HandBooks Professional Java-C-Scrip-SQL part 115 potx

As long as they implement the NSObject protocol, they can be used in contexts that depend on the methods that NSObject implements.. NSObject class -idinit Returns self.. NSObject class

Sets the isa pointer of the receiver to classObj, and returns the previous

value of the pointer This effectively converts the receiver to an instance of

the class represented by classObj This will only work if the receiver's class

is a descendant of the class represented by classObj, and instances of the two

classes have the same size If the call fails, it returns nil

1.10.3.8 Enforcing intentions

Each method in the following list generates a runtime error In each case, when you call the method, pass in the _cmd variable, which the runtime sets up to be the selector of the method you are in

-(id )subclassResponsibility:(SEL )sel

Call this in a method that you require subclasses to override and implement

-(id )notImplemented:(SEL )sel

Call this in a method that you have deliberately left unfinished

-(id )shouldNotImplement:(SEL )sel

Call this in a method that you inherit (and override) but that makes no sense for your subclass to support

1.10.3.9 Error handling

The following functions are used for error handling See Section 1.8 for more

information about their usage

-(id )doesNotRecognize:(SEL )sel

Generates a runtime error by invoking the -error: method on self The

runtime sends this message to an object when the object has failed to

respond to, or forward, a message

-(id )error:(const char *)format ,

Writes an error message to the system log (via the syslog( ) call) and exits

the program The runtime calls this method on self for a variety of error

conditions Override as described in Section 1.8 to customize error handling for a class

1.10.3.10 Archiving

The GNU runtime provides the type TypedStream for reading and writing objects

to and from archives Section 1.13 describes this type and its use in saving and restoring objects The following methods support archiving:

-(id)awake

Does nothing and returns self Called by the runtime when an object has

been reconstructed Override to customize behavior when your objects are restored

-(id )write:(TypedStream*)stream

Writes the receiver to the specified stream

-(id )read:(TypedStream*)stream

Sets the receiver's fields by reading from the specified stream

+(id )setVersion:(int )version

Sets the class version number

+(int)version

Returns the class's version number, or zero if it has never been set

+(int )streamVersion:(TypedStream*)stream

Returns the version number of the typed stream

1.10.4 The NSObject Class

NSObject supports most of the features of Object and adds others Often the

NSObject version of a method will have a more specifically typed return value For

example, -class is declared to return a Class and not just an id

In defining NSObject, Cocoa first declares the NSObject protocol (It can be

confusing that the class and protocol have the same name; they live in different namespaces.) This allows for other root classes As long as they implement the NSObject protocol, they can be used in contexts that depend on the methods that NSObject implements

The NSObject class implements the NSObject protocol along with other useful methods that its descendants may want but that are not part of the protocol and are not important for interchangeability

Cocoa also declares categories for NSObject, such as NSComparisonMethods, NSURLClient, and others, comprising dozens of additional methods This handbook discusses a few of these categories (NSCoding, NSKeyValueCoding) separately, but omits them from this section

In the following sections, the methods are grouped by topic, and marked as being part of the NSObject protocol or class

1.10.4.1 Creating, copying, and freeing objects

objects, and free the memory used by objects:

+(id)alloc

Allocates, clears, and returns a pointer to memory for a new instance of the

receiver Memory comes from the default zone Returns nil on failure

(NSObject class)

+(id )allocWithZone:(NSZone*)zone

Same as alloc, but uses a specific region of memory NSZone is not a class but a C type that you should treat as opaque You can get the zone of an object with the -zone method You may improve cache performance by

creating related objects in the same zone Passing in a nil parameter means

memory will be allocated from the default zone (NSObject class)

+(void)initialize

Sent by the runtime before the class or any of its descendants is sent a

message by the program Each class gets the message exactly once, and superclasses get the message before subclasses The NSObject

implementation does nothing Override (first passing the call to super) to

perform per-class setup (NSObject class)

-(id)init

Returns self This is the designated initializer for NSObject The Section 1.7

discusses overriding this method (NSObject class)

+(id)new

Calls +alloc, then -init on the resulting instance, and returns the result (NSObject class)

+(id )copyWithZone:(NSZone*)zone

Returns the receiver since class objects are unique Ignores the parameter Don't override (NSObject class)

+(id )mutableCopyWithZone:(NSZone*)zone

Returns the receiver since class objects are unique Ignores the parameter Don't override (NSObject class)

-(id)copy

Calls -copyWithZone: with a nil zone parameter NSObject doesn't

implement that method (only the class method of the same name) but many Cocoa classes do Your class will have to implement -copyWithZone: in order to use -copy The Section 1.7 discusses how to do this

-(id)mutableCopy

Calls -mutableCopyWithZone: with a nil zone parameter NSObject doesn't

implement that method (only the class method of the same name) but many Cocoa classes do Your class will have to implement

-mutableCopyWithZone in order to use -mutableCopy The Section 1.7

discusses how to do this (NSObject class)

-(void)dealloc

Deallocates the memory used to store the receiver Override to release any additional resources managed by your object Don't call -dealloc directly if you are using retain/release memory management (See Section 1.12.)

(NSObject class)

+(void)load

Called by the runtime when a class or category is set up for use by the

runtime Deprecated in favor of the +initialize method (NSObject class) 1.10.4.2 Identifying objects and classes

Use the following methods to find out about an object's identity, and to compare it with other objects:

-(Class)class

Returns the class object for the receiver's class (NSObject protocol)

-(Class)superclass

Returns the class object of the receiver's parent class (NSObject protocol) -(NSString*)description

Returns a string describing the state of the receiver The NSObject

implementation prints the class name and the runtime address of the

instance Override to provide a string the runtime will use when you print an object using the %@ formatting descriptor with NSLog( ) (NSObject

protocol)

-(id)self

Returns the receiver You can get a class object from the class's name with this method:

Class circleClass = [Circle self];

(NSObject protocol)