A singleton is simply a class that is instantiated exactly once. Singletons typically represent a system component that is intrinsically unique, such as the window manager or file system. Making a class a singleton can make it difficult to test its clients, as it’s impossible to substitute a mock implementation for a singleton unless it implements an interface that serves as its type.

表示不理解啊。。

singleton如何向整个系统提供实例？