iOS 如何创建单例

// YOURCLASSNAME.h

@interface YOURCLASSNAME : NSObject

+(instancetype) sharedInstance;

@end

// YOURCLASSNAME.m

@implementation YOURCLASSNAME

+ (instancetype)sharedXXX {
static YOURCLASSNAME *_sharedInstance = nil;
static dispatch_once_t oncePredicate;
dispatch_once(&oncePredicate, ^{
// alloc & init work
// [[YOURCLASSNAME alloc] init];
});

return _sharedInstance;
}

@end

+ (instancetype)sharedManager {
static AFNetworkActivityIndicatorManager *_sharedManager = nil;
static dispatch_once_t oncePredicate;
dispatch_once(&oncePredicate, ^{
_sharedManager = [[self alloc] init];
});

return _sharedManager;
}