Understanding Foundation

Description: The Foundation framework provides the "nuts and bolts" classes for both iPhone OS and Mac OS programming, and an understanding of the Foundation framework is essential for building great software on the Mac, iPhone, and iPad. Learn about the wide-ranging capabilities of the Foundation framework and discover how to best use features like collections, strings, archiving, notifications, preferences, bundles, and more.

  • Foundation provides building block classes
    • Fundamental types used by all applications
    • Assembled by higher level software (all applications on the system)
  • Introduces consistent conventions
  • Raises lower-level concepts
    • Foundation wraps up complex OS level tasks, so you can focus on your problem domain instead of nuts and bolts of how exactly to do things

Where is Foundation?

  • Foundation is just another framework used in your app
  • Foundation itself relies on other frameworks such as CFNetwork and CoreFoundation
  • Foundation is also used by other frameworks such as UIKit and AppKit

Building blocks


  • A place to put your objects
  • Most commonly used features:
    • Iterating
    • Sorting
    • Filtering

Three main collections:

CollectionPrime Directive
NSArray / NSMutableArrayPreserves order
NSDictionary / NSMutableDictionaryMaps key objects to value objects
NSSet / NSMutableSetUnique, unordered objects

Collection Iteration

  • for loop
NSUInteger count = [array count];
for (NSUInteger i = 0; i < count; i++) {
  id value = [array objectAtIndex:i];

NSArray *values = [dictionaryOrSet allObjects];
count = [values count];
for (NSUInteger i = 0; i < count; i++) {
  id value = [values objectAtIndex:i];
  • Using NSEnumerator with while loop

An enumerator is a Foundation object that can keep track of your location and iteration for you.

NSEnumerator *e = [arrayOrSet objectEnumerator];
while (id object = [e nextObject]) {
  // extra step required for index

NSEnumerator *e = [dictionary keyEnumerator];
while (id key = [e nextObject]) {
  id value = [e objectForKey:key];
  // or, use objectEnumerator and iterate objects directly
  • Fast enumeration with for...in
    • It's fast for two reasons:
      1. it actually performs faster, as it uses some implementation details of how these collections are created to retrieve groups of objects and actually have better performance.
      2. because you have to write less code
for (id object in arrayOrSet) {
  // no easy access to index for array here

for (id key in dictionary) {
  // requires extra step to get object
  id value = [dictionary objectForKey:key];
  • Blocks (new in Snow Leopard and iOS 4)
[array enumerateObjectsUsingBlock:^(id object, NSUInteger index, BOOL *stop) {
  // your code here

[dictionary enumerateKeysAndObjectsUsingBlock:^(id key, id object, BOOL *stop) {
  // your code here

[set enumerateObjectsUsingBlock:^(id object, BOOL *stop) {
  // your code here

Recommended iteration method:

  • Mac OS X 10.0: for loop or NSEnumerator
  • Mac OS X 10.5, iPhone OS 2: for loop or fast enumeration
  • Mac OS X 10.6, iOS 4: Fast enumeration or Block enumeration

Sorting an NSArray

Determine sort order using: - C function - Objective-C method - NSSortDescriptor - class that lets you specify which properties on an array or an object in an array that you want to sort on. - Blocks

  • For immutable or mutable arrays, return a new object
  • For mutable arrays, method to sort in place

Example where we sort an array of strings by length:

NSMutableArray *names = ...; // array of NSString objects
[names sortUsingComparator:^(id left, id right) {
  NSComparisonResult result;
  NSUInteger lLen = [left length], rLen = [right length];
  if (lLen < rLen) {
    result = NSOrderedAscending;
  } else if (lLen > rLen) {
    result = NSOrderedDescending;
  } else {
    result = NSOrderedSame;
  return result;


⚠️ do not mutate a collection while enumerating it, this causes an exception.

To filter a collection, either:

  • Mutate a copy
  • Gather changes on the side, then apply

Example of the latter:

NSMutableArray *files = ...; // array of NSString objects
NSIndexSet *toRemove = [files indexesOfObjectsPassingTest:^(id obj, NSUInteger idx, BOOL *stop) {
  if ([obj hasPrefix:@"."]) return YES;
    return NO;

[files removeObjectsAtIndexes:toRemove];

More Collection features

  • Searching
  • Apply selector to each item
  • NSArray: Slicing and concatenation
  • NSSet: Intersection, union, set subtraction


  • NSString is an object container for most text on the system
    • Array of Unicode characters
    • Treat as opaque container - NSString has a ton of methods on it that let you manipulate strings and do other kinds of string operations.
  • Common uses:
    • Comparison
    • Searching
    • Converting encodings

Comparing Strings

-(NSComparisonResult)compare:(NSString *)string;
-(NSComparisonResult)localizedCompare:(NSString *)string;

/// This encapsulates what Apple thinks are the best practices for the options 
/// to compare strings with if you're going to display them in a manner to the user.
/// This will take into account the user's current locale, all of their preferences 
/// and so on.
-(NSComparisonResult)localizedStandardCompare:(NSString *)string;
-(NSComparisonResult)compare:(NSString *)string


NSString *str1 = @"string A";
NSString *str2 = @"string B";
NSComparisonResult result = [str1 compare:str2];
// result is: NSOrderedAscending

NSArray *strings = [NSArray arrayWithObjects:@"Larry", @"Curly", @"Moe", nil];
NSArray *sortedStrings = [strings sortedArrayUsingSelector:@selector(localizedCompare:)];
// result is: "Curly", "Larry", "Moe"

Searching Strings

-(NSRange)rangeOfString:(NSString *)aString;
-(NSRange)rangeOfString:(NSString *)aString
                 locale:(NSLocale *)locale;
  • NSRange has a location and length

Why do we need to know the length of the range? Isn't it the same as aString?
Because of different unicode representations. There might be cases where the same string matches strings with different lengths, for example José can be stored as José or as Jose´ ( is called a decomposed character).

As of iPhone OS 3.2, NSStringCompareOptions supports NSRegularExpressionSearch:

str = @"Going going gone!";
found = [str rangeOfString:@"go(\\w*)"
                     range:NSMakeRange(0, [str length])];
// found.location = 6, found.length = 5

String Encodings

  • An encoding is a map of numbers to characters
  • When you create a string from data, it's always important to know what encoding that data is stored in
/// From NSData to NSString
NSData *data = ...;
NSString *inString = [[NSString alloc] initWithData:data 

/// From NSString to NSData
NSString *outString = @"For Windows";
NSData *converted = [outString dataUsingEncoding:NSUTF16StringEncoding];

If you need a char * to pass through a system call (like Open), you should use NSString's convenience method fileSystemRepresentation.

  • this will get you a character pointer that points to the data in the correct encoding for the system
  • the data that's pointed to by that char * is autoreleased
const char *fileName = [outString fileSystemRepresentation];

More String Features

  • printf-style formatting
  • Enumeration of substrings, lines, and paragraphs (can do this in a language-independent way)
  • Replacement of substrings
  • Path completion
  • Mutability (via NSMutableString)

Dates and times

  • Encapsulate complexity of date and time calculations
  • Automatically handle user preferences
  • Common uses:
    • Represent a date in a calendar
    • Find amount of time between two dates
    • Format a date or number correctly for user in any locale

Main classes


  • An invariant point in time
  • Calendar and time zone independent (you can't present an NSDate to a user without knowing a calendar and time zone)
  • Time is measured as seconds since a reference point, a Double


  • Defines beginning, length, and divisions of a year
  • Example: Gregorian calendar, Hebrew calendar


  • Simple object structure
  • Contains storage for year, month, day, etc.
  • Exact meaning of properties depends on calendar

Example how many days there are between Christmas and Black Friday in 2010:

// Find Christmas
NSCalendar *cal = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

NSDateComponents *components = [[NSDateComponents alloc] init];
[components setYear:2010];
[components setMonth:12];
[components setDay:25];

NSDate *christmas = [cal dateFromComponents:components];

// Find Thanksgiving
NSDateComponents *tComps = [[NSDateComponents alloc] init];
[tComps setYear:2010];
[tComps setMonth:11];
[tComps setWeekday:5]; // 5 is Thursday in NSGregorianCalendar
[tComps setWeekdayOrdinal:4];

NSDate *thanksgiving = [cal dateFromComponents:tComps];

// Find Black Friday (the day after Thanksgiving (shows how to add date components to dates)
NSDateComponents *toAdd = [[NSDateComponents alloc] init];
[toAdd setDay:1];

NSDate *blackFri = [cal dateByAddingComponents:toAdd
// Find number of days between dates
NSDateComponents *diff = [cal components:NSDayCalendarUnit
NSInteger days = [diff day]; // days == 29

More Dates and times features

  • Find day of the week for a date
  • Time zone calculations
  • Converting between calendars


  • used for converting values to and from strings

The talk focuses on two main formatters:

  • NSDateFormatter
    • Convert date to or from a string
  • NSNumberFormatter
    • Convert number to or from a string

Each class has a set of pre-defined styles, or use your own.

Get String from Date:

NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
[fmt setTimeStyle:NSDateFormatterNoStyle];
[fmt setDateStyle:NSDateFormatterLongStyle];
NSLog(@"Thanksgiving is: %@", [fmt stringFromDate:thanksgiving]);
// > Thanksgiving is: November 25, 2010

// Here I used the current system time zone and calendar, 
// because I haven't specified them to the formatter.

If you're parsing a string:

NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
[fmt setDateFormat:@"dd/MM/yyyy HH:mm"];
[fmt setTimeZone:[NSTimeZone timeZoneWithName:@"America/Los_Angeles"]];
[fmt setCalendar:cal];
NSDate *date = [formatter dateFromString:@"10/06/2010 9:30"];

Format strings conform to Unicode Standard TR35-6

Persistence and archiving

Store data to disk for later retrieval:

  • Property lists
  • User preferences
  • Keyed archives
  • Large, complex data structures

Property lists - NSPropertyListSerialization

  • Stores a small amount of structured data
  • Cross-platform and human readable (XML format)
    • Plists also have a binary format that is better performing. So you can choose which format to use depending on what your requirements are
  • Property list allowed types:
    • NSArray, NSDictionary
    • NSString
    • NSDate
    • NSNumber (integer, floating point, boolean)
    • NSData (PLists are designed to handle small amount of data)

Store a Plist:

NSDictionary *colors = [NSDictionary dictionaryWithObjectsAndKeys:@"Verde", @"Green", @"Rojo",@"Red", @"Amarillo", @"Yellow", nil];

NSError *error = nil;
NSData *plist = [NSPropertyListSerialization dataWithPropertyList:colors
if (!plist) [NSApp presentError:error];

Note the error handling pattern:

  • If plist is nil, then the error parameter has been filled out
  • If plist is not nil, the error parameter that you pass in will be completely untouched, Foundation will not modify it in any way

Read a PList:

NSData *readData = [NSData dataWithContentsOfURL:urlOfFile];
NSDictionary *newColors = [NSPropertyListSerialization propertyListWithData:readData
if (!newColors) [NSApp presentError:error];

User preferences - NSUserDefaults

  • Stores small values representing user preferences
  • Organized by domain, which is a group of defaults
  • Domains searched in specific order:
    • Arguments to executable (volatile)
    • Application (stored in the user home directory)
    • Global (system-wide location)
    • Language (for locale-specific preferences, volatile)
    • Registration (factory settings of your application, volatile)

Set Registration Domain Defaults

+ (void)initialize {
  NSDictionary *appDefaults = [NSDictionary
      [NSNumber numberWithBool:YES], @"FrogBlastVentCore",
      @"iPad", @"DefaultDeviceName",
      [NSNumber numberWithInt:3], @"PiValue", nil];

  NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];

  [defaults registerDefaults:appDefaults];

Read User Default

BOOL doIt = [defaults boolForKey:@"FrogBlastVentCore"];
// doIt == YES

Read Executable Arguments

// Run your app with:
// MyApp.app/Contents/MacOS/MyApp -ConferenceName WWDC
NSString *argPref = [defaults stringForKey:@"ConferenceName"];
// argPref is "WWDC"

Set User Default

[defaults setBool:NO forKey:@"FrogBlastVentCore"];

// later...
doIt = [defaults boolForKey:@"FrogBlastVentCore"];
// doIt == NO

Keyed archives

  • for storing an arbitrary graph of objects
  • its primary design purpose is to enable easy backward and forward compatibility
    • because in a keyed archive, the way it works is you associated keys with your object values. You can add a key in a future version of your app, and the past versions of your app don't have to read that key, and vice versa.
  • Allows for substitutions during encode and decode
  • Objects do not need to be property list types
  • Object must implement NSCoding protocol
    • The way that the keyed archiver knows how to encode and decode your objects is by the NSCoding protocol that your objects implement.

Implementing NSCoding.
Let's say that we have the following Robot definition:

@interface Robot : NSObject <NSCoding> {
  NSString *name;
  Robot *nemesis;
  NSInteger model;
@property (copy) NSString *name;
@property (retain) Robot *nemesis;
@property NSInteger model;

To implement NSCoding, I need to fill out two methods:

// To encode our object, we need to tell the keyed archiver 
// how to store the data that we have as our properties:
- (void)encodeWithCoder:(NSCoder *)coder {
  [coder encodeObject:name forKey:@"name"];
  [coder encodeObject:nemesis forKey:@"nemesis"];
  [coder encodeInteger:model forKey:@"model"];

- (id)initWithCoder:(NSCoder *)coder {
  self = [super init];
  name = [[coder decodeObjectForKey:@"name"] copy];
  nemesis = [[coder decodeObjectForKey:@"nemesis"] retain];
  model = [coder decodeIntegerForKey:@"model"];
  return self;


// Note how r1 and r2 reference each other, but keyed archiver will handle that.
Robot *r1 = [[Robot alloc] init], *r2 = [[Robot alloc] init];
r1.name = @"Bender"; r1.nemesis = r2; r1.model = 22;
r2.name = @"Flexo";  r2.nemesis = r1; r2.model = 22;

NSData *data = [NSKeyedArchiver archivedDataWithRootObject:r2];


Robot *r3 = [NSKeyedUnarchiver unarchiveObjectWithData:data];

NSLog(@"Nemesis is: %@", r3.nemesis.name);
// Nemesis is: Bender

Large, complex data structures - Core Data


  • Support for key-value coding and key-value observing
  • Validation of values and consistency of relationships
  • Change tracking and undo
  • Sophisticated queries
  • Incremental editing

Persistence Cheat Sheet

Data TypeRecommended Persistence Method
User preferencesNSUserDefaults
Small files, cross-platformNSPropertyListSerialization
Object graph with cycles, non-property list typesNSKeyedArchiver
Large data set, object relational databaseCore Data
Specialized dataCustom Format

Files and URLs

  • NSURL is the preferred way to reference files and resources
  • URLs are used with the file system through NSFileManager
  • URLs are used with a network resource through the URL loading classes

Creating URLs

// Local URLs
NSURL *file = [NSURL fileURLWithPath:@"/Users/tony/file.txt"];
NSURL *up = [file URLByDeletingLastPathComponent];
NSURL *file2 = [up URLByAppendingPathComponent:@"file2.txt"];

// Web URLs
NSURL *aapl = [NSURL URLWithString:@"http://www.apple.com"];


  • Group code and resources
  • Include code for different platforms and architectures
  • Simplify loading localized resources

Loading a Bundle Resource

NSBundle *bundle = [NSBundle mainBundle];
NSURL *url = [bundle URLForResource:@"localizedImage"

Bundle vs. Packages

BundleFile PackageBundle + File Package
Primary ClassNSBundleNSFileWrapperNSBundle
UseCode and ResourcesUser DocumentsApplication Code and resources
For Example.framework.rtfd.app
  • the idea of file packages is to treat a directory that contains all the appropriate resources and attachments to the text and so forth, as one document in the finder. So the user can't lose associated pieces of their documents.
    • double-clicking on a rich text document will open its default editor.

More Bundle Features

  • Bundles can load other bundles (plug in system)
  • You can locate the various components of a bundle
  • You can locate list all its resources of a type or in a subdirectory
  • Support for localized strings

Operation queues

  • Object-oriented way to describe work
  • Simplifies design of concurrent applications

Two main classes:

  • Operations
  • Operation Queues

Operations - NSOperation

  • Encapsulates a work unit
  • you use it by subclassing and overriding the -main to define your work
  • Or use one of the Foundation-provided subclasses:
    • NSInvocationOperation - lets you specify a target and selector
    • NSBlockOperation - lets you specify your work as a block

Operation Queues - NSOperationQueue

  • Maintains list of operations to perform
  • Runs operations concurrently or serially

Using an NSOperation Subclass

@interface MyOp : NSOperation
@implementation MyOp
- (void)main {
  // Your work goes here

// elsewhere...
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
MyOp *op = [[MyOp alloc] init];
[queue addOperation:op];

Other NSOperation Features

  • You can set up dependencies between operations even if those operations are in different queues.
  • You can set up priorities of operations within a queue
  • Key-value observing compatible properties (so you can find out when it's running, when it's finished, etc.)
  • you can have a block run when you're operation is finished

Missing anything? Corrections? Contributions are welcome 😃

Written by

Federico Zanetello

Federico Zanetello

Software engineer with a strong passion for well-written code, thought-out composable architectures, automation, tests, and more.