源代码：
destroy(destroyChild = true) {
super.destroy(destroyChild);
this._bitmap && this._bitmap.destroy();
this._text && this._text.destroy(destroyChild);
this._bitmap = null;
this._text = null;
this._clickHandler = null;
this._labelColors = this._sources = this._strokeColors = null;
}
destroy方法，对 _clickHandler 置空处理，如果this._clickHandler 不为null是，没有将hangdler回收
