dispatch_once单例初始化

static GHCache *instance = nil;

/**

*  单例，静态初始化方法

*

*  @return 返回一个单例

*/

+ (GHCache*)shareCache{

static dispatch_once_t onceToken;

dispatch_once(&onceToken, ^{

instance = [[self alloc] init];

});

return instance;

}