Swift 5.5, iOS 12+，没有更多的第三方库。

请检查这个要点https://gist.github.com/dimohamdy/5166ba6c88f6954fa6b23bc9f28cbe12

使用

 startNetworkReachabilityObserver()
 
  if Reachability.shared.isConnected {
       print("no internet connection")   
  }

Code

import Network
import Foundation

class Reachability {

    static var shared = Reachability()
    lazy private var monitor = NWPathMonitor()

    var isConnected: Bool {
        return monitor.currentPath.status == .satisfied
    }

    func startNetworkReachabilityObserver() {
        monitor.pathUpdateHandler = { path in
            if path.status == .satisfied {
                NotificationCenter.default.post(name: Notifications.Reachability.connected.name, object: nil)
            } else if path.status == .unsatisfied {
                NotificationCenter.default.post(name: Notifications.Reachability.notConnected.name, object: nil)
            }
        }
        let queue = DispatchQueue.global(qos: .background)
        monitor.start(queue: queue)
    }
}

从iOS 12开始，NWPathMonitor取代了Reachability。用这个:

import Network


struct Internet {
 
 private static let monitor = NWPathMonitor()
 
 static var active = false
 static var expensive = false
 
 /// Monitors internet connectivity changes. Updates with every change in connectivity.
 /// Updates variables for availability and if it's expensive (cellular).
 static func start() {
  guard monitor.pathUpdateHandler == nil else { return }
  
  monitor.pathUpdateHandler = { update in
   Internet.active = update.status == .satisfied ? true : false
   Internet.expensive = update.isExpensive ? true : false
  }
  
  monitor.start(queue: DispatchQueue(label: "InternetMonitor"))
 }
 
}

在使用:

Internet.start()

if Internet.active {
 // do something
}
  
if Internet.expensive {
 // device is using Cellular data or WiFi hotspot
}

在项目中创建一个新的Swift文件，命名为Reachability.swift。将以下代码剪切并粘贴到其中以创建您的类。

import Foundation
import SystemConfiguration

public class Reachability {

    class func isConnectedToNetwork() -> Bool {

        var zeroAddress = sockaddr_in(sin_len: 0, sin_family: 0, sin_port: 0, sin_addr: in_addr(s_addr: 0), sin_zero: (0, 0, 0, 0, 0, 0, 0, 0))
        zeroAddress.sin_len = UInt8(sizeofValue(zeroAddress))
        zeroAddress.sin_family = sa_family_t(AF_INET)

        let defaultRouteReachability = withUnsafePointer(&zeroAddress) {
            SCNetworkReachabilityCreateWithAddress(kCFAllocatorDefault, UnsafePointer($0))
        }

        var flags: SCNetworkReachabilityFlags = SCNetworkReachabilityFlags(rawValue: 0)
        if SCNetworkReachabilityGetFlags(defaultRouteReachability!, &flags) == false {
             return false
        }

        let isReachable = flags == .Reachable
        let needsConnection = flags == .ConnectionRequired

        return isReachable && !needsConnection

    }
}

你可以使用下面的代码检查项目中的任何地方的互联网连接:

if Reachability.isConnectedToNetwork() {
    println("Internet connection OK")
} else {
    println("Internet connection FAILED")
}

如果用户没有连接到互联网，您可能希望向他们显示一个警告对话框来通知他们。

if Reachability.isConnectedToNetwork() {
    println("Internet connection OK")
} else {
    println("Internet connection FAILED")
    var alert = UIAlertView(title: "No Internet Connection", message: "Make sure your device is connected to the internet.", delegate: nil, cancelButtonTitle: "OK")
    alert.show()
}

解释:

我们正在制作一个可重用的公共类和方法，可以在项目的任何地方使用，以检查互联网连接。我们需要添加基础和系统配置框架。

在公共类Reachability中，方法isConnectedToNetwork() -> Bool{}将返回一个关于互联网连接的Bool值。我们使用if循环对case执行所需的操作。我希望这足够了。干杯!

更新版本的@martin的答案Swift 5+使用组合。它还包括iOS 14的不可用原因检查。

import Combine
import Network

enum NetworkType {
    case wifi
    case cellular
    case loopBack
    case wired
    case other
}

final class ReachabilityService: ObservableObject {

    @Published var reachabilityInfos: NWPath?
    @Published var isNetworkAvailable: Bool?
    @Published var typeOfCurrentConnection: NetworkType?

    private let monitor = NWPathMonitor()
    private let backgroundQueue = DispatchQueue.global(qos: .background)

    init() {
        setUp()
    }

    init(with interFaceType: NWInterface.InterfaceType) {
        setUp()
    }

    deinit {
        monitor.cancel()
    }
}

private extension ReachabilityService {

    func setUp() {

        monitor.pathUpdateHandler = { [weak self] path in
            self?.reachabilityInfos = path
            switch path.status {
            case .satisfied:
                print("ReachabilityService: satisfied")
                self?.isNetworkAvailable = true
                break
            case .unsatisfied:
                print("ReachabilityService: unsatisfied")

                if #available(iOS 14.2, *) {
                    switch path.unsatisfiedReason {

                    case .notAvailable:
                        print("ReachabilityService: unsatisfiedReason: notAvailable")
                        break
                    case .cellularDenied:
                        print("ReachabilityService: unsatisfiedReason: cellularDenied")
                        break
                    case .wifiDenied:
                        print("ReachabilityService: unsatisfiedReason: wifiDenied")
                        break
                    case .localNetworkDenied:
                        print("ReachabilityService: unsatisfiedReason: localNetworkDenied")
                        break
                    @unknown default:
                        print("ReachabilityService: unsatisfiedReason: default")
                    }
                } else {
                    // Fallback on earlier versions
                }

                self?.isNetworkAvailable = false
                break
            case .requiresConnection:
                print("ReachabilityService: requiresConnection")
                self?.isNetworkAvailable = false
                break
            @unknown default:
                print("ReachabilityService: default")
                self?.isNetworkAvailable = false
            }
            if path.usesInterfaceType(.wifi) {
                self?.typeOfCurrentConnection = .wifi
            } else if path.usesInterfaceType(.cellular) {
                self?.typeOfCurrentConnection = .cellular
            } else if path.usesInterfaceType(.loopback) {
                self?.typeOfCurrentConnection = .loopBack
            } else if path.usesInterfaceType(.wiredEthernet) {
                self?.typeOfCurrentConnection = .wired
            } else if path.usesInterfaceType(.other) {
                self?.typeOfCurrentConnection = .other
            }
        }

        monitor.start(queue: backgroundQueue)
    }
}

用法:

在视图模型中:

private let reachability = ReachabilityService()

init() {
    reachability.$isNetworkAvailable.sink { [weak self] isConnected in
        self?.isConnected = isConnected ?? false
    }.store(in: &cancelBag)
}

在你的控制器中:

viewModel.$isConnected.sink { [weak self] isConnected in
    print("isConnected: \(isConnected)")
    DispatchQueue.main.async {
        //Update your UI in here
    }
}.store(in: &bindings)

如果有人已经在使用Alamofire，那么-

struct Connectivity {
  static let sharedInstance = NetworkReachabilityManager()!
  static var isConnectedToInternet:Bool {
      return self.sharedInstance.isReachable
    }
}

用法:

if Connectivity.isConnectedToInternet {
     print("Connected")
 } else {
     print("No Internet")
}

为了解决评论中提到的4G问题，我使用@AshleyMills的可达性实现作为参考，重写了Swift 3.1的可达性:

更新:Xcode 10.1•Swift 4或更高版本

Reachability.swift文件

import Foundation
import SystemConfiguration

class Reachability {
    var hostname: String?
    var isRunning = false
    var isReachableOnWWAN: Bool
    var reachability: SCNetworkReachability?
    var reachabilityFlags = SCNetworkReachabilityFlags()
    let reachabilitySerialQueue = DispatchQueue(label: "ReachabilityQueue")
    init(hostname: String) throws {
        guard let reachability = SCNetworkReachabilityCreateWithName(nil, hostname) else {
            throw Network.Error.failedToCreateWith(hostname)
        }
        self.reachability = reachability
        self.hostname = hostname
        isReachableOnWWAN = true
        try start()
    }
    init() throws {
        var zeroAddress = sockaddr_in()
        zeroAddress.sin_len = UInt8(MemoryLayout<sockaddr_in>.size)
        zeroAddress.sin_family = sa_family_t(AF_INET)
        guard let reachability = withUnsafePointer(to: &zeroAddress, {
            $0.withMemoryRebound(to: sockaddr.self, capacity: 1) {
                SCNetworkReachabilityCreateWithAddress(nil, $0)
            }
        }) else {
            throw Network.Error.failedToInitializeWith(zeroAddress)
        }
        self.reachability = reachability
        isReachableOnWWAN = true
        try start()
    }
    var status: Network.Status {
        return  !isConnectedToNetwork ? .unreachable :
                isReachableViaWiFi    ? .wifi :
                isRunningOnDevice     ? .wwan : .unreachable
    }
    var isRunningOnDevice: Bool = {
        #if targetEnvironment(simulator)
            return false
        #else
            return true
        #endif
    }()
    deinit { stop() }
}

extension Reachability {

    func start() throws {
        guard let reachability = reachability, !isRunning else { return }
        var context = SCNetworkReachabilityContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
        context.info = Unmanaged<Reachability>.passUnretained(self).toOpaque()
        guard SCNetworkReachabilitySetCallback(reachability, callout, &context) else { stop()
            throw Network.Error.failedToSetCallout
        }
        guard SCNetworkReachabilitySetDispatchQueue(reachability, reachabilitySerialQueue) else { stop()
            throw Network.Error.failedToSetDispatchQueue
        }
        reachabilitySerialQueue.async { self.flagsChanged() }
        isRunning = true
    }

    func stop() {
        defer { isRunning = false }
        guard let reachability = reachability else { return }
        SCNetworkReachabilitySetCallback(reachability, nil, nil)
        SCNetworkReachabilitySetDispatchQueue(reachability, nil)
        self.reachability = nil
    }

    var isConnectedToNetwork: Bool {
        return isReachable &&
               !isConnectionRequiredAndTransientConnection &&
               !(isRunningOnDevice && isWWAN && !isReachableOnWWAN)
    }

    var isReachableViaWiFi: Bool {
        return isReachable && isRunningOnDevice && !isWWAN
    }

    /// Flags that indicate the reachability of a network node name or address, including whether a connection is required, and whether some user intervention might be required when establishing a connection.
    var flags: SCNetworkReachabilityFlags? {
        guard let reachability = reachability else { return nil }
        var flags = SCNetworkReachabilityFlags()
        return withUnsafeMutablePointer(to: &flags) {
            SCNetworkReachabilityGetFlags(reachability, UnsafeMutablePointer($0))
            } ? flags : nil
    }

    /// compares the current flags with the previous flags and if changed posts a flagsChanged notification
    func flagsChanged() {
        guard let flags = flags, flags != reachabilityFlags else { return }
        reachabilityFlags = flags
        NotificationCenter.default.post(name: .flagsChanged, object: self)
    }

    /// The specified node name or address can be reached via a transient connection, such as PPP.
    var transientConnection: Bool { return flags?.contains(.transientConnection) == true }

    /// The specified node name or address can be reached using the current network configuration.
    var isReachable: Bool { return flags?.contains(.reachable) == true }

    /// The specified node name or address can be reached using the current network configuration, but a connection must first be established. If this flag is set, the kSCNetworkReachabilityFlagsConnectionOnTraffic flag, kSCNetworkReachabilityFlagsConnectionOnDemand flag, or kSCNetworkReachabilityFlagsIsWWAN flag is also typically set to indicate the type of connection required. If the user must manually make the connection, the kSCNetworkReachabilityFlagsInterventionRequired flag is also set.
    var connectionRequired: Bool { return flags?.contains(.connectionRequired) == true }

    /// The specified node name or address can be reached using the current network configuration, but a connection must first be established. Any traffic directed to the specified name or address will initiate the connection.
    var connectionOnTraffic: Bool { return flags?.contains(.connectionOnTraffic) == true }

    /// The specified node name or address can be reached using the current network configuration, but a connection must first be established.
    var interventionRequired: Bool { return flags?.contains(.interventionRequired) == true }

    /// The specified node name or address can be reached using the current network configuration, but a connection must first be established. The connection will be established "On Demand" by the CFSocketStream programming interface (see CFStream Socket Additions for information on this). Other functions will not establish the connection.
    var connectionOnDemand: Bool { return flags?.contains(.connectionOnDemand) == true }

    /// The specified node name or address is one that is associated with a network interface on the current system.
    var isLocalAddress: Bool { return flags?.contains(.isLocalAddress) == true }

    /// Network traffic to the specified node name or address will not go through a gateway, but is routed directly to one of the interfaces in the system.
    var isDirect: Bool { return flags?.contains(.isDirect) == true }

    /// The specified node name or address can be reached via a cellular connection, such as EDGE or GPRS.
    var isWWAN: Bool { return flags?.contains(.isWWAN) == true }

    /// The specified node name or address can be reached using the current network configuration, but a connection must first be established. If this flag is set
    /// The specified node name or address can be reached via a transient connection, such as PPP.
    var isConnectionRequiredAndTransientConnection: Bool {
        return (flags?.intersection([.connectionRequired, .transientConnection]) == [.connectionRequired, .transientConnection]) == true
    }
}

func callout(reachability: SCNetworkReachability, flags: SCNetworkReachabilityFlags, info: UnsafeMutableRawPointer?) {
    guard let info = info else { return }
    DispatchQueue.main.async {
        Unmanaged<Reachability>
            .fromOpaque(info)
            .takeUnretainedValue()
            .flagsChanged()
    }
}

extension Notification.Name {
    static let flagsChanged = Notification.Name("FlagsChanged")
}

struct Network {
    static var reachability: Reachability!
    enum Status: String {
        case unreachable, wifi, wwan
    }
    enum Error: Swift.Error {
        case failedToSetCallout
        case failedToSetDispatchQueue
        case failedToCreateWith(String)
        case failedToInitializeWith(sockaddr_in)
    }
}

使用

在你的AppDelegate.swift didFinishLaunchingWithOptions方法中初始化它，并处理任何可能发生的错误:

import UIKit
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        do {
            try Network.reachability = Reachability(hostname: "www.google.com")
        }
        catch {
            switch error as? Network.Error {
            case let .failedToCreateWith(hostname)?:
                print("Network error:\nFailed to create reachability object With host named:", hostname)
            case let .failedToInitializeWith(address)?:
                print("Network error:\nFailed to initialize reachability object With address:", address)
            case .failedToSetCallout?:
                print("Network error:\nFailed to set callout")
            case .failedToSetDispatchQueue?:
                print("Network error:\nFailed to set DispatchQueue")
            case .none:
                print(error)
            }
        }
        return true
    }
}

和一个视图控制器示例:

import UIKit
class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        NotificationCenter.default
            .addObserver(self,
                         selector: #selector(statusManager),
                         name: .flagsChanged,
                         object: nil)
        updateUserInterface()
    }
    func updateUserInterface() {
        switch Network.reachability.status {
        case .unreachable:
            view.backgroundColor = .red
        case .wwan:
            view.backgroundColor = .yellow
        case .wifi:
            view.backgroundColor = .green
        }
        print("Reachability Summary")
        print("Status:", Network.reachability.status)
        print("HostName:", Network.reachability.hostname ?? "nil")
        print("Reachable:", Network.reachability.isReachable)
        print("Wifi:", Network.reachability.isReachableViaWiFi)
    }
    @objc func statusManager(_ notification: Notification) {
        updateUserInterface()
    }
}

示例项目