AFNetworkReachabilityManager 是对 SystemConfiguration 模块的封装，苹果的文档中也有一个类似的项目 Reachability 这里对网络状态的监控跟苹果官方的实现几乎是完全相同的。

AFNetworkReachabilityManager 的使用和实现

AFNetworkReachabilityManager 的使用还是非常简单的，只需要三个步骤，就基本可以完成对网络状态的监控。

初始化 AFNetworkReachabilityManager
调用 startMonitoring 方法开始对网络状态进行监控
设置 networkReachabilityStatusBlock 在每次网络状态改变时, 调用这个 block

初始化 AFNetworkReachabilityManager

在初始化方法中，使用 SCNetworkReachabilityCreateWithAddress 或者 SCNetworkReachabilityCreateWithName 生成一个 SCNetworkReachabilityRef 的引用。

+ (instancetype)managerForDomain:(NSString *)domain {
    SCNetworkReachabilityRef reachability = SCNetworkReachabilityCreateWithName(kCFAllocatorDefault, [domain UTF8String]);

    AFNetworkReachabilityManager *manager = [[self alloc] initWithReachability:reachability];

    return manager;
}

+ (instancetype)managerForAddress:(const void *)address {
    SCNetworkReachabilityRef reachability = SCNetworkReachabilityCreateWithAddress(kCFAllocatorDefault, (const struct sockaddr *)address);
    AFNetworkReachabilityManager *manager = [[self alloc] initWithReachability:reachability];

    return manager;
}

这两个方法会通过一个域名或者一个 sockaddr_in 的指针生成一个 SCNetworkReachabilityRef
调用 - [AFNetworkReachabilityManager initWithReachability:] 将生成的 SCNetworkReachabilityRef 引用传给 networkReachability
设置一个默认的 networkReachabilityStatus

- (instancetype)initWithReachability:(SCNetworkReachabilityRef)reachability {
    self = [super init];
    if (!self) {
        return nil;
    }

    _networkReachability = CFRetain(reachability);
    self.networkReachabilityStatus = AFNetworkReachabilityStatusUnknown;

    return self;
}

当调用 CFBridgingRelease(reachability) 后，会把 reachability 桥接成一个 NSObject 对象赋值给 _networkReachability ，然后释放原来的CoreFoundation 对象。

监控网络状态

在初始化 AFNetworkReachabilityManager 后，会调用 startMonitoring 方法开始监控网络状态。

- (void)startMonitoring {
    [self stopMonitoring];

    if (!self.networkReachability) {
        return;
    }

    __weak __typeof(self)weakSelf = self;
    AFNetworkReachabilityStatusBlock callback = ^(AFNetworkReachabilityStatus status) {
        __strong __typeof(weakSelf)strongSelf = weakSelf;

        strongSelf.networkReachabilityStatus = status;
        if (strongSelf.networkReachabilityStatusBlock) {
            strongSelf.networkReachabilityStatusBlock(status);
        }

    };

    SCNetworkReachabilityContext context = {0, (__bridge void *)callback, AFNetworkReachabilityRetainCallback, AFNetworkReachabilityReleaseCallback, NULL};
    SCNetworkReachabilitySetCallback(self.networkReachability, AFNetworkReachabilityCallback, &context);
    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);
        }
    });
}

1、先调用 - stopMonitoring 方法，如果之前设置过对网络状态的监听，使用 SCNetworkReachabilityUnscheduleFromRunLoop 方法取消之前在 Main Runloop 中的监听

- (void)stopMonitoring {
    if (!self.networkReachability) {
        return;
    }

    SCNetworkReachabilityUnscheduleFromRunLoop(self.networkReachability, CFRunLoopGetMain(), kCFRunLoopCommonModes);
}

2、创建一个在每次网络状态改变时的回调,每次回调被调用时,重新设置 networkReachabilityStatus 属性,调用 networkReachabilityStatusBlock

__weak __typeof(self)weakSelf = self;
    AFNetworkReachabilityStatusBlock callback = ^(AFNetworkReachabilityStatus status) {
        __strong __typeof(weakSelf)strongSelf = weakSelf;

        strongSelf.networkReachabilityStatus = status;
        if (strongSelf.networkReachabilityStatusBlock) {
            strongSelf.networkReachabilityStatusBlock(status);
        }

    };

3、创建一个 SCNetworkReachabilityContext

typedef struct {
     CFIndex		version;
     void *		__nullable info;
     const void	* __nonnull (* __nullable retain)(const void *info);
     void		(* __nullable release)(const void *info);
     CFStringRef	__nonnull (* __nullable copyDescription)(const void *info);
 } SCNetworkReachabilityContext;

 SCNetworkReachabilityContext context = {
     0,
     (__bridge void *)callback,
     AFNetworkReachabilityRetainCallback,
     AFNetworkReachabilityReleaseCallback,
     NULL
 };

其中的 callback 就是上一步中的创建的 block 对象
这里的 AFNetworkReachabilityRetainCallback 和 AFNetworkReachabilityReleaseCallback 都是非常简单的 block，在回调被调用时，只是使用 Block_copy 和 Block_release 这样的宏
传入的 info 会以参数的形式在 AFNetworkReachabilityCallback 执行时传入 static const void * AFNetworkReachabilityRetainCallback(const void *info) { return Block_copy(info); } static void AFNetworkReachabilityReleaseCallback(const void *info) { if (info) { Block_release(info); } }

4、当目标的网络状态改变时，会调用传入的回调

SCNetworkReachabilitySetCallback(
     (__bridge SCNetworkReachabilityRef)networkReachability,
     AFNetworkReachabilityCallback,
     &context
 );

5、在 Main Runloop 中对应的模式开始监控网络状态

SCNetworkReachabilityScheduleWithRunLoop(
     (__bridge SCNetworkReachabilityRef)networkReachability,
     CFRunLoopGetMain(),
     kCFRunLoopCommonModes
 );

6、获取当前的网络状态，调用 callback

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0),^{
     SCNetworkReachabilityFlags flags;
     if (SCNetworkReachabilityGetFlags((__bridge SCNetworkReachabilityRef)networkReachability, &flags)) {
         AFPostReachabilityStatusChange(flags, callback);
     }
 });

设置 networkReachabilityStatusBlock 以及回调

在 Main Runloop 中对网络状态进行监控之后，在每次网络状态改变，就会调用 AFNetworkReachabilityCallback 函数：

static void AFNetworkReachabilityCallback(SCNetworkReachabilityRef __unused target, SCNetworkReachabilityFlags flags, void *info) {
    AFPostReachabilityStatusChange(flags, (__bridge AFNetworkReachabilityStatusBlock)info);
}

这里会从 info 中取出之前存在 context 中的 AFNetworkReachabilityStatusBlock。

__weak __typeof(self)weakSelf = self;
AFNetworkReachabilityStatusBlock callback = ^(AFNetworkReachabilityStatus status) {
    __strong __typeof(weakSelf)strongSelf = weakSelf;

    strongSelf.networkReachabilityStatus = status;
    if (strongSelf.networkReachabilityStatusBlock) {
        strongSelf.networkReachabilityStatusBlock(status);
    }

};

取出这个 block 之后，传入 AFPostReachabilityStatusChange 函数：

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

调用 AFNetworkReachabilityStatusForFlags 获取当前的网络可达性状态
在主线程中异步执行上面传入的 callback block（设置 self 的网络状态，调用 networkReachabilityStatusBlock）
发送 AFNetworkingReachabilityDidChangeNotification 通知.

static AFNetworkReachabilityStatus AFNetworkReachabilityStatusForFlags(SCNetworkReachabilityFlags flags) {
    BOOL isReachable = ((flags & kSCNetworkReachabilityFlagsReachable) != 0);
    BOOL needsConnection = ((flags & kSCNetworkReachabilityFlagsConnectionRequired) != 0);
    BOOL canConnectionAutomatically = (((flags & kSCNetworkReachabilityFlagsConnectionOnDemand ) != 0) || ((flags & kSCNetworkReachabilityFlagsConnectionOnTraffic) != 0));
    BOOL canConnectWithoutUserInteraction = (canConnectionAutomatically && (flags & kSCNetworkReachabilityFlagsInterventionRequired) == 0);
    BOOL isNetworkReachable = (isReachable && (!needsConnection || canConnectWithoutUserInteraction));

    AFNetworkReachabilityStatus status = AFNetworkReachabilityStatusUnknown;
    if (isNetworkReachable == NO) {
        status = AFNetworkReachabilityStatusNotReachable;
    }
#if	TARGET_OS_IPHONE
    else if ((flags & kSCNetworkReachabilityFlagsIsWWAN) != 0) {
        status = AFNetworkReachabilityStatusReachableViaWWAN;
    }
#endif
    else {
        status = AFNetworkReachabilityStatusReachableViaWiFi;
    }

    return status;
}

因为 flags 是一个 SCNetworkReachabilityFlags，它的不同位代表了不同的网络可达性状态，通过 flags 的位操作，获取当前的状态信息 AFNetworkReachabilityStatus。

typedef CF_OPTIONS(uint32_t, SCNetworkReachabilityFlags) {
	kSCNetworkReachabilityFlagsTransientConnection	= 1<<0,
	kSCNetworkReachabilityFlagsReachable		= 1<<1,
	kSCNetworkReachabilityFlagsConnectionRequired	= 1<<2,
	kSCNetworkReachabilityFlagsConnectionOnTraffic	= 1<<3,
	kSCNetworkReachabilityFlagsInterventionRequired	= 1<<4,
	kSCNetworkReachabilityFlagsConnectionOnDemand	= 1<<5,	// __OSX_AVAILABLE_STARTING(__MAC_10_6,__IPHONE_3_0)
	kSCNetworkReachabilityFlagsIsLocalAddress	= 1<<16,
	kSCNetworkReachabilityFlagsIsDirect		= 1<<17,
#if	TARGET_OS_IPHONE
	kSCNetworkReachabilityFlagsIsWWAN		= 1<<18,
#endif	// TARGET_OS_IPHONE

	kSCNetworkReachabilityFlagsConnectionAutomatic	= kSCNetworkReachabilityFlagsConnectionOnTraffic
};

AFNetworking学习之路(四)

监控网络状态

AFNetworkReachabilityManager 的使用和实现

初始化 AFNetworkReachabilityManager

监控网络状态

设置 networkReachabilityStatusBlock 以及回调

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