目录1、原来容易堵塞的代码2、采用异步模式的代码3、小结如题。之所以要采用异步方式，是因为外部接口不可控，无法预测。如果对方不在线，因而无法访问，只有靠超编程时抛出异常，容易造成堵塞。
I display elements in a hierarchy, clicking one displays the next set of elements in the hirearchy. Each element has a tag called \"level\" which has some value which is 1-.... (whatever the number of
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