在开发中，有时候我们需要获取这些信息：

手机是否联网
当前网络是WiFi还是蜂窝
那么我总结一下具体的使用场景有哪些？肯定有遗漏：

聊天列表，需要实时监控当前的网络是不是可达的，如果不可达，则出现不能联网的提示

在线视屏播放，需要判断当前的网络状态，如果不是WiFi，应该给出流量播放的提示

对于比较重要的网络请求，在请求出错的情况下，判断网路状态，找出请求失败原因。

可以把请求进行缓存后，当监听到网络连接成功后发送。举个例子，每次进app都要把位置信息发给服务器，如果发送失败后，发现是网络不可达造成的失败，那么可以把这个请求放入到一个队列中，在网络可达的时候，开启队列任务。

当网络状态变化时，实时的给用户提示信息

获取某个节点或地址是不是可达的

但是，极其不建议在发请求前，先检测当前的网络是不是可达。因为手机的网络状态是经常变化

SCNetworkReachabilityFlags

SCNetworkReachabilityFlags是获取网络状态最核心的东西。我们来看看它有哪些内容：

作用

SCNetworkReachabilityFlags能够判断某个指定的网络节点名称或者地址是不是可达的，也能判断该节点或地址是不是需要先建立连接，也可以判断是不是需要用户手动去建立连接。

注意：这里所说的连接分为用编程手段连接和用手动建立连接两种

我们只列举出跟本类相关的一些选项：

(1)kSCNetworkReachabilityFlagsReachable 表明当前指定的节点或地址是可达的。注意：可达不是代表节点或地址接受到了数据，而是代表数据能够离开本地,因此。就算是可达的，也不一定能够发送成功

(2)kSCNetworkReachabilityFlagsConnectionRequired 表明要想和指定的节点或地址通信，需要先建立连接。比如说拨号上网。注意：对于手机来说，如果没有返回该标记，就说明手机正在使用蜂窝网路或者WiFi

(3)kSCNetworkReachabilityFlagsConnectionOnTraffic 表明要想和指定的节点或地址通信，必须先建立连接，但是在当前的网络配置下，目标是可达的。注意：任何连接到指定的节点或地址的请求都会触发该标记，举个例子，在很多地方需要输入手机，获取验证码后才能联网，就是这个原理

(4)kSCNetworkReachabilityFlagsConnectionOnDemand 表明要想和指定的节点或地址通信，必须先建立连接，但是在当前的网络配置下，目标是可达的。但是建立连接必须通过CFSocketStream APIs才行，其他的APIs不能建立连接

(5)kSCNetworkReachabilityFlagsInterventionRequired 表明要想和指定的节点或地址通信，必须先建立连接，但是在当前的网络配置下，目标是可达的。需要用户手动提供一些数据，比如密码或者token

(6)kSCNetworkReachabilityFlagsIsWWAN 表明是不是通过蜂窝网络连接

上边的这些选项，会在下边的一个核心方法中使用到，我们在下边的代码中在给出说明。

ConnectionType

/// Defines the various connection types detected by reachability flags.
    ///
    /// - ethernetOrWiFi: The connection type is either over Ethernet or WiFi.
    /// - wwan:           The connection type is a WWAN connection.
    public enum ConnectionType {
        case ethernetOrWiFi
        case wwan
    }

对于手机而言，我们需要的连接类型就两种，一种是蜂窝网络，另一种是WiFi网络。因此在设计NetworkReachabilityManager的时候，通过上边的枚举获取当前的网络连接类型。

NetworkReachabilityStatus

/// Defines the various states of network reachability.
    ///
    /// - unknown:      It is unknown whether the network is reachable.
    /// - notReachable: The network is not reachable.
    /// - reachable:    The network is reachable.
    public enum NetworkReachabilityStatus {
        case unknown
        case notReachable
        case reachable(ConnectionType)
    }

网络状态明显要比网络类型范围更大，因此又增加了两个选项，一个表示当前的网络是未知的，另一个表示当前的网路不可达。

综上所述，我们的目的就是拿到这个NetworkReachabilityStatus，那么NetworkReachabilityManager是如何把NetworkReachabilityStatus传递出来的呢？ 答案就是闭包，

/// A closure executed when the network reachability status changes. The closure takes a single argument: the
    /// network reachability status.
    public typealias Listener = (NetworkReachabilityStatus) -> Void

swift的闭包，我们已经很熟悉了，在开发中，首先初始化NetworkReachabilityManager，然后设置Listener，第三部开启监控，这个开启监控的方法会在下边讲到。

Properties

在NetworkReachabilityManager中，属性分为public和private，我们先看public部分：

/// Whether the network is currently reachable.
    public var isReachable: Bool { return isReachableOnWWAN || isReachableOnEthernetOrWiFi }

    /// Whether the network is currently reachable over the WWAN interface.
    public var isReachableOnWWAN: Bool { return networkReachabilityStatus == .reachable(.wwan) }

    /// Whether the network is currently reachable over Ethernet or WiFi interface.
    public var isReachableOnEthernetOrWiFi: Bool { return networkReachabilityStatus == .reachable(.ethernetOrWiFi) }

    /// The current network reachability status.
    public var networkReachabilityStatus: NetworkReachabilityStatus {
        guard let flags = self.flags else { return .unknown }
        return networkReachabilityStatusForFlags(flags)
    }

    /// The dispatch queue to execute the `listener` closure on.
    public var listenerQueue: DispatchQueue = DispatchQueue.main

    /// A closure executed when the network reachability status changes.
    public var listener: Listener?

public表明我们可以通过NetworkReachabilityManager实例直接获得的属性，能够让我们很方便的获取我们想要的数据。我们对这些属性做一些简单的说明：
(1)isReachable: Bool 当前网络是可达的，要么是蜂窝网络，要么是WiFi连接

(2)isReachableOnWWAN: Bool 表明当前网络是通过蜂窝网络连接

(3)isReachableOnEthernetOrWiFi: Bool 表明当前网络是通过WiFi连接

(4)networkReachabilityStatus: NetworkReachabilityStatus 返回当前的网络状态，这也是上边3个判断的基础

(5)listenerQueue 监听listener在那个队列中调用，默认的是主队列
listener: Listener 监听闭包，当网络状态发生变化时会调用

上边这些public属性有的是只读的，有的不是，我们在看看private属性：

(6)flags: SCNetworkReachabilityFlags? 主要目的是获取flags，在上边我们介绍过，网络状态就是根据flags判断出来的是通过下边的方法获取到的：

@available(iOS 2.0, *)
  public func SCNetworkReachabilityGetFlags(_ target: SCNetworkReachability, _ flags: UnsafeMutablePointer<SCNetworkReachabilityFlags>) -> Bool

(7)reachability: SCNetworkReachability 必不可少的对象，有了它才能获取flags

(8)previousFlags: SCNetworkReachabilityFlags 用于记录当前的flags，在收到系统的callBack方法后，通过比较现在的flags和previousFlags来判断是不是要调用listener函数

Initialization

关于初始化，NetworkReachabilityManager提供了三种选择：

通过指定host

/// Creates a `NetworkReachabilityManager` instance with the specified host.
    ///
    /// - parameter host: The host used to evaluate network reachability.
    ///
    /// - returns: The new `NetworkReachabilityManager` instance.
    public convenience init?(host: String) {
        guard let reachability = SCNetworkReachabilityCreateWithName(nil, host) else { return nil }
        self.init(reachability: reachability)
    }

通过init方法会默认的设置为指向0.0.0.0

/// Creates a `NetworkReachabilityManager` instance that monitors the address 0.0.0.0.
    ///
    /// Reachability treats the 0.0.0.0 address as a special token that causes it to monitor the general routing
    /// status of the device, both IPv4 and IPv6.
    ///
    /// - returns: The new `NetworkReachabilityManager` instance.
    public convenience init?() {
        var address = sockaddr_in()
        address.sin_len = UInt8(MemoryLayout<sockaddr_in>.size)
        address.sin_family = sa_family_t(AF_INET)

        guard let reachability = withUnsafePointer(to: &address, { pointer in
            return pointer.withMemoryRebound(to: sockaddr.self, capacity: MemoryLayout<sockaddr>.size) {
                return SCNetworkReachabilityCreateWithAddress(nil, $0)
            }
        }) else { return nil }

        self.init(reachability: reachability)
    }

通过指定SCNetworkReachability

private init(reachability: SCNetworkReachability) {
        self.reachability = reachability
        self.previousFlags = SCNetworkReachabilityFlags()
    }

deinit

 deinit {
        stopListening()
   }

上边的代码表明，在NetworkReachabilityManager被销毁的时候，会停止监控，因此在开发中就要额外注意这一点，最好让控制器强引用它。

startListening

在开发中，对于开发某个功能，我有时候会称为开发某种能力类，我们可以采取自上而下的方法，我先定义出最基本的伪代码，对于网络监控我们的伪代码就应该是下边这样的
(1)创建一个监控者
(2)设置监控回调事件
(3)开始监控
(4)停止监控
在这里讲点额外的编程技巧，上边的4个伪代码我们可以成为子程序，每个子程序都应该有一定的内聚性要求，就是说每个子程序最好能够实现一个单一的功能。子程序会出现成对出现的情况，比如开始和停止，等等。那么我们现在要讲的就是第三步，开始监控。

@discardableResult
    public func startListening() -> Bool {
        var context = SCNetworkReachabilityContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
        context.info = Unmanaged.passUnretained(self).toOpaque()

        let callbackEnabled = SCNetworkReachabilitySetCallback(
            reachability,
            { (_, flags, info) in
                let reachability = Unmanaged<NetworkReachabilityManager>.fromOpaque(info!).takeUnretainedValue()
                reachability.notifyListener(flags)
            },
            &context
        )

        let queueEnabled = SCNetworkReachabilitySetDispatchQueue(reachability, listenerQueue)

        listenerQueue.async {
            self.previousFlags = SCNetworkReachabilityFlags()
            self.notifyListener(self.flags ?? SCNetworkReachabilityFlags())
        }

        return callbackEnabled && queueEnabled
    }

@discardableResult表明可以忽略返回值。其实开始监控网络状态就分为两部：

(1)设置Callback回调函数
(2)设置Callback回调队列

当然必要的前提是必须初始化了一个reachability。

这里有一些很有意思的东西，可能我们在swift中是不常见的。比如：Unmanaged.passUnretained(self).toOpaque(),比如:let reachability = Unmanaged<NetworkReachabilityManager>.fromOpaque(info!).takeUnretainedValue()

/// A type for propagating an unmanaged object reference.
///
/// When you use this type, you become partially responsible for
/// keeping the object alive.
public struct Unmanaged<Instance : AnyObject> {

    /// Unsafely turns an opaque C pointer into an unmanaged class reference.
    ///
    /// This operation does not change reference counts.
    ///
    ///     let str: CFString = Unmanaged.fromOpaque(ptr).takeUnretainedValue()
    ///
    /// - Parameter value: An opaque C pointer.
    /// - Returns: An unmanaged class reference to `value`.
    public static func fromOpaque(_ value: UnsafeRawPointer) -> Unmanaged<Instance>

    /// Unsafely converts an unmanaged class reference to a pointer.
    ///
    /// This operation does not change reference counts.
    ///
    ///     let str0: CFString = "boxcar"
    ///     let bits = Unmanaged.passUnretained(str0)
    ///     let ptr = bits.toOpaque()
    ///
    /// - Returns: An opaque pointer to the value of this unmanaged reference.
    public func toOpaque() -> UnsafeMutableRawPointer

    /// Creates an unmanaged reference with an unbalanced retain.
    ///
    /// The instance passed as `value` will leak if nothing eventually balances
    /// the retain.
    ///
    /// This is useful when passing an object to an API which Swift does not know
    /// the ownership rules for, but you know that the API expects you to pass
    /// the object at +1.
    ///
    /// - Parameter value: A class instance.
    /// - Returns: An unmanaged reference to the object passed as `value`.
    public static func passRetained(_ value: Instance) -> Unmanaged<Instance>

    /// Creates an unmanaged reference without performing an unbalanced
    /// retain.
    ///
    /// This is useful when passing a reference to an API which Swift
    /// does not know the ownership rules for, but you know that the
    /// API expects you to pass the object at +0.
    ///
    ///     CFArraySetValueAtIndex(.passUnretained(array), i,
    ///                            .passUnretained(object))
    ///
    /// - Parameter value: A class instance.
    /// - Returns: An unmanaged reference to the object passed as `value`.
    public static func passUnretained(_ value: Instance) -> Unmanaged<Instance>

    /// Gets the value of this unmanaged reference as a managed
    /// reference without consuming an unbalanced retain of it.
    ///
    /// This is useful when a function returns an unmanaged reference
    /// and you know that you're not responsible for releasing the result.
    ///
    /// - Returns: The object referenced by this `Unmanaged` instance.
    public func takeUnretainedValue() -> Instance

    /// Gets the value of this unmanaged reference as a managed
    /// reference and consumes an unbalanced retain of it.
    ///
    /// This is useful when a function returns an unmanaged reference
    /// and you know that you're responsible for releasing the result.
    ///
    /// - Returns: The object referenced by this `Unmanaged` instance.
    public func takeRetainedValue() -> Instance

    /// Performs an unbalanced retain of the object.
    public func retain() -> Unmanaged<Instance>

    /// Performs an unbalanced release of the object.
    public func release()

    /// Performs an unbalanced autorelease of the object.
    public func autorelease() -> Unmanaged<Instance>
}

这里提供一个文章地址[HandyJSON] 设计思路简析,关于swift中指针的使用可以参考这篇文章。很强大啊。
在上边的开始监控中有一个函数：notifyListener
，这个函数的目的就是通知监听者，也就是触发回调函数。

func notifyListener(_ flags: SCNetworkReachabilityFlags) {
        guard previousFlags != flags else { return }
        previousFlags = flags

        listener?(networkReachabilityStatusForFlags(flags))
    }

networkReachabilityStatusForFlags

这个函数是根据flags获取状态的核心函数，但是我觉得没什么好说的，在开发中用的也不多，我们把代码粘一下，然后重点来说说swift中运算符==重载

func networkReachabilityStatusForFlags(_ flags: SCNetworkReachabilityFlags) -> NetworkReachabilityStatus {
        /// 这里的contains函数要传递的值是OptionSet自身，因此.reachable换成SCNetworkReachabilityFlags.reachable也是可以的,reachable是一个静态方法
        /// flags.contains(.reachable)如果是true，就代表有网络连接
        guard flags.contains(.reachable) else { return .notReachable }

        var networkStatus: NetworkReachabilityStatus = .notReachable

        if !flags.contains(.connectionRequired) { networkStatus = .reachable(.ethernetOrWiFi) }

        if flags.contains(.connectionOnDemand) || flags.contains(.connectionOnTraffic) {
            if !flags.contains(.interventionRequired) { networkStatus = .reachable(.ethernetOrWiFi) }
        }

        #if os(iOS)
            if flags.contains(.isWWAN) { networkStatus = .reachable(.wwan) }
        #endif

        return networkStatus
    }

运算符重载

要想重载==,需要实现Equatable协议：

public protocol Equatable {

/// Returns a Boolean value indicating whether two values are equal.
///
/// Equality is the inverse of inequality. For any values `a` and `b`,
/// `a == b` implies that `a != b` is `false`.
///
/// - Parameters:
///   - lhs: A value to compare.
///   - rhs: Another value to compare.
public static func ==(lhs: Self, rhs: Self) -> Bool

}

其实，这种思想还是很重要的，在开发中可以通过这种方式来判断两个模型是不是相同，等等很多种使用场景。我简单的把Apple文档中的注释说明部分翻译一下。

==和!=是对立统一的关系，我们自定义了==,同理，!=也就支持了。在swift中，很多基本的数据类型都支持了Equatable协议。

Equatable协议的一个典型的应用场景就是判断一个集合中是否包含某个值。在swift中，如果集合中的值都实现了Equatable协议，那么就可以通过contains(:)方法来判断是不是包含该值。这也说明了contains(:)内部实现应该是通过==来实现的。使用contains(_:)方法的好处就是省去了我们遍历数据，然后再进行判断的繁琐步骤。我们看个例子：

///     let students = ["Nora", "Fern", "Ryan", "Rainer"]
///
///     let nameToCheck = "Ryan"
///     if students.contains(nameToCheck) {
///         print("\(nameToCheck) is signed up!")
///     } else {
///         print("No record of \(nameToCheck).")
///     }
///     // Prints "Ryan is signed up!"

需要把==声明成为自定义类型的静态方法

假如说我们有一个街道地址的结构体：

///     struct StreetAddress {
        ///         let number: String
        ///         let street: String
        ///         let unit: String?
        ///
        ///         init(_ number: String, _ street: String, unit: String? = nil) {
        ///             self.number = number
        ///             self.street = street
        ///             self.unit = unit
        ///         }
        ///     }

我们让StreetAddress实现Equatable协议：

///
///     extension StreetAddress: Equatable {
///         static func == (lhs: StreetAddress, rhs: StreetAddress) -> Bool {
///             return
///                 lhs.number == rhs.number &&
///                 lhs.street == rhs.street &&
///                 lhs.unit == rhs.unit
///         }
///     }
///

接下来我们就能使用系统的contains(_:)方法来判断一个集合中是不是包含摸个街道地址了。

///
///     let addresses = [StreetAddress("1490", "Grove Street"),
///                      StreetAddress("2119", "Maple Avenue"),
///                      StreetAddress("1400", "16th Street")]
///     let home = StreetAddress("1400", "16th Street")
///
///     print(addresses[0] == home)
///     // Prints "false"
///     print(addresses.contains(home))
///     // Prints "true"
///

有了上边的知识，我们在看看NetworkReachabilityManager是怎么用的：

extension NetworkReachabilityManager.NetworkReachabilityStatus: Equatable {}

/// Returns whether the two network reachability status values are equal.
///
/// - parameter lhs: The left-hand side value to compare.
/// - parameter rhs: The right-hand side value to compare.
///
/// - returns: `true` if the two values are equal, `false` otherwise.
public func ==(
    lhs: NetworkReachabilityManager.NetworkReachabilityStatus,
    rhs: NetworkReachabilityManager.NetworkReachabilityStatus)
    -> Bool
{
    switch (lhs, rhs) {
    case (.unknown, .unknown):
        return true
    case (.notReachable, .notReachable):
        return true
    case let (.reachable(lhsConnectionType), .reachable(rhsConnectionType)):
        return lhsConnectionType == rhsConnectionType
    default:
        return false
    }
}

在swift中，static函数还可以像上边这么用，把函数写到类的代码块之外，当然，上边的代码也可以这么写：
extension NetworkReachabilityManager.NetworkReachabilityStatus: Equatable {

public static func ==(
    lhs: NetworkReachabilityManager.NetworkReachabilityStatus,
    rhs: NetworkReachabilityManager.NetworkReachabilityStatus)
    -> Bool
{
    switch (lhs, rhs) {
    case (.unknown, .unknown):
        return true
    case (.notReachable, .notReachable):
        return true
    case let (.reachable(lhsConnectionType), .reachable(rhsConnectionType)):
        return lhsConnectionType == rhsConnectionType
    default:
        return false
    }
}

}