objective-c init方法的写法

Thus, we arrive at the rules that all stylish Objective-C programmers follow when writing initializers:

• If a class has several initializers, only one should do the real work. That method is known as the
  designated initializer. All other initializers should call, either directly or indirectly, the designated
  initializer.
• The designated initializer will call the superclass’s designated initializer before initializing its
  instance variables.
• If the designated initializer of your class has a different name than the designated initializer of
  its superclass, you must override the superclass’s designated initializer so that it calls the new
  designated initializer.
• If you have several initializers, clearly document which is the designated initializer in the header.

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