Generating a Client

To register your application with Publication Subscription, create a client object. Sending applicationClient to the PSClient class returns a client object for the current application, as the following code shows.

PSClient *client = [PSClient applicationClient];

You can also create a client object to inspect feeds of another application. To create this client object, you need the bundle identifier of the other application. The bundle identifier is a string using the reverse-DNS naming convention that uniquely identifies each application that uses Publication Subscription. Rather than using the applicationClient method to create the new client object, send clientForBundleIdentifier: to the PSClient class and pass the bundle identifier.

For example, for your application to view the feeds that Mail subscribes to, you would use:

PSClient *mailClient = [PSClient clientForBundleIdentifier:@"com.apple.mail"];

Using a Subscribed Feed

After your application has its client object, it can subscribe to feeds. First, create an NSURL object containing the URL to the feed. Then, use the client object method addFeedWithURL: to subscribe the client to the feed and return a newly initialized PSFeed object. The code should look like Listing 3-1. Keep in mind that the addFeedWithURL: method does not create duplicate entries in the database. If you add the same feed twice, you receive the same data from the original database entry.

Listing 3-1  Subscribing a client to a feed

NSURL    *url    = [NSURL URLWithString:

                          @"http://www.apple.com/main/rss/hotnews/hotnews.rss"];

PSFeed   *feed   = [client addFeedWithURL:url];

Some feeds may require a user name and password for users to access them through HTTP authentication. Store the authorization information with the feed by setting the login property of the feed object for the user name, and use the setPassword: method to store the password in the user’s default keychain. Publication Subscription also uses the same cookie database as Safari, so if the user is logged into the site of the feed through Safari, Publication Subscription also has access. If you don’t store the authorization information with the feed and the user is not logged in through Safari, the PubSub Agent requests the authorization information from the user. Listing 3-2 shows how to store the user name and password in the feed object.

Listing 3-2  Accessing a protected feed

// Place user name and password in NSString objects

NSString *login = @"username";

NSString *password = @"password";

// Store the authorization information in the feed

feed.login = username;

[feed setPassword: password];

Using a Feed Without a Subscription

You can create a feed object without subscribing to the feed. In this situation, you need to use the feed object method initWithURL: and pass the URL of the feed. See Listing 3-3.

Listing 3-3  Initializing a PSFeed object without a subscription

NSURL    *url    = [NSURL URLWithString:

                          @"http://www.apple.com/main/rss/hotnews/hotnews.rss"];

PSFeed   *feed   = [[PSFeed alloc] initWithURL:url];

NSError *error;

[feed refresh:&error];

If you do this, the feed is not automatically updated as a subscribed feed would be. Instead, you are using the Publication Subscription framework to parse the feed.

Storing a Feed’s Preferences

A client can subscribe to multiple feeds, but not all feeds are updated at the same interval. You may have one feed that needs to be checked every five minutes, while another feed only needs to be updated every three hours. Similarly, you may want the time it takes an entry to expire to depend on the feed. The feed settings objects allow you to customize the preferences for each feed. Each feed object is instantiated with a group of default settings.

To customize the preferences for a feed, obtain a feed settings object by either using the settings property of a feed object, or the PSFeedSettings defaultFeedSettings class method. Then adjust the settings as you see fit with the different properties. Store the updated feed settings in the feed object.

Listing 3-4 retrieves the settings object, sets the entries to expire after 30 days, sets the feed to update every 30 minutes, and then puts the settings object back in the feed.

Listing 3-4  Modifying the settings of a feed

// Retrieve PSFeedSettings object from feed

PSFeedSettings *feedSettings = feed.settings;

// Set entries to expire after 30 days (60 s/m * 60m/h * 24 h/d * 30 d)

feedSettings.expirationInterval = 2592000;

// Set feed to check for updates every 30 minutes (60 s/m * 30m)

feedSettings.refreshInterval = 1800;

// Store settings back in the feed

feed.settings = feedSettings;

You can also set the feed settings for a client. If you do, a feed that uses the default settings inherits the client’s settings instead of the default ones. However, any settings unique to a feed overrides the settings of the client.

You now know how to create a feed object for any web feed and adjust the preferences for them. The next chapter, Viewing and Retrieving Content, explains how to obtain the entries and other information in the feed.