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Threadsafe Lazy Property Initialization

I was looking for a safe way to initialize a property on individual instances of an object. “Safe” as in “thread-safe”, because nowadays you never know… with GCD it could just happen that an object is used from multiple threads concurrently.

I felt the need to chronicle the findings here one the different approaches I tried.

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Caching Caches

While doing some performance tuning on the iCatalog.framework I stumbled upon a method of about 7 statements where a single line was responsible for more than a third of all CPU time.

This basically was only getting the path to the app’s Library/Caches folder. By itself this statement looks very innocent and I had it in about a dozen places all around the app. But it turns out that if you calling it hundreds or thousands of times then the time it takes to search for the Caches (and Documents) path sums up enormously.

Interestingly it does not seem like Apple implemented any caching for them so they seem to use around the same time all the time. But these values are prime candidates for caching because they won’t change while your app is running. Also the objc function call to get a cached version of the paths is several orders of magnitude faster than determining it in the first place.

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Decompressing Files into Memory

As a hobby project I am working on uncovering hidden treasures that exist on all your iOS devices. Hidden, because there is no Objective-C API for them, Existing, because Apple includes a great deal of open source libraries in iOS, compiled as a dynamic library.

What items exist you can see if you check out what dylibs are there to be seen in “Link Binary with Libraries”. Most entries beginning with lib and ending with dylib can be used. Some people have reported getting rejected for adding the static variants of libraries like libxslt or libarchive, but that’s probably because Apple sees these symbols as duplicate to the ones contained in the dynamic libraries.

We previously looked at libxml2 for parsing HTML (and part 2), today we’ll familiarize ourselves with zlib for decompressing .gz and .zip files.

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Fast Folder Nuking on iOS

I got a strange bug report last week for iCatalog. Deleting of outdated catalogs takes too long, if we couldn’t show a HUD with a spinner while the deletion occurs. That was definitely one of these HUUUUU?! moments. I always thought that file deletion is instant on Unix since only an entry in a file table needs to be removed.

I grabbed an iPad 1 and deleted a 160 MB catalog. Only to find that the whole deletion – a simple NSFileManager removeItemAtPath – took 50 seconds. Uhm, no that is far from ideal to be blocking the main thread and interface for that long.

I played around a bit and over the course of the day, with some great help from several GCD experts on twitter, I pieced together a solution that might interest you if you ever have to delete large amounts of files in an instant. Before Cocoa, on Carbon, OSX offered a method called FSPathMoveObjectToTrashAsync, this is sort of the equivalent for iOS.

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Xcode Build Rules

When I moved the default CSS rules into a separate file I was facing the old dilemma: how can I embed this in the static library but still be able to easily add contents to it via Xcode. I previously explained how you can turn any file into a c-Byte-array. But this still required manual work.

I did a bit of researching and found that on regular Linux systems people seem to have a tool named objcopy which can copy files as their are into an object file (.o) which can be linked together to the final binary by the linker. But unfortunately this tool does not come with Xcode. So it is out of the question because I want everybody to be able to build DTCoreText.

Xcode Build Rules come to the rescue. They can automate any kind of preprocessing you like and it turns out they are an easy solution for this very problem.

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GitHub Fork, Fix, Pull Request

In this post I will to demonstrate how you can contribute to an Open Source project hosted on GitHub. I previously blogged about how you can make and apply patches, which is certainly one way. But on GitHub there is an even cooler method, one that is also less work on the project maintainer.

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Taming HTML Parsing with libxml (2)

The parsing of HTML necessary for my DTCoreText open source project is done entirely with the sophisticated use of NSScanner. But it has been long on my list to rewrite all of this parsing with a the industry standard libxml2 which comes preinstalled on all iOS devices. Not only is this potentially much faster in dealing with large chunks of HTML. It probably is also more intelligent in correcting structural errors of the HTML code you throw at it.

In part 1 of this series I showed you how to add the libxml2 library to your project and explained the basic concepts of using libxml2 for parsing any odd HTML snippet into a DOM. In this here part 2 we will create an Objective-C based wrapper around libml2 so that we can use it just like NSXMLParser, only for HTML.

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Helping Xcode Find Library Headers

In my previous article about Sub-Projects in Xcode I showed you how you can have an Xcode project as a dependency. The advantage of this is that you can update your library from within the same project and debug into the sub-project’s code.

One thing that was not immediately obvious was how you deal with the problem that Xcode cannot find your sub-project’s library headers. Hence this writeup.

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How to Make and Apply Patches

Sometimes you want to tell somebody how you fixed a problem in their code, but for some reason the code is not on github so you cannot send them a pull request. If you felt really smart then you might put the changes you made into an e-mail, like “in file1.m:102 you change it to x, in file2.m:54 you make change y”. Thought this doesn’t really help the developer you are trying to help. Even when following your change instructions to the letter it is a tedious and error-prone method of applying your changes. Thanks to M. Douglas McIlroy, Adjunct Professor at Dartmouth Colleague – one of the early Unix pioneers – there is a better way. He invented the form of diff that we are using today to get the Difference between two versions of a file. And Larry Wall who invented the patch command which can take a diff file and effectively apply it to files. Read more

Command Line Tools Tutorial (1)

Honestly I was very much excited when I found that I can use my current knowledge of Objective-C and Foundation classes like NSString to also build nifty little tools. Previously I had to resort to bash script to perform one-off operations on files that where too tedious to do manually. But knowing what I am going to show you in this article will enable you to also write these littler helpers.

I believe that beginners should rather start with writing a couple of command line tools before diving into building user interface driven apps. Commend line tools are linear and thus their function is easier to grasp. They are more akin to “functional programming” then “object oriented programming” if you will.

I am going to show you what goes into building a simple command line tool and you be the judge whether you agree with my assessment.

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