1.VC生命周期
A页面显示
loadView -> viewDidLoad->viewWillAppear->viewDidAppear
A页面跳转B页面
A loadView ->A viewDidLoad->B viewWillDisappear->A viewWillAppear->B viewDidDisappear->A viewDidAppear
B页面返回A页面
B viewWillDisappear->A viewWillAppear->B viewDidDisappear->A viewDidAppear->B dealloc
2.创建单例 创建.h文件，定义宏

ifndef XLXSingleton_h

define XLXSingleton_h

// @interface

define singleton_interface(className) \

• (instancetype)shared##className;

// @implementation

define singleton_implementation(className) \

static className *_instance; \

• (instancetype)shared##className
  {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
  _instance = [[self alloc] init];
  });
  return _instance;
  } \
• (instancetype)allocWithZone:(NSZone *)zone
  {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
  _instance = [super allocWithZone:zone];
  });
  return _instance;
  } \

• (instancetype)copyWithZone:(NSZone *)zone{
  return _instance;
  } \
• (instancetype)mutableCopyWithZone:(NSZone *)zone{
  return _instance;
  }

endif /* XLXSingleton_h */