如何检测网页的加载进度。从而编写进度条

对于如何检测网页加载进度这个问题，百度了一下，没有好的答案。可以说是没有答案。但是找到了一个开源项目。
pace.js

如何想直接用现成的进度条。这个库很丰富，现在想要分析一下它的源代码来了解，如何检测网页加载进度，方便我们编写自己的js组件 。

pace.js 代码不多 。才1千行。

start: When pace is initially started, or as a part of a restart
stop: When pace is manually stopped, or as a part of a restart
restart: When pace is restarted (manually, or by a new AJAX request)
done: When pace is finished
hide: When the pace is hidden (can be later than done, based on ghostTime and minTime)

上面是它的进度条的生命周期。

经过分析，它的核心方法在go这个函数里。

 Pace.go = function() {    var start;    Pace.running = true;    bar.render();    start = now();    cancelAnimation = false;    return animation = runAnimation(function(frameTime, enqueueNextFrame) {      var avg, count, done, element, elements, i, j, remaining, scaler, scalerList, sum, _j, _k, _len1, _len2, _ref2;      remaining = 100 - bar.progress;      count = sum = 0;      done = true;      for (i = _j = 0, _len1 = sources.length; _j < _len1; i = ++_j) {        source = sources[i];        scalerList = scalers[i] != null ? scalers[i] : scalers[i] = [];        elements = (_ref2 = source.elements) != null ? _ref2 : [source];        for (j = _k = 0, _len2 = elements.length; _k < _len2; j = ++_k) {          element = elements[j];          scaler = scalerList[j] != null ? scalerList[j] : scalerList[j] = new Scaler(element);          done &= scaler.done;          if (scaler.done) {            continue;          }          count++;          sum += scaler.tick(frameTime);        }      }      avg = sum / count;      bar.update(uniScaler.tick(frameTime, avg));      if (bar.done() || done || cancelAnimation) {        bar.update(100);        Pace.trigger('done');        return setTimeout(function() {          bar.finish();          Pace.running = false;          return Pace.trigger('hide');        }, Math.max(options.ghostTime, Math.max(options.minTime - (now() - start), 0)));      } else {        return enqueueNextFrame();      }    });  };

因为在这个函数我们看到了，以下几个 关键方法的调用

     bar.render();     remaining = 100 - bar.progress;     bar.update(100);     Pace.trigger('done');     bar.finish();     Pace.running = false;     return Pace.trigger('hide');

表现了进度条从生成到隐藏。

那么推导出来关键的变量在于bar.progress的变化原因

找到了

 for (i = _j = 0, _len1 = sources.length; _j < _len1; i = ++_j) {        source = sources[i];        scalerList = scalers[i] != null ? scalers[i] : scalers[i] = [];        elements = (_ref2 = source.elements) != null ? _ref2 : [source];        for (j = _k = 0, _len2 = elements.length; _k < _len2; j = ++_k) {          element = elements[j];          scaler = scalerList[j] != null ? scalerList[j] : scalerList[j] = new Scaler(element);          done &= scaler.done;          if (scaler.done) {            continue;          }          count++;          sum += scaler.tick(frameTime);        }      }

看SOURCE_KEYS

SOURCE_KEYS = {
ajax: AjaxMonitor,
elements: ElementMonitor,
document: DocumentMonitor,
eventLag: EventLagMonitor
};
这是一个页面的组成部分。
上面的那个两层循环就是在循环它们的每个元素是否加载完成 。

他们每个监视器都有一个tracker

 function EventLagMonitor() {      var avg, interval, last, points, samples,        _this = this;      this.progress = 0;      avg = 0;      samples = [];      points = 0;      last = now();      interval = setInterval(function() {        var diff;        ...        if (++points >= options.eventLag.minSamples && avg < options.eventLag.lagThreshold) {          _this.progress = 100;          return clearInterval(interval);        } else {          return _this.progress = 100 * (3 / (avg + 3));        }      }, 50);    }

  XHRRequestTracker = (function() {    function XHRRequestTracker(request) {      var event, size, _j, _len1, _onreadystatechange, _ref2,        _this = this;      this.progress = 0;      if (window.ProgressEvent != null) {        size = null;        request.addEventListener('progress', function(evt) {          if (evt.lengthComputable) {            return _this.progress = 100 * evt.loaded / evt.total;          } else {            return _this.progress = _this.progress + (100 - _this.progress) / 2;          }        }, false);        _ref2 = ['load', 'abort', 'timeout', 'error'];        for (_j = 0, _len1 = _ref2.length; _j < _len1; _j++) {          event = _ref2[_j];          request.addEventListener(event, function() {            return _this.progress = 100;          }, false);        }      } else {

 SocketRequestTracker = (function() {    function SocketRequestTracker(request) {      var event, _j, _len1, _ref2,        _this = this;      this.progress = 0;      _ref2 = ['error', 'open'];      for (_j = 0, _len1 = _ref2.length; _j < _len1; _j++) {        event = _ref2[_j];        request.addEventListener(event, function() {          return _this.progress = 100;        }, false);      }    }    return SocketRequestTracker;

所以检测页面加载进度的思路如下

SOURCE_KEYS = {
ajax: AjaxMonitor,
elements: ElementMonitor,
document: DocumentMonitor,
eventLag: EventLagMonitor
};
获取以上类别的所有元素。为每个Monitor创建一个Tracker。然后为每个元素addeventListener(“progress”,function(){ 获取进度})。
然后遍历所有元素的进度。
如果加载完成就Continue然后去执行一下动画
否则就进度剩余的部分++;
在循环开始有一个done变量，与所有的元素的.done进行&=，如果最后结果是true，那么进度条就完成 ，可以hide了。
而这个检测进度的方法是不断调用自身的。直到done=true;