介紹

1、官方文檔寫法

static AccountManager *DefaultManager = nil;
 
+ (AccountManager *)defaultManager {
    if (!DefaultManager) DefaultManager = [[self allocWithZone:NULL] init];
    return DefaultManager;
}

2、來自 objcolumnist

+ (AccountManager *)sharedManager
{
        static AccountManager *sharedAccountManagerInstance = nil;
        static dispatch_once_t predicate;
        dispatch_once(&predicate, ^{
                sharedAccountManagerInstance = [[self alloc] init]; 
        });
    return sharedAccountManagerInstance;
}

該寫法滿足三個特性：
1）線程安全。
2）滿足靜態分析器的要求。
3）相容了ARC

樣例

@interface BmobIMNetwork ()<GCDAsyncSocketDelegate>

@property (strong, nonatomic) GCDAsyncSocket *gcdAsyncSocket;
@property (strong, nonatomic) NSMutableData *packet;

@end

@implementation BmobIMNetwork

+ (BmobIMNetwork *)sharedBmobIMNetwork
{
    static BmobIMNetwork *sharedBmobIMNetworkInstance = nil;
    static dispatch_once_t predicate;
    dispatch_once(&predicate, ^{
        sharedBmobIMNetworkInstance = [[self alloc] initPritate];
    });
    return sharedBmobIMNetworkInstance;
}

- (instancetype)initPritate{
    self = [super init];
    if (self) {
        _gcdAsyncSocket = [[GCDAsyncSocket alloc] initWithDelegate:self delegateQueue:dispatch_get_main_queue()];
        _packet = [[NSData alloc] init];
    }
    return self;
}

- (instancetype)init{
    @throw [[NSException alloc] initWithName:@"Singleton" reason:@"Use + [BmobIMNetwork sharedBmobIMNetwork]" userInfo:nil];
    return nil;
}

參考

iOS 創建單例的兩種方法