变量的值是＆＃34;未定义＆＃34;而且我不知道为什么

Question

我的程序有问题 - ＆＃34; this.imageManager.get（＆＃34; cowboy＆＃34;）＆＃34;回馈＆＃34; undefined＆＃34;而且我不知道为什么。

1.有ImageMenager构造函数的代码

function ImageManager(placeholderDataUri) {
  this._images = {};
  if (placeholderDataUri) {
    this._placeholder = new Image();
    this._placeholder.src = placeholderDataUri;
  }
}

_p = ImageManager.prototype;

_p.load = function(images, onDone, onProgress) {
  // The images queue
  var queue = [];
  for (var im in images) {
    queue.push({
      key: im,
      path: images[im]
    });
  }
  if (queue.length == 0) {
    onProgress && onProgress(0, 0, null, null, true);
    onDone && onDone();
    return;
  }

  var itemCounter = {
    loaded: 0,
    total: queue.length
  };

  for (var i = 0; i < queue.length; i++) {
    this._loadItem(queue[i], itemCounter, onDone, onProgress);
  }
};

_p._loadItem = function(queueItem, itemCounter, onDone, onProgress) {
  var self = this;
  var img = new Image();

  img.onload = function() {
    self._images[queueItem.key] = img;
    self._onItemLoaded(queueItem, itemCounter, onDone, onProgress, true);
  };

  img.onerror = function() {
    self._images[queueItem.key] = self._placeholder ? self._placeholder : null;
    self._onItemLoaded(queueItem, itemCounter, onDone, onProgress, false);
  };
  img.src = queueItem.path;
};

_p._onItemLoaded = function(queueItem, itemCounter, onDone, onProgress, success) {
  itemCounter.loaded++;
  onProgress && onProgress(itemCounter.loaded, itemCounter.total, queueItem.key, queueItem.path, success);
  if (itemCounter.loaded == itemCounter.total) {
    onDone && onDone();
  }
};

/**
 * Returms the loaded image by the given value
 * @param key image alias
 */
_p.get = function(key) {
  return this._images[key];
};

2。有一个Renderer构造函数的代码

function Renderer() {
  var self = this;
  this.readyForDrawing = false;

  //przygotowanie imageManager do pracy 
  this.imageManager = new ImageManager();
  this.imageManager.load({
    "cowboy": "cowboy.png"
  }, this._change());
}

_p = Renderer.prototype;

_p.draw = function(ComponentObj) {
  var object = ComponentObj;
  alert(this.imageManager.get("cowboy")); //there is alert which display    "undefined"
  ctx.drawImage(this.imageManager.get("cowboy"), object.x, object.y, object.width, object.height);
};

_p._change = function() {
  this.readyForDrawing = true;
};

当renderer.readyForDrawing具有true值但是发生错误时，我创建了对象渲染器和调用函数renderer.draw（） -

  

未捕获的TypeError：无法执行＆＃39; drawImage＆＃39; on＆＃39; CanvasRenderingContext2D＆＃39;：提供的值不是＆＃39;（HTMLImageElement或HTMLVideoElement或HTMLCanvasElement或ImageBitmap）＆＃39;

this.imageManager.get("cowboy")的值未定义且我不知道为什么（我正在使用函数ImageManager一段时间，一切都很好）。