OC & Swift 单例

+ (NetworkTool *)sharedNetworkTool {
    
    static id instace;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instace = [[self alloc] init];
    });
    return instace;
}

   
    private static let instance = NetworkTool()
    
    class func sharedNetwork() -> NetworkTool{
        return instance
    }