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NSNotifications and Background Threads

I’m coming out of highly concentrated work (approx 87 hours) on my latest project. It’s an app that allows you to organize a betting pool for the upcoming FIFA World Cup.

The simplest method of communicating with our web-based API we found to be having the server send array or dictionary PLISTs which I could load and parse in a single line of code synchronously. So first I created all the API calls in synchronous blocking mode. When they where working I added a method by the same name prefixed with “async” and would have the blocking code executed on an NSOperationQueue.

If the synchronous method needed zero or one parameter then you can use NSInvocationOperation of calling it and have the queue work it off in the background. In some cases more than one parameter has to be passed. Here an NSInvocation has to constructed with multiple parameters, which I explained previously.

Once the API call is done processing it needs to tell the app about its result. This is done conveniently by sending NSNotifications. And in all the places in the UI where specific notifications should have an effect, you simply subscribe to the notifications by adding an observer for them to the default NSNotificationCenter. NSOperationQueue automatically uses multiple threads and takes care of the autorelease pool. So any operation might either run on the main thread or on a background thread.

Warning: Crash Ahead!

This causes a problem I have only ever seen happen on Simulator, so I’m not sure if it would also happen on the device. Generally you want the NSNotifications to be sent on the main thread as well, especially if they trigger UI activities like dismissing a modal login dialog. I don’t know if Apple will tweak NSNotificationCenter to send on the main thread in the future, but until they do, here’s my drop in solution.

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Parsing an RSS pubDate

Andreas Heller asks:

When parsing an RSS feed I get a date from pubDate, but as NSString and not NSDate. How do I get a date that I can use for sorting?

That’s a problem you face quite often when dealing with dates which are encoded in XML, be it an RSS feed or any other XML-based file format that you would be getting via HTTP GET. Contrary to other languages where any properly formatted date can be automatically parsed we have to do this ourselves in Cocoa.

Fortunately there is the NSDateFormatter class which can do it both ways: from date to nicely-formatted string as well as the other way around.

Let’s do like a Unix-pro and get ourselves some test data by getting some pubDates from my RSS feed. In terminal type:

curl www.drobnik.com/touch/feed/ | grep pubDate

This gets us the pubDates from the 10 latest articles on my blog. We see that WordPress encodes the pubDates in this format “Mon, 03 May 2010 18:54:26 +0000″, not really a very easy to parse one I concede. According to the RSS 2.0 spec this is supposed to be in RFC822 standard. If anyone would ask me, personally I think that the inventor of this date format should be poisoned, then hanged and maybe shot for good measure. Who in his right mind would create a date representation that does not allow for string sorting?

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Meta-Calling Multi-Param Methods

Objective-C has several powerful methods of working “Meta”. Besides of handling methods and objects themselves you can also save a method’s signature in a variable, and call it later. That’s a “selector”. Or you could construct an object representing the entire call of a method, including one or multiple parameters and then use this object in place of the actual call.

Why would one do such a complicated thing? Well, there are several pretty useful scenarios where it is useful to know this technique. The main benefit of this method is that you can construct method calls during runtime using information that you only get while your app is running.

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Physics 101 – UIKit app with Box2D for Gravity

Personally I was most at the edge of my seat at the “Voices that Matters Conference” in Seattle when Rod Strougo showed us how to make a physics-enabled game with Cocos2D and Box2D in under an hour. It really was as sexy as he sounds. Eros did what every good TV-cook would do, he had most things already prepared. On the flight home I wanted to see if I could just take the physics part (without Cocos2D) and make a UIKit app with it.

The goal of this experiment was to have a UIView with multiple square subviews of different sizes that would start falling as soon as the app starts. The first tricky part is how to add the latest version of the Box2D physics engine to your iPhone project. Then we need to mediate between the different units and coordinate systems of UIKit and Box2D. Finally when we got it all running, we also want to add  the current gravity vector to affect the boxes.

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Back (Almost)

My trip to Seattle was great, some of the talks on the conference where indeed worth my while and my host Kevin proved to be extremely hospitable. Something that did almost shock me in the beginning was being approached by half a dozen people individually who came to me to thank me for my blog. I don’t see myself as a celebrity, but I guess with 500 people stumbling upon your online ramblings, chances are that somebody will recognize you, especially if you attend an iPhone-related conference.

These are a few of the nice people that I had the presence of mind to get a picture with after they thanked me and told me that they are reading my blog. Thank you! I appreciate your appreciation. :-)

I did not get a chance to write anything useful since Seattle for several reasons. First I lost 2 days due to travelling back. One day was literally stolen from me because I had to travel back to Vienna to deal with the aftermath of a break-in into my jumpbase there. The following two days where a blur, I only faintly remember buying a parasol for the deck that our father-in-law built for us while I was away.

Most of the day yesterday I spent with writing invoices for the iPads I brought home for 3 colleagues and rewriting DTAboutViewController to be allow for server-side updating of any plist. And now it’s already weekend, the only useful thing I could do was write a lengthy post for my German personal blog and this one article here.

Please be patient with me while I still try to get back on firm ground. On the plane from Seattle I managed to put Box2D into a UIKit app to have buttons be “physical”. The recipe for this will come soon.

Seattle

Provided that my stroke of luck continues, I will be in Seattle, WA, USA this week. Currently it looks like I was extremely lucky having booked my flight leaving Tuesday instead of Monday. Had I booked Monday’s, my flight would have been fallen prey to volcanish ash from Ireland.

Towards the weekend I’ll be at the Voices That Matter iPhone Developer’s Conference and until then I hope to meet up with several fellow iPhone geeks who happen to live in the general area. Probably do a fair bit of sightseeing, as I have not been to Seattle before. And if possible fill the empty half of my suitcase with iPads. ;-)

So if you are in the general area, drop me a note. If you require my services please be patient until I return.

Dr. Touch #16 – "i of the Volcano"

Dr. Touch is waiting to see if he can fly to the US, there’s a volcano in the way. At the same time Apple unleashes hell in their developer agreement, delay of the international iPad release and lightning-fast new laptops. Today Mike Vallez is my special guest, we’ll talk about iPhone app marketing and his new eBook Secrets to Effective iPhone App Marketing.

Play

Show notes (aka my script) after the break.

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How to setup an SVN repo in Xcode

If you purchase a component from the Dr. Touch’s Parts Store you will receive a quick start guide which includes the access details to the Subversion repository that keeps my code. The address of the repository uses HTTPS protocol, which confuses some people into thinking that they could use a browser to download the files.

That’s why I made this video showing how to properly set up your Xcode to get the source for any part onto your harddisk.

The advantage of following this guide is that you end up with a “working copy” of the contents of the repository. That means that you can simply pick “SCM – Update Entire Project” and you get your copy updated to the latest version.

There are generally several methods how you could add external code to your own project, besides of simply copying it. The problem with just copying is that it’s quite hard to keep all the individual copies of a framework updated in all of your projects.

In my case I am continuing development on most of my components if I add them to one of my own apps, but I don’t want to have to make all additions that I put into the new project into the component project a second time. Development is boring if you have to repeat steps. Surely there has to be a method to have the updates stream back from the app project to the component project.

In fact, there is. The technique to use is called …

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Making Your Own iPhone Frameworks

Previously I explained how to make a static library that simultaneously supports multiple different CPUs. At that time I was ignorant to the reason why there are no means to make iPhone frameworks. But I did not give up searching, especially since apparently a few individuals seemed to have pulled it off without properly documenting their method for others to duplicate.

The only guide I found is from two years ago, but I was able to verify that it works and so I will document it in an article so that many more people can use this great mechanism. But first, let’s briefly explore what it really IS that Apple does not want us to do. The generic statement that you can read on thousands of pages is “Apple does not want you to make iPhone frameworks”.

But is that really so? It turns out, this statement is not entirely accurate or rather specific enough to be the truth.

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GeoCorder 1.0.5

Seems there is an ever-growing community about my little tool GeoCorder which I originally made to get GPS data for testing. It evolved into a light-weight utility many people use for geotagging DSLR photos.

A user informed me that I messed up with the 1.0.4 version of the free variant. Seems both versions got out of sync. The only difference should be that the regular version costs $1 and the free versions has a banner ad. So I went in and fixed a couple of things.

CHANGES

  • Fixed some memory leaks
  • Updated AdMob framework in free version
  • replaced a deprecated function
  • fixed the free/paid inconsistency

Now all the new features from 1.0.4 are also in the free version as it was intended.

UPDATE April 16th: I had to self-reject my binary because it appears that there was a bug in the ZIP routine. I also added an extra switch to not record events when you are not moving. I removed the 1 Meter setting for the distance Filter because it looks like CLLocationManager ignores that. This gets replaced by the switch.

UPDATE April 17th: Self-Rejected another time. User asked me to change the XML Version to 1.0 for compatibility.