learn-objective-c-on-the-mac.markdown

Learn Objective-C on the Mac

by Mark Dalrymple and Scott Knaster

Chapter 2: Extensions to C

• pg 8: Files ending in .m are handled by the Objective-C compiler.
• pg 9: The #import directive is like #include, but doesn't require the programmer to use an #ifndef directive so that a file is included only once.
• pg 9: A framework is a collection of parts -- header files, libraries, images, sounds, and more -- collected together into a single unit.
• pg 10: Each framework has a master header file that includes all the framework's individual header files.
• pg 13: BOOL is just a type definition for the signed character type, where YES is 1 and NO is 0.
• pg 16: Don't compare a BOOL value directly to YES, as some methods might return values that assume that anything non-zero is treated as YES.

Chapter 3: Introduction to Object-Oriented Programming

• pg 39: id is a generic type that's used to refer to any kind of object; it's simply a pointer.
• pg 41: Each object has a reference to its class; when an object receives a message, the object delegates to its class, which finds and executes the correct code.
• pg 46: The argument type or return type in a method signature is always in parentheses.
• pg 47: If a method takes an argument, it has a colon; if it takes no arguments, it has no colons.
• pg 48: Defining a method solely in the @implementation directive does not make it private; the method is still accessible due to the language's dynamic nature.

Chapter 4: Inheritance

• pg 62: There is no support for multiple inheritance, but you can reap some of its benefits by using categories and protocols.
• pg 68: NSObject declares one instance variable, named isa, which holds the pointer to the object's class.
• pg 69: To fix the fragile base class problem, the 64-bit Objective-C runtime in Leopard use indirection for determining ivar locations instead of adding offsets to self.
• pg 70: To call the superclass version of a method, send an appropriate message to super.

Chapter 5: Composition

• pg 75: The description method in your class defines how it is printed by NSLog with the %@ format specifier.
• pg 76: Pointers are automatically initialized to nil (a zero value) when declared.
• pg 77: When a constructor calls its superclass constructor with [super init], it's convention to assign the result to self.
• pg 80: Getter methods shouldn't start with get; in Cocoa, this means the value is returned via a pointer passed in as an argument.

Chapter 6: Source File Organization

• pg 88: Using a .mm file as an extension tells the compiler you've written your code in Objective-C++, which allows using C++ and Objective-C together.
• pg 90: In Xcode, groups allow organizing files together but do not equate to directories on your hard drive.
• pg 92: In an #import statement, a header file in angle brackets is a read-only system header file, while one in quotes is local to your project.
• pg 95: A class is forward referenced with @class, which is put between the import statements and the @interface keyword.

Chapter 7: More About Xcode

• pg 106: When typing a class or method name, press the escape key to see all code completion options.
• pg 108: To move between placeholders after code completion, press Control and forward slash.
• pg 111: Edit all in Scope will allow renaming a local variable in a function, while the Refactor option allows renaming an entire class.
• pg 115: To search for classes, methods, or properties, press Command, Option, Shift, and D.
• pg 119: Comments with TODO:, FIXME:, !!!: and ???: are indexed automatically by the function menu.
• pg 120: The split box beneath the lock will horizontally split the window; holding Option will split the window vertically.
• pg 122: Holding Control and double-clicking on a symbol will issue a documentation seach for that symbol.
• pg 124: Disable a breakpoint by clicking on it symbol, or remove it by dragging the symbol out of the gutter.
• pg 129: Hold control and press period to cycle forward through code completion options; adding shift will cycle backward.

Chapter 8: A Quick Tour of the Foundation Kit

• pg 131: Cocoa is two different frameworks: Foundation and Application Kit, where the former has useful low-level, data-oriented classes and types.
• pg 135: Declaring a method with a plus sign makes it a class method.
• pg 136: The isEqualToString method of a NSString compares string values; using == will only compare the pointers.
• pg 141: Because inheritance works with class methods and instance methods, stringWithFormat is available on class NSMutableString.
• pg 143: A class name with CF means it's part of the Core Foundation framework and written in C; CF and Cocoa objects are "toll-free bridged," so they can be used interchangeably.
• pg 147: When NSEnumerator returns nil, iteration has finished, and so you can't store nil in arrays.
• pg 149: The initializer lists for both NSArray and NSDictionary have nil as their last value.
• pg 150: Extending NSString, NSArray, or NSDictionary is problematic because they're implemented as class clusters, or many classes behind a common interface.
• pg 152: NSValue allows putting structs into an NSArray or NSDictionary; the @encode compiler directive creates the required string describing the type for you.

Chapter 9: Memory Management

• pg 163: The retain method returns an id, so you can chain it with other messages.
• pg 167: If a setter method retains its argument and then releases its current value, it guards against the retain count hitting 0 when passing the current value as the argument.
• pg 169: An autorelease pool is created on an event, and destroyed when the event ends.
• pg 172: If you get a hold of an object in any way other than new, alloc, or copy, assume it has a retain count of 1 and has been autoreleased.
• pg 175: Autorelease pools are kept in a stack; sending an autorelease message puts the receiver into the topmost pool.
• pg 177: For iPhone programming, Apple recommends you avoid using autorelease in your own code, and convenience functions that give you autoreleased objects.

Chapter 10: Object Initialization

• pg 179: The alloc method initializes all memory to zero, all numbers as 0, all booleans as NO, and all pointers as nil.
• pg 181: If a new object is returned from a superclass init method, self must be updated so any subsequent instance variable references affect the right memory locations.
• pg 183: The init methods are ordinary methods that follow a naming convention; you can provide as many convenience initializers as needed.
• pg 198: All initializer methods of a class use the designated initializer to do initialization work, and subclasses use the superclass' designated initializer as well.

Chapter 11: Properties

• pg 205: The @property keyword in the header file includes the type, while the @synthesize keyword does not.
• pg 208: Always specify copy in properties for string attributes; a common error is to use a string from a UI component, which are mutable and change as the user types.
• pg 212: A default decoration for the @property keyword is assign; alternatively you can use retain or copy.
• pg 213: Passing nil to a setter generated from the property will release the previous value if needed.

Chapter 12: Categories

• pg 226: Using categories you can split a class' implementation across multiple files, and even among frameworks.
• pg 227: To simulate a private method, exclude it from the .h file, and add it to a category at the top of the implementation file while leaving the definition in the header's @implementation part.

Chapter 13: Protocols

• pg 235: You adopt a protocol by listing its name in your class' interface declaration; then your class conforms to the protocol.
• pg 239: When implementing copyWithZone, send the allocWithZone message to [self class] to create a class of the right subtype.
• pg 242: A copyWithZone method should send a copyWithZone message to its parent class if available, but send a copy message to objects that compose it.
• pg 246: Specifying protocol names on instance variables and methods arguments ensures that the assigned variables conform to those protocols.

Chapter 14: Introduction to the AppKit

• pg 252: IBOutlet is defined to be nothing, so it disappears upon compiling; IBAction is defined to be void, and so the associated method returns nothing.
• pg 253: Nib files, which stands for NeXT Interface Builder, are binary files that serialize objects, and .xib files are XML that are compiled into nib files.
• pg 260: When making connections in Interface Builder, drag from the object that needs to know something to the object it needs to know about.
• pg 262: When an object recreated from a nib file is initialized, all its IBOutlet instances are nil; you must wait until awakeFromNib is called to do any work with them.

Chapter 15: File Loading and Saving

• pg 266: NSTimeInterval is simply a typedef of double representing some interval of seconds.
• pg 268: Property list files can be converted between XML and compressed binary format using the plutil command.
• pg 270: The NSCoder class allows for converting objects to NSData and back; NSData, along with NSArray, NSDictionary, NSString, NSNumber, and NSDate, can be used in property lists.
• pg 273: If a parent class adopts NSCoding, initWithCoder should call the superclass implementation of initWithCoder first; otherwise, simply call init.

Chapter 16: Key-Value Coding

• pg 280: KVC automatically boxes and unboxes scalar values into NSValue or NSNumber when neccessary.
• pg 281: Methods like valueForKey and setValue:forKey will use getters and setters if they exist, and will access the variables directly otherwise.
• pg 283: If a key path includes an array or to-many attribute, the remaining part of the key is sent to every object in the array, and the values are returned in an array.
• pg 288: KVC operators are always used with valueForKeyPath.
• pg 294: When printing, "<null>" is the [NSNull null] object which could be returned as a value from a NSDictionary, while "(null)" is nil and could mean the key did not exist.

Chapter 17: NSPredicate

• pg 301: You cannot use $VARIABLE for key paths in a NSPredictate string, only values; use the %K format specifier to substitute key paths.
• pg 303: Curly braces denote an array, even though they are printed as having parentheses.
• pg 305: String relational operators can be decorated with [c] for case-insensitivity, and [d] for diacritic-insensitivity (i.e. removing accents), or both.