code: function() {
  			var self = this;
  			self.cxt = null;
  			self.canvas.off(startEventName, self._startHandler);
  			self.canvas.off(moveEventName, self._moveHandler);
  			self.canvas.off(endEventName, self._endHandler);
  			self.holder.innerHTML = '';
  			self.canvas = null;
  		}
docs: 释放资源