有向图编辑器 有向图中涉及到节点和有向边，分别用Node和Arrow来建模，考虑到Node和Arrow要被多个其他对象调用，直接处理开销比较大。可以利用设计模式中的代理模式创建NodeProxy和ArrowProxy实现对Node和Arrow的操作。-Directed graph editor Involved in a directed graph with nodes and directed edges, respectively, to the Node and Arrow mo