How to Authenticate Users with Facebook Graph API in Swift

Integrate Facebook SDK

• First, ensure that you have integrated the Facebook SDK into your Swift project. You can do this via CocoaPods by adding `pod 'FacebookCore'` to your `Podfile`, followed by installing the pod with `pod install`.
• After installation, import the Facebook SDK into your `AppDelegate.swift`:

import FBSDKCoreKit

• In your `AppDelegate`, configure the SDK by overriding the `application(_:didFinishLaunchingWithOptions:)` method:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    ApplicationDelegate.shared.application(
        application,
        didFinishLaunchingWithOptions: launchOptions
    )
    return true
}

Implement User Authentication

• To handle user authentication, you need to import `FBSDKLoginKit` in the relevant view controller.
• Implement the login button and its delegate methods:

import FBSDKLoginKit

class YourViewController: UIViewController, LoginButtonDelegate {

    override func viewDidLoad() {
        super.viewDidLoad()

        let loginButton = FBLoginButton()
        loginButton.center = view.center
        loginButton.delegate = self
        view.addSubview(loginButton)
    }

    func loginButton(_ loginButton: FBLoginButton, didCompleteWith result: LoginManagerLoginResult?, error: Error?) {
        if let error = error {
            print("Error occurred during login: \(error.localizedDescription)")
            return
        }

        // Check for cancellation
        guard let token = AccessToken.current else {
            print("User cancelled login.")
            return
        }

        print("Login successful, token: \(token.tokenString)")
    }

    func loginButtonDidLogOut(_ loginButton: FBLoginButton) {
        print("User logged out")
    }
}

• Ensure that you have the necessary permissions in your Facebook App Settings and request them if needed, such as `email` or `public_profile`:

loginButton.permissions = ["email", "public_profile"]

Handling App Switching

• To properly handle sign-in via Facebook, enable app switching by overriding the `application(_:open:options:)` method in your `AppDelegate.swift`:

func application(_ app: UIApplication, open url: URL, options: [UIApplication.OpenURLOptionsKey : Any] = [:]) -> Bool {
    return ApplicationDelegate.shared.application(app, open: url, options: options)
}

Access User Information

• Once the user is authenticated, you can access their profile information using the Graph API. Use the following example to access user info:

if let token = AccessToken.current, !token.isExpired {
    // Use GraphRequest to fetch user data
    let request = GraphRequest(graphPath: "me", parameters: ["fields": "id, name, email"])
    request.start { _, result, error in
        if let error = error {
            print("Failed to fetch user data: \(error.localizedDescription)")
            return
        }

        if let result = result as? [String: Any] {
            print("Fetched user: \(result)")
        }
    }
}

Handle Errors and Exceptions

• Always add error handling in your implementation to manage scenarios where the login or data request fails.
• Consider displaying appropriate alerts to inform the user about what went wrong, such as loss of network connection or authorization issues.
• Check for a valid and non-expired `AccessToken` before making any Graph API calls to ensure smooth user experience.