This is the first post of a series of tutorials for creating an event viewer for iOS and Android. In this post, we’ll examine the high-level application architecture. The project is located at https://github.com/gergelykoncz/Events.iOS.
The objective of the app is to display events in a view similar to the built-in daily calendar view. Since there are no public APIs for this neither on iOS nor on Android, we’ll roll out our own solutions, but first let’s plan the application.
We’ll work with only one entity (for now), which I decided to call Event. The event class looks like this:
As you can see, it has an identifier, a start and end date, a summary, a description, and a flag indicating if it’s an all day event. Nothing particularly difficult.
Events will come from a web service in JSON format, but it’s an implementation detail. I don’t really care where they come from at this point, but I’ve a firm belief that every class should do one and only one thing (the Single Responsibility Principle), so there should be a class that I can call and it gives me back some events. This is in fact an already named pattern, called Repository. Thus I created a class named EventRepository, which will serve my events whatever way it finds best.
Note: I don’t want to get too enterprisey at this point, but if we’d have more sources for our events, then we could create some more repositories (one for each source, with a common interface/base class), and use a Facade as a coordinator class which would delegate the event retrieval for based on whatever logic we find best.
So let’s review this EventRepository. As I stated before, the events come from a web service, in JSON format. Since it uses the network and this can (and will) introduce so much delay that our app would be very unresponsive if we would want to wait until data is loaded (if it’ll be loaded at all). That’s why I use NSURLConnection in an asynchronous fashion. When someone calls EventRepository to load the dates, it builds an NSURLConnection, set its delegate to itself and fires the request. After this the main thread is free to do whatever meaningful work instead of waiting for the request to complete.
There are two common cases at this point: the request fails, or it succeeds. In both cases we should notify the caller, but first let’s see the cases in detail.
If the request succeeds, we parse the result as JSON, using the NSJSONSerialization class. Since our API returns an array of events, we iterate over each JSON object in the array, and create an instance of Event using a different class, EventMapper. As its name suggests, EventMapper is a mapper. You can find this concept under many names (here is a discussion on the topic, and here’s a recent post of mine regarding mappers), the gist of it is that it is a class that it builds entities based on some input.
After we have our collection of events, we simply notify the caller through a delegate. If anything goes wrong, we use the same delegate as well. There are multiple ways of doing this, we could have used the NSNotificationCenter, but the Cocoa framework uses the Delegation pattern more extensively, and I got very used to it. The concept is exactly the same as with EventRepository and NSURLConnection. EventRepository simply delegates the task of retrieving events to NSURLConnection, and provides a way (in this case, a reference) to NSURLConnection to notify it when it’s done.
EventRepository offers this thing in its loadEventsForDate method. It accepts a date, naturally, and a delegate object which it’ll notify if it has a result (or any error occurs in the process).
Here’s the protocol (EventsDelegate) that is being used:
And we’re almost done! There’s a final piece without our work does nothing: we need a ViewController. And our ViewController has to implement the EventsDelegate so that it can be notified by EventRepository. The example class EventListViewController is for demonstration purposes only. It won’t render our shiny events in the promised daily calendar view format (that is the topic of an upcoming post), rather it uses a mere UITableView. Here is the source:
As you can see, by using the right patterns and careful delegation, we ended up with a short and maintainable view controller. In fact, less than ten lines (blanks included) are responsible for loading events and reporting errors. Most of the class deals with the UITableView delegates.
I hope that I managed to emphasize how clean your design can become when you use the appropriate patterns and concepts. I think that by creating smaller classes with only truly one goal you can end up with a more maintainable (and thus, cost-effective) codebase. Of course there are some vital functions missing here, like error handling and logging, but I tried to make the examples more concise.
But don’t forget that the point of this is not the admiration of patterns and best practices, but to make an app that works. I’ll follow up this post (and the GitHub project) when I implemented the view I promised.