While building 55Prophets for iOS (you can download it here), I wanted to allow deep linking into the app. For example, I wanted users to be able to share links to specific questions. When someone accessed the link, I wanted the app to take them directly to that question, instead of dropping them into the home screen that appears on a normal app launch. However, the question page is typically several layers deep into a navigation stack. So how do you link directly into a view controller like this?

To accomplish this, first you need to register a url scheme for your application. Apple provides documentation on how to do this here. Once you set this up, an iOS device with your app installed will launch your app any time a user touches a link with your url scheme (e.g. myappurlscheme://leagues/123).

To centralize deep linking, I created a class called FFDeepLinker that would process all links into the app. This class contains a singleton instance that can be used to process links. The FFDeepLinker.h file looks something like this:

@interface FFDeepLinker : NSObject

  @property (nonatomic, strong) UIWindow *appWindow;
  @property (nonatomic, strong) NSManagedObjectContext *managedObjectContext;

  +(instancetype)sharedLinker;
  +(void)setSharedLinker:(FFDeepLinker *)linker;

  -(void)handleUrl:(NSURL *)url;

@end

I then set up the shared singleton instance in the app delegate's - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions method. The setup looks something like this:

FFDeepLinker *deepLinker = [[FFDeepLinker alloc] init];
deepLinker.appWindow = self.window;
deepLinker.managedObjectContext = // the main core data context containing the app's data;
[FFDeepLinker setSharedLinker:deepLinker];

When your app is launched via one of these types of links, it will automatically call -(BOOL)application:(UIApplication *)application openURL:(NSURL *)url sourceApplication:(NSString *)sourceApplication annotation:(id)annotation in your app delegate. In this method, I simply call [[FFDeepLinker sharedLinker] handleUrl:url]; to process the url.

Inside FFDeepLinker is where the magic happens. There, I keep a dictionary that contains url pattern regular expressions as the keys and a corresponding method name as the value. It looks something like this:

+(NSDictionary *)deepLinkPatterns{
  return @{
    @"//leagues/(\\\\d+)/join" : @"handleJoinLeagueURL:",
    @"//leagues/(\\\\d+)/questions/(\\\\d+)" : @"handleShowQuestionURL:",
    @"//leagues/(\\\\d+)/comments" : @"handleShowLeagueCommentsURL:",        
    @"//leagues/(\\\\d+)/questions/(\\\\d+)/review" : @"handleShowReviewQuestionURL:"
  };
}

Each of the entries in this dictionary corresponds to one type of potential deep link that the app handles. The -(void)handleUrl:(NSURL *)url method cycles through the regular expressions and checks for a match against the url that we are opening. If it finds a match, it captures the variable path components (typically ID's of resources) and calls the corresponding method. It looks like this:

-(void)handleUrl:(NSURL *)url{
  NSString *linkString = [url resourceSpecifier];
  NSError *error = NULL;
  for (NSString *pattern in [[FFDeepLinker deepLinkPatterns] allKeys]) {
    NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:pattern
options:NSRegularExpressionCaseInsensitive error:&error];

    NSTextCheckingResult *result = [regex firstMatchInString:linkString options:NSMatchingAnchored
range:NSMakeRange(0, linkString.length)];

    if (result) {
      NSMutableArray *captures = [NSMutableArray array];
      for (int i=1; i<result.numberOfRanges; i++) {
        NSRange range = [result rangeAtIndex:i];
        NSString *capture = [linkString substringWithRange:range];
        [captures addObject:capture];
    }

    NSString *selector = [[FFDeepLinker deepLinkPatterns] objectForKey:pattern];
    [self performSelector:NSSelectorFromString(selector) withObject:captures];
    return;
  }
}

So, if we open a url with the pattern //leagues/123/questions/456, it will match to //leagues/(\\\\d+)/questions/(\\\\d+) and call the method -(void)handleShowQuestionURL:(NSArray *)capturedComponents with an array containing the strings "123" and "456". Within -(void)handleShowQuestionURL:(NSArray *)capturedComponents, I then load the league with ID=123 and question with ID=456 out of core data, create the appropriate series of view controllers, and present the nav stack to the user:

MembershipsViewController *membershipsVC = [self.appWindow.rootViewController.storyboard instantiateViewControllerWithIdentifier:@"MembershipsViewController"];

LeagueTabBarController *leagueVC = [self.appWindow.rootViewController.storyboard instantiateViewControllerWithIdentifier:@"LeagueTabBarController"];
leagueVC.membership = membership;

QuestionsViewController *questionsVC = [self.appWindow.rootViewController.storyboard instantiateViewControllerWithIdentifier:@"QuestionsViewController"];
questionsVC.membership = membership;

AnswersViewController *answersVC = [self.appWindow.rootViewController.storyboard instantiateViewControllerWithIdentifier:@"AnswersViewController"];
answersVC.question = question;
answersVC.membership = membership;

UINavigationController *navController = [[UINavigationController alloc] init];
[navController setViewControllers:@[membershipsVC, leagueVC, questionsVC, answersVC] animated:NO];

if(self.appWindow.rootViewController.presentedViewController != nil){
  [self.appWindow.rootViewController dismissViewControllerAnimated:NO completion:^{}];
}

[self.appWindow.rootViewController presentViewController:navController animated:NO completion:^{}];

Now, users can have links directly to questions, and the normal navigation back-buttons will be preserved and operate as expected.

P.S. If you enjoyed this post, feel free to hit me up on twitter @bcroesch.