分层结构的原则：the dependencies between subsystems are from the top down: layers pictured near the top depend on lower layers, but subsystems nearer the bottom do not depend on higher layers.
这种子系统之间的依赖性只能是从上到下，也就是图中top的子系统依赖bottom的子系统，反之则不行。
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发表于 2017-4-10 14:45:45