AFNetworking 中 GCD 的使用(1)

AFNetworkReachabilityManager 负责监听网络状态

异步到主线程，通知在主线程发送 通知的发送，接受只能在自己的线程中

static void AFPostReachabilityStatusChange(SCNetworkReachabilityFlags flags, AFNetworkReachabilityStatusCallback block) {
    AFNetworkReachabilityStatus status = AFNetworkReachabilityStatusForFlags(flags);
    dispatch_async(dispatch_get_main_queue(), ^{
        AFNetworkReachabilityManager *manager = nil;
        if (block) {
            manager = block(status);
        }
        NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];
        NSDictionary *userInfo = @{ AFNetworkingReachabilityNotificationStatusItem: @(status) };
        [notificationCenter postNotificationName:AFNetworkingReachabilityDidChangeNotification object:manager userInfo:userInfo];
    });
}

保证多线程安全只初始化一次

+ (instancetype)sharedManager {
    static AFNetworkReachabilityManager *_sharedManager = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _sharedManager = [self manager];
    });

    return _sharedManager;
}

global queue 队列获取网络状态，之后在AFPostReachabilityStatusChange 中，转入main thread 发送通知

- (void)startMonitoring {
  
    ...
    SCNetworkReachabilityScheduleWithRunLoop(self.networkReachability, CFRunLoopGetMain(), kCFRunLoopCommonModes);
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND,        0),^{
        SCNetworkReachabilityFlags flags;
        if (SCNetworkReachabilityGetFlags(self.networkReachability, &flags)) {
            AFPostReachabilityStatusChange(flags, callback);
        }
    });
}

总结：此文件对GCD的使用比较简单，只是保证一次初始化，异步到主线程队列