Objective-C 中，@class & #import 的区别
Key points:
"#Import" links the header file it contains. Everything in the header, including property definitions, method declarations and any imports in the header are made available. Import provides the actual definitions to the linker.
@class by contrast just tells the linker not to complain it has no definition for a class. It is a "contract" that you will provide a definition for the class at another point.
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