this); // takes `play` from prototype object
};

AnimatedCanvas.prototype.play = function() {
  if (this.position <= 190) {
    this.draw.clearRect(0, 0, 400, 10);
    this.draw.fillRect(this.position, 0, 10, 10);
    this.position += 2;
    this.animationId = window.requestAnimationFrame(this.play);
  }
};

您可能希望将取消添加到您的原型(prototype)中以便能够停止您的动画，例如:

AnimatedCanvas.prototype.cancel = function() {
  if (this.animationId) {
    window.cancelAnimationFrame(this.animationId);
  }
};

...但关键是，它在问题中描述的用例中没有用。