javascript - 请解释此代码段

Question

我在浏览 html5 canvas 示例时遇到了这段代码。我已经粘贴了代码，并在我有疑问的地方给出了评论。

if(window.addEventListener) {

  window.addEventListener('load', function () {
  var canvas, context, tool;

  function init () {
    canvas = document.getElementById('imageView');
    if (!canvas) {
      alert('Error: I cannot find the canvas element!');
      return;
    }

    if (!canvas.getContext) {
      alert('Error: no canvas.getContext!');
      return;
    }

     context = canvas.getContext('2d');
    if (!context) {
      alert('Error: failed to getContext!');
      return;
    }

        tool = new tool_pencil();

    canvas.addEventListener('mousedown', ev_canvas, false);
    canvas.addEventListener('mousemove', ev_canvas, false);
    canvas.addEventListener('mouseup',   ev_canvas, false);
  }

   function tool_pencil () {
    var tool = this;
    this.started = false;


    this.mousedown = function (ev) {
        context.beginPath();
        context.moveTo(ev._x, ev._y);
        tool.started = true;
    };

      this.mousemove = function (ev) {
      if (tool.started) {
        context.lineTo(ev._x, ev._y);
        context.stroke();
      }
    };

       this.mouseup = function (ev) {
      if (tool.started) {
        tool.mousemove(ev);
        tool.started = false;
      }
    };
  }

    function ev_canvas (ev) {
    if (ev.layerX || ev.layerX == 0) { 
      ev._x = ev.layerX;
      ev._y = ev.layerY;
    } else if (ev.offsetX || ev.offsetX == 0) { 
      ev._x = ev.offsetX;
      ev._y = ev.offsetY;
    }

    //Please explain the follwing set of line
    var func = tool[ev.type];
    if (func) {
      func(ev);
    }
  }

  init();

}, false); }

Answer 1

结合其他答案，它正在做的是将所有回调放在一个对象中。结果是func是tool.onmousedown, tool.onmouseup等