5 Time-saving Objective-C tips every developer should know

Here’s a quick fire list of 5 time-saving Objective-C tips that every developer should know. Perfect for making time for that extra coffee!

1. Enum shorthand

Enums at their simplest are labelled sets of integers. Where an inexperienced programmer might use a number to represent the download state of an image lets say, e.g. 0 = queued, 1 = downloading, 2 = downloaded, 3 = complete… When using these numbers in actual code a simple slip of the finger on the keyboard and they could easily type an extra digit in their if statement or assignment. These types of errors don’t usually show up when compiling. Even worse, these are usually obscure errors that only reveal themselves at run-time and only then when things don’t work as expected often requiring considerable debugging effort.

This is where enums step in. Enums are really handy and allow you to associate symbols or ‘labels’ with integers. If you type an enum value that doesn’t exist the compiler steps in and reports an error at compile-time instead and Xcode will probably suggest a fix for you if it was a typo! The other benefit of enums is that you can quickly add new values into them and assuming your code doesn’t save the integer to disk your code will automatically work with the new values.

There are a couple of different ways to define enums but the easiest way which will also provide Xcode with some extra compiling hints is as follows:

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How to call a block after a delay

On iOS and on OS X you sometimes need the User Interface to update after a short delay. The old way of doing it was calling the performSelector:withObject:afterDelay: selector on any NSObject subclass but that requires defining a new method in your class and you can only pass one object as a parameter.

Instead, you can use dispatch_after from the Grand Central Dispatch APIs to execute code within a block after a certain time interval. Don’t be afraid, it might be low-level C but you can cut and paste and just put your code inside and it will retain the variable scope that blocks usually do!

double delayInSeconds = 0.5;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){

  // Your code here

});

Calculating UNIX file permissions

Permissions Mac App A few years ago I wrote a simple but handy Mac app that calculates unix file permissions using a matrix of check boxes. I wrote it because I wanted to better understand how those octal values get calculated and to expand my experience of writing Mac apps.

I was also learning some crazy assembly code at the time too so I was also making sense of putting bitwise operations to task. Rather than let this code languish on my hard drive, I thought I’d share…
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Core Data Objects in Wrong Sections

NSFetchedResultsController is a really handy class. Use one of the default Core Data templates in Xcode and you’ll very quickly have a nice list of managed objects in a table view. With a few more lines of code you can get the NSFetchedResultsController to group your objects by sections. You do this by specifying a key-path in the class’s constructor method but there is another step that if overlooked will cause some confusion.

In a sample app I’ve created a food table that lists food in categories.

FetchedResultsController method grouping sections using a key-path:

Objective-C:

- (NSFetchedResultsController *)fetchedResultsController {
	if (fetchedResultsController != nil) {
		return fetchedResultsController;
	}

	// Create and configure a fetch request with the food entity.
	NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
	NSEntityDescription *entity = [NSEntityDescription entityForName:@"RWFood" inManagedObjectContext:managedObjectContext];
	[fetchRequest setEntity:entity];

	// Create the sort descriptors array.
	NSSortDescriptor *nameDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
	NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:nameDescriptor, nil];
	[fetchRequest setSortDescriptors:sortDescriptors];

	// Create and initialize the fetch results controller.
	NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:@"category" cacheName:@"Food"];
	self.fetchedResultsController = aFetchedResultsController;
	fetchedResultsController.delegate = self;

	// Memory management.
	[aFetchedResultsController release];
	[fetchRequest release];
	[nameDescriptor release];
	[sortDescriptors release];

	return fetchedResultsController;
}

Swift:


lazy var fetchedResultsController: NSFetchedResultsController<rwfood> = {
    let fetchRequest: NSFetchRequest<rwfood> = RWFood.fetchRequest()
    let sortDescriptor = NSSortDescriptor(key: "name", ascending: true)
    fetchRequest.sortDescriptors = [sortDescriptor]
    let aFetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: managedObjectContext, sectionNameKeyPath: "category", cacheName: "Food")
    aFetchedResultsController.delegate = self
    do {
        try aFetchedResultsController.performFetch()
    } catch let error {
        print("Unable to perform fetch: \(error)")
    }
    return aFetchedResultsController
}()

Specify a key-path

Save and quit the app a few times and you’ll see the objects seem to be in the wrong sections. If you look closer you’ll see that the objects are actually sorted in ascending name order. On looking at the code, it seems this is exactly what we asked the program to do! After some testing it also seems to show up more often if the table is a grouped one.

As per the docs, after you specify a key-path to group each section with you also need to make sure the first sort descriptor is sorting this key-path. Add a sort descriptor and everything will work as expected.

Revised fetchedResultsController method with missing sort descriptor:

Objective-C:

- (NSFetchedResultsController *)fetchedResultsController {

	if (fetchedResultsController != nil) {
		return fetchedResultsController;
	}

	// Create and configure a fetch request with the plant entity.
	NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
	NSEntityDescription *entity = [NSEntityDescription entityForName:@"RWPlant" inManagedObjectContext:managedObjectContext];
	[fetchRequest setEntity:entity];

	// Create the sort descriptors array.
	NSSortDescriptor *typeDescriptor = [[NSSortDescriptor alloc] initWithKey:@"type" ascending:YES];
	NSSortDescriptor *nameDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
	NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:typeDescriptor, nameDescriptor, nil];
	[fetchRequest setSortDescriptors:sortDescriptors];

	// Create and initialize the fetch results controller.
	NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:@"type" cacheName:@"Plants"];
	self.fetchedResultsController = aFetchedResultsController;
	fetchedResultsController.delegate = self;

	// Memory management.
	[aFetchedResultsController release];
	[fetchRequest release];
        [categoryDescriptor release];
	[nameDescriptor release];
	[sortDescriptors release];

	return fetchedResultsController;
}

Swift:


lazy var fetchedResultsController: NSFetchedResultsController = {
    let fetchRequest: NSFetchRequest<rwplant> = RWPlant.fetchRequest()
    let typeDescriptor = NSSortDescriptor(key: "type", ascending: true)
    let nameDescriptor = NSSortDescriptor(key: "name", ascending: true)
    fetchRequest.sortDescriptors = [typeDescriptor, nameDescriptor]
    let aFetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: managedObjectContext, sectionNameKeyPath: "type", cacheName: "Plants")
    aFetchedResultsController.delegate = self
    do {
        try aFetchedResultsController.performFetch()
    } catch let error {
        print("Unable to perform fetch: \(error)")
    }
    return aFetchedResultsController
}()

Re-ordering Core Data Objects on iOS4

The Core Data framework on the iPhone is incredibly powerful. It’s also incredibly efficient and part of that is because a collection of objects only have the order that you implicitly give them. For example you typically might give an Event object a timestamp and when you pull out all the events you might sort on that timestamp.

The NSFetchedResultsController is the main power house when dealing with such a scenario and is great for the master part of a master/detail data relationship. Its main purpose is to manage the results returned from a fetch request similar to the above and provide data for a UITableView via delegate methods. It reacts on the model level so if you delete an Event object, the NSFetchedResultsController informs its delegate and so updates the UITableView automatically. It’s very clever indeed!

As I found out earlier yesterday, the problem comes when you want to re-order the objects in a user-defined way. Instead of sorting on a timestamp, I wanted the user to be able to specify that one object should occur before the other… I’m working on an app that lets you place waypoints down on a map. Timestamps in waypoints aren’t much use. It’s much more critical that they have a specific order.

After some Googling I came across a useful article on CocoaIsMyGirlfriend. This helped me 90% of the way but I had problems when re-ordering. When you re-order the objects, the UITableView would move cells about seemingly at random. This is because NSFetchedResultsController is model-driven. When you re-order something using the tableview methods the view is already correct (because you’ve dragged and dropped the cell there – it’s a user-driven change) and so when the delegate detects your index changes, it walks all over your view believing the cells to be in their original position. The trick is to ‘disable’ the UI updates with a boolean in your delegate methods. Set the boolean just before your re-indexing and unset it afterwards.

For an example, see this stack overflow link on re-ordering.

Also have a look at what the Apple docs say on user-driven updates on the NSFetchedResultsController.