Coding Practice

 
29
Jan
2013
 

Down with Magic Strings!

by Patrick Hughes

Developing iOS apps in Xcode is pretty great. With Objective-C and llvm we get type checking and autocompletion of all our classes and method names which is a nice improvement over my favorite dynamic languages. Unfortunately there are still some places where the compiler can’t help us. There are various resources we load from files like images, nibs & xibs and other resources which we need to specify by name, like a view controller we want to load from a storyboard.
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15
May
2012
 

Unit Testing with Core Data

by Saul Mora

Whether you subscribe to Test Driven Development (TDD) or another testing practice, when it comes automated unit testing with Core Data, things can be a little tricky. But if you keep it simple, and take things step by step, you can get up and running with unit testing using Core Data fairly quickly. We’ll explore the what, how and why of unit testing with Core Data. We’ll also be using the helper library MagicalRecord. MagicalRecord not only lets us get up and running faster, but helps to cut down on the noise in our tests.

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7
Jan
2011
 

Passing around a NSManagedObjectContext on iOS

by Marcus Zarra

**This article is reprinted from the MDN**

The documentation on Core Data for the iPhone has lead to some confusion about how best to use Core Data on a Cocoa Touch device. One particular section seems to be the most confusing, specifically:

> A view controller typically shouldn’t retrieve the context from a global object such as the application delegate. This tends to make the application architecture rigid. Neither should a view controller typically create a context for its own use. This may mean that operations performed using the controller’s context aren’t registered with other contexts, so different view controllers will have different perspectives on the data.

> When you create a view controller, you pass it a context. You pass an existing context, or (in a situation where you want the new controller to manage a discrete set of edits) a new context that you create for it. It’s typically the responsibility of the application delegate to create a context to pass to the first view controller that’s displayed.

The idea behind this section is the issue of rigidity. Ideally, each view controller should be an island on its own. It should not rely on its parent, nor should it rely on the Application Delegate. Once a view controller is pushed onto the screen it should ideally be its own master.

## Why Rigidity is bad

It is fairly common when designing a Cocoa Touch application to “hard code” everything. Take the following navigation controller design:

![Navigation Controller Design](http://www.cimgf.com/wp-content/uploads/2011/01/Image1.png “Standard Navigation Controller Design”)

When this design, it is common to code each view controller and make it “aware” of its parent. In that design, it would be common to see view controller B call methods or call back (to its delegate) view controller A. While there is nothing technically wrong with this design, it is very rigid. It is nearly impossible to either move view controller B to another location in the stack or to reuse view controller B somewhere else. This is the trap that the documentation is trying to help new developers avoid.

## Solution One

Again using a standard/normal navigation controller design, it is expected that the detail flows from left to right. The left most (or root) view controller contains the most vague information and the right most (or deepest) view controller contains the greatest detail.

![UIFetchedResultsController](http://www.cimgf.com/wp-content/uploads/2011/01/Image2.png “UIFetchedResultsController”)

In this case then the best solution is to use a `UIFetchedResultsController`. This controller can be considered a thin layer between the view controllers and the Core Data bits. The advantage is that the `UIFetchedResultsController` is designed to work with tables. The other advantage is that your least detailed view (the root most likely) can listen as the delegate of the `UIFetchedResultsController` for changes and update itself.

In this design, however, instead of passing around a context, you would hand off just the entity that the child view controller needs to know about. The Core Data Recipes example provided by Apple illustrates this design quite well.

How does this break rigidity? Each view controller, from the root on down, only knows what is passed into it. The root gets the `UIFetchedResultsController` passed into it. The child views only get the items it cares about passed into it. None of them care what their parent view controller is. There is no call back to a parent.

## Solution two

What happens when we don’t have a typical navigation controller design? Perhaps a child view can pop up a modal view that displays different information. Perhaps a child view, for whatever reason needs to access information that cannot be directly passed into it every time.

In these cases there are a few different options.

### View already has a `NSManagedObject`

Following our example above, lets say that view controller C needs to create a new object. Perhaps it is a detail view of a recipe and the user wants to add a new recipe type (perhaps she is a vegan and just discovered there is no vegan type in the list). In this case we have passed in an entity (the recipe) but not a reference to the `NSManagedObjectContext`. Fortunately this solution is easy to fix. The `NSManagedObject` retains a reference to its `NSManagedObjectContext` internally and we can access it. Therefore we can easily retrieve the `NSManagedObjectContext` from the `NSManagedObject` and create the new Type entity and pass it to the modal child or whatever our design calls for.

This again avoids rigidity because the view controller that represents the entity does not need to call up to a parent object or the `UIApplication` delegate. It is self contained and only manages view controllers that are down stream from it.

### View does not have a `NSManagedObject`

In this situation things are *slightly* more complicated. In this case we want to create a `@property` for the `NSManagedObjectContext` and require that our creator set the property.

@interface MyViewController : ViewController
{
NSManagedObjectContext *moc;
}

@property (nonatomic, retain) NSManagedObjectContext *moc;

@end

Again, the view controller is an island of its own because it does not care where that `NSManagedObjectContext` came from. All it knows is that it is required for the view to function. It does not care of it is a new `NSManagedObjectContext` specifically created for its use (perhaps for a cancelable edit tree) or if it is the same `NSManagedObjectContext` that has been passed around since the launch of the application. All it knows is that it has the elements it needs to perform its function.

By making the `NSManagedObjectContext` a settable property we can also transplant the view easily. If, at some point in the project lifecycle, we decide that it makes more sense to have the following design:

![Modal View Controller](http://www.cimgf.com/wp-content/uploads/2011/01/Image3.png “Modal View Controller”)

Taking from Apple’s Recipes Application, perhaps we decide that moving from the table view directly to the image of the recipe is more pleasing to the users and that when they want to see how to make it they can “flip” the image over and see the detail.

Making this change with each view controller being an island is quite simple. We just rearrange the views without having to worry too much about breaking the application.

## Solution three

Up until now we have been looking at just a navigation controller design. But what about tab bars? In the situation of a tab bar we again want to avoid rigidity because it is even more common that tabs will get moved around.

The solution to this is to again use a `@property` for the `NSManagedObjectContext` and require that the creator set this property before the view is displayed on screen. If you are creating the tabs in code this is trivial because you are already calling init on the view controller and you can add one more line of code after the init to set the property.

If the user interface is being developed mostly in Interface Builder it is slightly more tricky. Personally I am not a fan of creating navigation controllers or tab bar controllers in Interface Builder. However if that is the design then I would recommend referencing the view controllers as properties and passing along the context upon initialization of the application. It may be possible to do this entirely in Interface Builder but I am not comfortable enough to recommend that as a solution.

## Conclusion

The overall idea behind this article is to keep each view controller separate from anything up stream or in a different silo. This will make the design far more flexible in the long run. Any time that you feel the “need” to pass in a parent view controller to a child view controller, reconsider the design. Consider using a `@protocol` delegate design or NSNotification calls instead. Keep each view controller as its own private island.

 
2
May
2010
 

My current Prefix.pch file

by Marcus Zarra

I have posted and discussed this file a few times but as with all things it has been touched, tweaked, and generally improved upon.

In this article we will discuss the latest iteration of my `Prefix.pch` file. As with anything I post, it is available for you to use as you see fit.

## The File

For those who don’t want to read the entire post, here is the file:

#ifdef DEBUG
  #define DLog(...) NSLog(@"%s %@", __PRETTY_FUNCTION__, [NSString stringWithFormat:__VA_ARGS__])
  #define ALog(...) [[NSAssertionHandler currentHandler] handleFailureInFunction:[NSString stringWithCString:__PRETTY_FUNCTION__ encoding:NSUTF8StringEncoding] file:[NSString stringWithCString:__FILE__ encoding:NSUTF8StringEncoding] lineNumber:__LINE__ description:__VA_ARGS__]
#else
  #define DLog(...) do { } while (0)
  #ifndef NS_BLOCK_ASSERTIONS
    #define NS_BLOCK_ASSERTIONS
  #endif
  #define ALog(...) NSLog(@"%s %@", __PRETTY_FUNCTION__, [NSString stringWithFormat:__VA_ARGS__])
#endif

#define ZAssert(condition, ...) do { if (!(condition)) { ALog(__VA_ARGS__); }} while(0)

This does not *replace* the Prefix.pch that comes with your project but it does go at the top of every project that I work on. The rest of this post we will review what this does.
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18
Feb
2010
 

Creating a NSManagedObject that is Cross Platform

by Marcus Zarra

An interesting question came up on Stackoverflow today so I decided to expound upon it in a short blog post.

A situation that I believe we are going to be seeing more and more often is one where application developers are writing multiple “versions” of their applications to be used on the desktop, their iPhone and now the iPad.

Because of that situation, it is becoming even more important that we write as much portable code as possible. Fortunately, our model can be completely portable between the two platforms.

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4
Sep
2008
 

Cocoa Tutorial: Creating your very own framework

by Marcus Zarra

There are numerous situations where creating your own framework is advantageous. Perhaps you have a block of code that you use repeatedly in many different projects. Perhaps you are building a plug-in system for your application and want the infrastructure to be available both to the application and to any plugins that are coded.

A Cocoa framework is another project type in Xcode. The end result is a bundle, similar to an application bundle. Inside of this bundle is the compiled code you wrote and any headers that you want exposed. The headers are important. Without them, just like any other piece of Objective-C code, it is very hard to code against.

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3
Jun
2008
 

Version Control Makes You A Better Programmer

by Matt Long

I’m a believer. I’ve used version control before, but Marcus has convinced me that with a little known version control system called Git, written by Linus Torvalds (the creator of Linux), version control is not just about versioning, it’s about expressing yourself with your code and collaborating with others, seamlessly.

As memory serves the only time I’ve used version control in a meaningful way the system I was using was Visual SourceSafe from Microsoft. I know. Blech! It’s awful! I’ve pulled code from many a CVS or Subversion repository, but I’ve never really used them in the way they are intended to be used. Now, thanks to Marcus, I realize that version control isn’t just about versioning any more. It’s a whole methodology/ideology that makes better programmers. Here is what I mean.
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29
May
2008
 

Coding Practice: Cleaning up the default Core Data project

by Marcus Zarra

In this entry I am going to do something a little different. Instead of showing some wicked code, I am going to reformat one of Apple’s default templates.

Apple writes some amazing code. Unfortunately a lot of their templates demonstrate simply terrible coding practices. What is worse, people take these templates and assume that they are the proper practice for coding in Objective-C and propagate that poor code into their own projects.

The first example I am going to tackle is the AppDelegate class that is auto generated in the Core Data template. This is not the Core Data Document template but the standard application template.

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23
Apr
2008
 

Cocoa Tutorial: Don’t Be Lazy With NSDecimalNumber (Like Me)

by Marcus Zarra

NSDecimalNumber is Objective-C’s solution to numbers that need to be very precise. The documentation defines it as:

NSDecimalNumber, an immutable subclass of NSNumber, provides an object-oriented wrapper for doing base-10 arithmetic. An instance can represent any number that can be expressed as mantissa x 10^exponent where mantissa is a decimal integer up to 38 digits long, and exponent is an integer from –128 through 127.

NSDecimalNumber

If you are dealing with currency at all, then you should be using NSDecimalNumber. However, since it is immutable and definitely not a primitive then it is difficult to use right? Well — yes — a bit. But if you do not want to see your $9.50 item displayed as $9.49999994 or something then you are better off using NSDecimalNumber right from the beginning. Otherwise you are going to be converting to it later and that is a LOT more painful.

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13
Apr
2008
 

Git and XCode: A git build number script

by Marcus Zarra

Git has been gaining in popularity with Cocoa developers as well as open source developers. As I work it into my development workflow, one item that was missing was the automatic injection of the build number into the application bundle.

There are a few scripts floating around that perform this trick for subversion, but git handles build numbers a bit differently and it appears that no one has bothered to publish one. As is known, subversion uses an incrementing integer for build numbers. This makes it very easy to determine which build number came first and makes it very useful for a non-public version number. Git, however, uses a hash for each build number which is not incrementing and therefore not very useful for determining version numbers. However, it is still very useful for pulling up a specific build when a crash report is received, etc.

Therefore, with the help of Matt Long’s perl-fu, I have updated Daniel Jalkut’s subversion perl script to work with git. Since the build numbers are not sequential, I would not recommend using them for Sparkle. Therefore, in my own build process for iWeb Buddy, I hand select the version number (for example 1.0.4) and then use the short hash from git as the CFBundleVersion number. Normally this number is displayed in parens after the primary build number but, at least in iWeb Buddy, I have removed it from the display entirely. Since it is no longer a sequential number it would only potentially confuse users and it displays in the crash reports anyway.

The updated script is as follows:

# Xcode auto-versioning script for Subversion by Axel Andersson
# Updated for git by Marcus S. Zarra and Matt Long

use strict;

# Get the current git commit hash and use it to set the CFBundleVersion value
my $REV = `/opt/local/bin/git show --abbrev-commit | grep "^commit"`;
my $INFO = "$ENV{BUILT_PRODUCTS_DIR}/$ENV{WRAPPER_NAME}/Contents/Info.plist";
    
my $version = $REV;
if( $version =~ /^commit\s+([^.]+)\.\.\.$/ )
{ 
	$version = $1;
}
else
{
	$version = undef;
}
die "$0: No Git revision found" unless $version;
    
open(FH, "$INFO") or die "$0: $INFO: $!";
my $info = join("", );
close(FH);
    
$info =~ s/([\t ]+CFBundleVersion<\/key>\n[\t ]+).*?(<\/string>)/$1$version$2/;
    
open(FH, ">$INFO") or die "$0: $INFO: $!";
print FH $info;
close(FH);

Since git is distributed, there is no need to be online to produce a build. The script will grab the current abbrev-commit hash and will inject it into the current build’s Info.plist file.

 
4
Apr
2008
 

Cocoa Tutorial: Using NSError to Great Effect

by Marcus Zarra

Error handling is rarely fun. I find myself often re-coding a method after I realize that I need to handle one error condition or another. Usually, error handling involves either try/catch or some return code strategy. Neither of those is pretty or easy to maintain. For Objective-C development, however, there is another option — NSError.
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2
Apr
2008
 

Cocoa Tutorial: Fixing Memory Leaks With Instruments

by Matt Long

Leaks IconAs I am getting toward what I think is the end of coding for an application I hope to release soon, the nitty gritty work of fixing leaks, optimizing code, and squashing bugs has become the majority of what I’m doing now. Gone is the fun part of the application development process where I was creating features and solving new problems. It is now drudgery and focusing requires diligence. I know that the rewards are worth it as these final steps are what give an application stability and make it shine, but getting through it can be nothing but toil. Fortunately with the developer tools that shipped with Leopard, Apple has made this work much easier to handle in a little application called Instruments.
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26
Mar
2008
 

Cocoa Tutorial: awakeFromNib vs applicationDidFinishLaunching

by Marcus Zarra

When developing an application in Objective-C and using Cocoa, there is a lot of “magic” that happens in the background. As we get more comfortable with the language and the APIs, we begin to discover the source of that magic and understand not only WHY it works but HOW it works.

One of those areas is the initialization and callbacks from the nib files to my code. Normally, when I want a controller to do something after the NIB/XIB has loaded, I add the method -(void)awakeFromNib and know that I will receive a call when all of the connections into the NIB/XIB are complete. But on what object does this get called and how?

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1
Mar
2008
 

Does Objective-C Perform Autoboxing on Primitives?

by Marcus Zarra


This article is inaccurate.


The writer was smoking crack or something when he wrote it and has not been able to duplicate his tests since. This article is left here for historical reasons.

One of the things about Objective-C that I find extremely useful is the ability to resolve a method call at runtime. In addition this same functionality allows us to do some fairly creative things with callbacks, passing messages between threads, etc.

However there is a bit of a trick when it comes to passing primitives though some of these methods. For example, one method that I use quite frequently is performSelectorOnMainThread:withObject:waitUntilDone:. How exactly does one pass a BOOL or an int to this method?
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