GitHub OAuth

Objectives

• Set up your application to use the OAuth2 protocol to access a user's GitHub account

Introduction

GitHub's developer site provides a very succinct description of OAuth and why to use it.

OAuth2 is a protocol that lets external applications request authorization to private details in a user's GitHub account without getting their password. This is preferred over Basic Authentication because tokens can be limited to specific types of data, and can be revoked by users at any time.

IMPORTANT: Use GitHub's OAuth API documentation throughout this lab to understand the details of constructing and handling the requests and responses necessary for completing the process. Implementing the protocol requires joint effort from the application (client), the user (resource owner), and the website (resource server). Here is a brief summary of steps to use GitHub OAuth:

1. Register your application with GitHub to receive a Client ID and a Client Secret.
2. Set up an Authorization callback URL on GitHub.
3. Set up a URL Scheme in Xcode for your application.
4. Direct user from within your application to GitHub for authorization.
5. Handle callback from GitHub containing a temporary code.
6. Use code to authenticate user and receive access token.
7. Save the access token in your application.
8. Use the access token to make requests for user account information.

So why use OAuth in your application? Since your application will use resources from your user's GitHub account, following the OAuth protocol grants you access to those resources. Additionally, the user will not have to be authenticated by GitHub AND your application. This saves your poor user from the agony of remembering another username and password.

This project is similar to the previous GitHub related labs, however it has been updated and organized a bit differently. Here's a run down of what's right out of the box:

• Model
• Repository class is used to create repo objects.
• RepositoryDataStore class stores Repository objects.
• View
• RepositoryTableViewCell class is the reusable cell for the RepositoryTableViewController. This class also handles starring requests.
• Controllers
• AppController class handles which view controller is displayed.
• LoginViewController class directs the user for authorization and authentication.
• RepositoryTableViewController class displays repositories and facilitates starring a repository.
• Networking
• GitHubAPIClient class interacts with the GitHub API.
• Utility
• Constants contains a struct of static storyboard IDs.
• Extensions contains a NSURL extension for parsing query items and a Notification.Name extension of static notification names.
• Secrets (you will need to add this file).
• Pods
• Locksmith is a protocol-oriented library for working the keychain (e.g., save user's access token).
• Etc.

For now, you can run the application to see some animated octocats. They are cheering for you so let's get started!

1. Set up your callback URL

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• Head on over to GitHub. Reference this screenshot to help you along the way.
• If you don't have a Client ID and a Client Secret set up from previous labs, go to Settings > OAuth Applications > Developer Applications > Register and start registering your new application.
• Whether you are registering a new application or have your application selected, find the header at the bottom of the form titled Authoriation callback URL.
• Enter some text following this format: gitHubOAuthLab-12345://callback. The first section before the colon, gitHubOAuthLab-12345, can be whatever you want (e.g., "gitHubOAuthSuperDuperLab"). It's intended to be unique to your application.
• Head on over to your project in Xcode and select your project in the the Project Navigator. Reference this screenshot to help you along the way.
• In the editor, select your project target, then select Info and look at the bottom of the list for URL Types.
• Expand the URL Types section and click on the plus sign.
• Enter your URL Scheme using the unique name you created above (e.g., gitHubOAuthLab-12345) and press enter. This will update your Info.plist file with your new URL scheme.

2. Add your Secrets file

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• Create your Secrets file and add your Client ID and Client Secret

struct Secrets {
   static let clientID = ""
   static let clientSecret = ""
}

3. Update the GitHubRequestType enum to include an oauth request

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The organization of the GitHubAPIClient file is different from previous GitHub related labs. The GitHubRequestType enum has associated types that provide the URL and HTTP Method for each type of request. The nested enums inside GitHubRequestType and the buildParams(with:) function are used to construct the different URL components of each type of request.

• Begin by adding the oauth case to the GitHubRequestType enum.
• The oauth case is used to redirect users to request GitHub access.
• Add a static constant to BaseURL called standard with a value of "https://github.com".
• Add a static constant to Path called oauth with a value of "/login/oauth/authorize".
• Add a static constant to Query called oauth. Refer to the other queries already listed to understand how they are constructed. The string query should be constructed with the following parameters:
• "client_id"=Secrets.clientID
• "scope"="repo"
• Add the oauth case to the method computed property and return nil within the oauth case.
• Add the oauth case to the url computed property and return the complete URL. The complete URL should be the following, BaseURL.standard + Path.oauth + Query.oauth.

You might see that you have two errors in Xcode which state that there are other switch statements that are not exhaustive. For those switch statements,--feel free to add in default implementation or just mix in the .oauth case with another case for now as we'll be implementing this later.

4. Use SFSafariViewController to request authorization

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• Locate the loginButtonTapped(_:) IBAction method in the LoginViewController class.
• Inside the method, pass the value from GitHubRequestType.oauth.url to to initialize a SFSafariViewController.
• Note: The safari view controller streamlines the process of directing a user to GitHub by providing easy access to a stripped down version of the Safari web browser inside your application.
• Hint: Import the Safari Services framework to use SFSafariViewController. Also, you will need to reference the safari view controller from a couple of methods within the LoginViewController class.
• Present the controller.
• Run the application to see if your safari view controller is presented when the login button is tapped. It should direct the user to the GitHub site to enter login information (Don't bother entering your GitHub credentials yet).
• Stop the application.

5. Handle the callback from GitHub

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In the previous step the user is directed to GitHub using a safari view controller to provide authorization. Once the user successfully completes authorization, the callback you provided in your GitHub account is used to trigger the URL Scheme you provided in your project settings. Additionally, the safari view controller calls a UIApplicatioDelegate method called application(_:open:options:) that passes a URL containing a temporary code received from the GitHub callback.

• Add the application(_:open:options:) method to your AppDelegate file. This method looks like the following:

    func application(_ app: UIApplication, open url: URL, options: [UIApplicationOpenURLOptionsKey : Any] = [:]) -> Bool {

    }

• Get the value for the key UIApplicationOpenURLOptionsKey.sourceApplication from the options dictionary argument.
• If the value equals "com.apple.SafariViewService", return true.
• Hint: The value from the options dictionary is of type Any and needs to be a String in order to make the comparison.

Up until now you probably haven't used NotificationCenter but you're about to take a crash course. In the simplest terms, you can post a notification saying, "HEY! SOMETHING HAPPENED!" An observer of the notification will be notified somewhere else in the application (and would probably say to themselves, "Why are you yelling at me? 😥").

Here are the two notification statements you will use in your application:

// Post notification
NotificationCenter.default.post(name:object:)

// Add observer
NotificationCenter.default.addObserver(_:selector:name:object:)

Now that you are a notification's expert, let's continue.

• In the previous step you verified the value, "com.apple.SafariViewService" and returned true. Add a post notification immediately before you return true. Use the Notification.Name extension from your Extensions file to provide the name .closeSafariVC. Pass the value from the incoming url argument to the object parameter of the notification. This notification is posting a message to anyone listening that it's ok to close the SFSafariViewController you presented inside the LoginViewController. The notification also includes the url argument passed as an object.
• Note: As mentioned above, the incoming url argument of the application(_:open:options:) method contains a temporary code that we need to proceed with the GitHub authentication process.
• Head back to the LoginViewController class and add a method called safariLogin(_:) that takes one argument called notification of type Notification and returns nothing.
• Add a notification observer inside viewDidLoad() of the LoginViewController class.
• The observer is the LoginViewController.
• The selector is the method you just created above, safariLogin(_:).
• The name is the name you used for the post notification in the app delegate, .closeSafariVC.
• The object is nil.
• Inside safariLogin(_:) access the .object property of the notification argument to get the URL passed from the app delegate.
• Hint: The .object property is Any?. How can you turn that into a URL.
• Print the URL to the console.
• Dismiss the safari view controller.
• Run the application, provide your credentials to GitHub in the safari view controller, and authorize the application.
• The URL containing the temporary code should print to the debugger and the safari view controller should be dismissed.

6. Update the GitHubRequestType enum to include an access token request

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Now that you have received a URL containing a temporary code from GitHub, you can make a request to receive an access token. Head back to the GitHubAPIClient file to update the GitHubRequestType enum.

• Begin by adding the token case to the GitHubRequestType enum.
• The token case is used to request an access token.
• Update the token case to accept a URL argument named url as an associated type.
• The token request should use the standard static constant from the BaseURL enum.
• Add a static constant to Path called accessToken with a value of "/login/oauth/access_token".
• Instead of adding parameters to the Query enum, you are going to update the buildParams(with code: String) function that returns a dictionary of parameters. When requesting the access token, you will need to provide your "client_id", "client_secret", and the temporary "code" you received back from GitHub as parameters. The code argument in the function is the temporary code that needs to be included.
• Complete the function by returning the dictionary of parameters. The function should only return the dictionary for the token case. Otherwise, it should return nil.
• Add the token case to the method computed variable and return "POST".
• Add the token case to the url computed variable and return the complete URL.
• The URL should contain the standard base url and the accessToken path. The buildParams(with:) function will be used in the next step.

7. Update generateURLRequest(_:) to handle the token case of the GitHubRequestType enum

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Before you move forward, take a moment to look through GitHubAPIClient. Take note of each section and how the methods are organized.

• // MARK: Response Typealias
• Type aliases are handy for giving alias names to existing types. In this lab it's used as a means to organize the response from all GitHub requests.
• // MARK: Request
• Routes all GitHub related requests through one method using the GitHubRequestType.
• // MARK: Request Generation
• Returns a URLRequest based on the type of the request.
• // MARK: Session Generation
• Returns a default URLSession.
• // MARK: Response Generation
• Executes a data task using the request type, session, and request. Contains a completion handler call back to pass three different values (See the type aliases mentioned above).
• // MARK: Response Processing
• Separate methods for handling each type of request.
• // MARK: Token Handling
• Separate methods and a variable for handling the access token received from GitHub.
• // MARK: Error Handling
• An error enum to use for various errors that could occur during the request process or the response handling.

With all of that in mind, start by updating generateURLRequest(_:).

• Add the token case.
• Remember this case has an associated URL type. When the token case of the GitHubRequestType is used, the URL containing the temporary code is passed in. You should use the value of the associated type to extract the temporary code from the URL that's passed in. You can capture the value in the case declaration like this: case .token(url: let url).
• Declare a string constant called code where the value is the return of a URL extension in the Extensions file called getQueryItemValue(named:). To get the code from the URL, pass in "code" as the named: argument, the function will find the query item for the key "code" using URLComponents. The function returns the code as an optional string.
• Declare a constant called parameters where the value is the return of the buildParams(with:) function. The code constant you just created needs to be added to a dictionary of parameters. Remember that you updated the buildParams(with:) function in the previous step. Use this function to pass in the code string in order to build a completed parameters dictionary for the request.
• Create a URLRequest called request and update the .httpMethod string property of the request using the type argument (Use the previous cases as a reference).
• Use the addValue(_:forHTTPHeaderField:) function on URLRequest to add the two following values to the header:
• "application/json" for header field, "Accept".
  • Included to indicate to GitHub how the response should be formatted.
• "application/json" for header field, "Content-Type".
  • Included to indicate to GitHub how the parameters are formatted in the request.
• Use the .httpBody data property of URLRequest to add your parameters dictionary.
• The current format of parameters needs to be serialized using JSONSerialization before it's applied as the value to .httpBody. Use the class function on JSONSerialization called data(withJSONObject:options:) where the object is parameters and the options are an empty array.
• After applying the serialized parameters to .httpBody return the request. return nil if the serialization fails.

8. Update generateResponse(type:session:request:completionHandler:) to handle the token case of the GitHubRequestType enum

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• Add the token case to the switch statement. Call processToken(response:) to process the response. Use the other cases as a reference.

9. Update processToken(response:) to handle the received access token

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• Update the processToken(response:) method to get the "access_token" from the JSON dictionary (use the incoming Data? argument to implement the JSON serialization). Once you get the access token from the JSON dictionary, save it as a string using saveAccess(token: String). The string is saved using the Locksmith pod. If all goes well, your Response return should be all nil values, otherwise bubble up an error in your response.
• HINT: If you're unsure of how to construct the method, use the other response processing methods as a reference.

10. Make the token request inside the safariLogin(_:) method of the LoginViewController

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Now that the GitHubAPIClient file has been updated to handle the request for an access token, it's time to turn your attention back to the LoginViewController. If you remember, the LoginViewController presents a SFSafariViewController to begin the authorization process with GitHub.

When GitHub redirects to your application with a temporary code, application(_:open:options:) is called in the app delegate. This is where you posted a notification saying it's ok to close the safari view controller. The notification observer you added to the LoginViewController calls safariLogin(_:). This is the method where you retrieved the URL from the notification object property.

As mentioned previously, the URL containing the temporary code now needs to be passed in as a part of the request to the GitHubAPIClient using the token case of the GitHubRequestType enum.

• Call the request(_:completionHandler:) from GitHubAPIClient using the token request type. Pass the URL as the value of the associated type.
• If error is nil, add the following statement: NotificationCenter.default.post(name: .closeLoginVC, object: nil).
• NOTE: The completion handler has three arguments. For the token request, you are only concerned with whether or not an error has occurred. If there has not been an error, a notification will be posted to an observer in the AppController to close the LoginViewController and present the RepositoryTableViewController.
• Build and run the application. If everything is set up correctly, you should see the RepositoryTableViewController displaying a list of repositories.

Advanced

• Starring

• Update the starred(token:) static function of the private Query enum inside GitHubRequestType. The incoming token argument needs to be added to the return string in order for the application to star/unstar the repositories listed in the ReposTableViewController. Build and run the application to see if a star icon appears in each row next to the repository name. A user should be able to tap a star icon to star/unstar repositories.

• Logout

  • The RepositoryTableViewController has an IBAction for the log out button. This method needs to call deleteAccessToken() from GitHubAPIClient and use the optional Error return to determine whether to post a notification to the AppController to close the table view controller. deleteAccessToken() still needs to be defined. It should delete the token and return an error if one occurs.