通过设置环境变量，我们可以快速的查看项目的配置情况
设置环境变量
创建LGPerson和LGTeacher两个类，并实现load方法
Command + R执行打印日志
当我们在自己创建的类中实现了load方法时，在程序运行过程中就会打印实现了load方法的类，通过这种方法我们可以快速的定位项目中实现了load方法的类.
常用环境变量
DYLD_PRINT_STATISTICS : 监控并打印应用启动时间
OBJC_PRINT_LOAD_METHODS:打印实现load方法的类
OBJC_PRINT_IMAGES: log image and library names as they are loaded
OBJC_PRINT_IMAGE_TIMES: measure duration of image loading steps
OBJC_PRINT_LOAD_METHODS: log calls to class and category +load methods
OBJC_PRINT_INITIALIZE_METHODS: log calls to class +initialize methods
OBJC_PRINT_RESOLVED_METHODS: log methods created by +resolveClassMethod: and +resolveInstanceMethod:
OBJC_PRINT_CLASS_SETUP: log progress of class and category setup
OBJC_PRINT_PROTOCOL_SETUP: log progress of protocol setup
OBJC_PRINT_IVAR_SETUP: log processing of non-fragile ivars
OBJC_PRINT_VTABLE_SETUP: log processing of class vtables
OBJC_PRINT_VTABLE_IMAGES: print vtable images showing overridden methods
OBJC_PRINT_CACHE_SETUP: log processing of method caches
OBJC_PRINT_FUTURE_CLASSES: log use of future classes for toll-free bridging
OBJC_PRINT_PREOPTIMIZATION: log preoptimization courtesy of dyld shared cache
OBJC_PRINT_CXX_CTORS: log calls to C++ ctors and dtors for instance variables
OBJC_PRINT_EXCEPTIONS: log exception handling
OBJC_PRINT_EXCEPTION_THROW: log backtrace of every objc_exception_throw()
OBJC_PRINT_ALT_HANDLERS: log processing of exception alt handlers
OBJC_PRINT_REPLACED_METHODS: log methods replaced by category implementations
OBJC_PRINT_DEPRECATION_WARNINGS: warn about calls to deprecated runtime functions
OBJC_PRINT_POOL_HIGHWATER: log high-water marks for autorelease pools
OBJC_PRINT_CUSTOM_CORE: log classes with custom core methods
OBJC_PRINT_CUSTOM_RR: log classes with custom retain/release methods
OBJC_PRINT_CUSTOM_AWZ: log classes with custom allocWithZone methods
OBJC_PRINT_RAW_ISA: log classes that require raw pointer isa fields
OBJC_DEBUG_UNLOAD: warn about poorly-behaving bundles when unloaded
OBJC_DEBUG_FRAGILE_SUPERCLASSES: warn about subclasses that may have been broken by subsequent changes to superclasses
OBJC_DEBUG_NIL_SYNC: warn about @synchronized(nil), which does no synchronization
OBJC_DEBUG_NONFRAGILE_IVARS: capriciously rearrange non-fragile ivars
OBJC_DEBUG_ALT_HANDLERS: record more info about bad alt handler use
OBJC_DEBUG_MISSING_POOLS: warn about autorelease with no pool in place, which may be a leak
OBJC_DEBUG_POOL_ALLOCATION: halt when autorelease pools are popped out of order, and allow heap debuggers to track autorelease pools
OBJC_DEBUG_DUPLICATE_CLASSES: halt when multiple classes with the same name are present
OBJC_DEBUG_DONT_CRASH: halt the process by exiting instead of crashing
OBJC_DISABLE_VTABLES: disable vtable dispatch
OBJC_DISABLE_PREOPTIMIZATION: disable preoptimization courtesy of dyld shared cache
OBJC_DISABLE_TAGGED_POINTERS: disable tagged pointer optimization of NSNumber et al.
OBJC_DISABLE_TAG_OBFUSCATION: disable obfuscation of tagged pointers
OBJC_DISABLE_NONPOINTER_ISA: disable non-pointer isa fields
OBJC_DISABLE_INITIALIZE_FORK_SAFETY: disable safety checks for +initialize after fork
