Understand the class object meaning

Optimizations for autoRelease in ARC

Global block, stack block, heap block
Global blocks do not capture any state, and there is no need for any state to participate at runtime. The entire memory region used by the Block is fully determined during the mutation phase. Therefore, global blocks can be declared in global memory and do not need to be created on the stack each time they are used. In addition, the copy operation of a global block is an empty operation, because a global block can never be reclaimed by the system. This block is actually a singleton.
Do not use dispatch_get_CURRENT_queue

The initialize and load
The load method does not follow inheritance rules. If a class does not implement a load method, the system does not call it, regardless of whether its superclasses at all levels implement load. When there is load in both the class and the class, both types of code are called, with the class executing before the class. Initialize is lazy and is executed when the application uses the associated class. Otherwise, it will never be called.