Advanced Object- C Features

Transcription

1 Advanced Object- C Features

2 Advanced Features Proper6es Categories Protocols Delegates Selectors Key- Value Coding Predicators

3 Proper6es Provide access to object a?ributes Shortcut to implemen6ng ge?er/se?er methods Also allow you to specify: read- only versus read- write access memory management policy

4 Defining Proper6es #import Person : NSObject { // instance variables NSString *name; int age; // method declara6ons - (NSString *) name; (void)setname:(nsstring *)value; - // (int) property age; declara6ons (void)setage:(int)age; int (BOOL) canlegallyvot; (copy) NSString * (readonly) BOOL canlegallyvot; -

5 Synthesizing Person - (int)age return age; name; - - (BOOL)canLegallyVote (void)setage:(int)value { age return = value; (age > 17); - (NSString *)name { return - (void)setname:(nsstring *)value { if (value!= name) { [name release]; name = [value copy]; - (void)canlegallyvote {...

6 Property A?ributes Read- only versus read- int age; // read- write by (readonly) BOOL canlegallyvote; Memory management policies (only for object (assign) NSString *name; // pointer (retain) NSString *name; // retain (copy) NSString *name; // copy called

7 (nonatomic, copy) NSString *string; the synthesized se?er method is similar to the following: - (void)setstring:(nsstring *)newstring { if (string!= newstring) { [string release]; string = [newstring copy];

8 Releasing Proper6es Do you have to release a property? By using the retain (copy) se?er seman6c se`ng in statement you're asking the compiler to build a se?er that calls retain on the object (or copy a new one) Since you are retaining that object (even though it's via magically auto- generated code), you have to release it! Where to release? in - (void)dealloc.

9 #import X : X - (void)release { NSLog(@"Releasing %p, next count = %d", self, [self retaincount]- 1); [super release]; - (id)retain { NSLog(@"Retaining %p, next count = %d", self, [self retaincount]+1); return [super retain]; - (void)dealloc { NSLog(@"Dealloc %p", self); Y : NSObject { X* (nonatomic, retain) x; - (void)dealloc { [x release]; [super int main () { NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init]; Y* y = [[Y alloc] init]; X* x = [[X alloc] init]; y.x = x; [y release]; [x release]; [pool drain]; return 0;

10 Atomicity This a?ribute specifies that accessor methods are not atomic. (There is no keyword to denote atomic.) Nonatomic Specifies that accessors are non- atomic. By default, accessors are atomic. Without nonatomic, then in a reference counted environment a synthesized get accessor for an object property uses a lock and retains and autoreleases the returned value: [_internal lock]; // lock using an object- level lock id result = [[value retain] autorelease]; [_internal unlock]; return result; With nonatomic, then a synthesized accessor for an object property simply returns the value directly.

11 Property Names vs. Instance Variables Property name can be different than instance Person : NSObject { int age = Whether or not you specify the name of the instance can only use an instance variable from the current class, not a superclass.

12 Proper6es Mix and match synthesized and implemented name; - (void)setage:(int)value { age = value; // now do something with the new age Se?er method explicitly implemented Ge?er method s6ll synthesized

13 @interface MyClass : NSObject { NSMutableArray (nonatomic, copy) myarray; - (void)setmyarray:(nsmutablearray *)newarray { if (myarray!= newarray) [myarray release]; myarray = [newarray mutablecopy];

14 Proper6es In Prac6ce Newer APIs Older APIs use ge?er/se?er methods Proper6es used heavily throughout UIKit APIs Not so much with Founda6on APIs Objec6ve- C 2.0 features only used on Mac OS X 10.5 (Leopard) or later You can use either approach Proper6es mean wri6ng less code, but magic can some6mes be non- obvious

15 Dot Syntax and self When used in custom methods, be careful with dot syntax for proper6es defined in your class References to proper6es and ivars behave very Person : NSObject { NSString (copy) Person - (void)dosomething { name Fred ; // accesses ivar directly! self.name Fred ; // calls accessor method

16 Common Pisall with Dot Syntax What will happen when this code Person - (void)setage:(int)newage { self.age = This is equivalent Person - (void)setage:(int)newage { [self setage:newage]; // Infinite

17 Further Reading Objec6ve- C 2.0 Programming Language Defining a Class Declared Proper6es Memory Management Programming Guide for Cocoa

18 Categories A category allows you to add methods to an exis6ng class even to one to which you do not have the source. This is a powerful feature that allows you to extend the func6onality of exis6ng classes without subclassing. Using categories, you can also split the implementa6on of your own classes between several files.

19 A Category Example Considering you are wri6ng a crossword puzzle program that takes a series of string, determines the length of each string, and put those lengths into an NSArray or NSDic6onary. You will need to wrap each length in an NSNumber object before adding it into the NSArray or NSDic6onary. NSNumber *number; number = [NSNumber numberwithunsignedint: [string length]];

20 @interface NSString (NumberConvenience) - (NSNumber *) // NSString (NumberConvenience) - (NSNumber *) lengthasnumber { unsigned int length = [self length]; return ([NSNumber numberwithunsignedint: length]); // // NumberConvenience int main (int argc, const char *argv[]) { NSAutoreleasePool *pool; pool = [[NSAutoreleasePool alloc] init]; NSMutableDic6onary *dict; dict = [NSMutableDic6onary dic6onary]; [dict setobject: [@"hello" lengthasnumber] [dict setobject: [@"ilikefish" lengthasnumber] [dict setobject: [@"Once upon a 6me" lengthasnumber] upon a 6me"]; NSLog (@"%@", dict); [pool release]; return (0); // main

21 Basic Rules for Categories You can t add new instance variables to a class When name collide (your category name is the same as the an exis6ng methods), the category wins. Category methods can do anything that methods defined in the class proper can do. At run6me, there s no difference. The methods the category adds to the class are inherited by all the class s subclasses, just like other methods.

22 Usage of Categories To extend classes defined by other implementors. As an alterna6ve to a subclass. To distribute the implementa6on of a new class into separate source NSWindow: NSWindow NSWindow(NSToolbarSupport) To declare informal protocols.

23 Protocols Protocols declare methods that can be implemented by any class. Protocols are useful in at least three situa6ons: To declare methods that others are expected to implement To declare the interface to an object while concealing its class To capture similari6es among classes that are not hierarchically related

24 Protocol Examples For example, the following methods that report user ac6ons on the mouse could be gathered into a protocol: - (void)mousedown:(nsevent *)theevent; - (void)mousedragged:(nsevent *)theevent; - (void)mouseup:(nsevent *)theevent; Any class that wanted to respond to mouse events could adopt the protocol and implement its methods.

25 Protocols and Subclass/Categories Class and category interfaces declare methods that are associated with a par6cular class mainly methods that the class implements. Informal and formal protocols, on the other hand, declare methods that are independent of any specific class, but which any class, and perhaps many classes, might implement. A protocol is simply a list of method declara6ons, una?ached to a class defini6on. Objects can be grouped into types not just on the basis of similari6es due to the fact that they inherit from the same class, but also on the basis of their similarity in conforming to the same protocol.

26 Formal Protocols Declaring NSCoding - (void) encodewithcoder: (NSCoder *) acoder; - (void) initwithcoder: (NSCoder *) Adop6ng Car: NSObject<NSCoding> { //

27 Op2onal Protocol You can to par66on your protocol into sec6ons as you see fit. If you do not specify any keyword, the MyProtocol (void)anop6onalmethod; (void)anotherrequiredmethod;

28 Protocol Objects Like class objects, Protocol objects are created automa6cally from the defini6ons and declara6ons found in source code and are used by the run6me system. They re not allocated and ini6alized in program source code. Source code can refer to a Protocol object using direc6ve: Protocol *myxmlsupportprotocol Protocols that are declared but not used (except for type checking as described below) aren t represented by Protocol objects at run6me.

29 Conforming to a Protocol A class is said to conform to a formal protocol if it adopts the protocol or inherits from another class that adopts it. It s possible to check whether an object conforms to a protocol by sending it a conformstoprotocol: message. if (! [receiver conformstoprotocol:@protocol(myxmlsupport)] ) { // Object does not conform to MyXMLSupport protocol // If you are expec6ng receiver to implement methods declared in the // MyXMLSupport protocol, this is probably an error

30 Informal Protocols You can define an informal protocol by grouping the methods in a category NSObject ( MyXMLSupport ) - initfromxmlrepresenta6on:(nsxmlelement *)XMLElement; - (NSXMLElement Informal protocols are typically declared as categories of the NSObject class, since that broadly associates the method names with any class that inherits from NSObject. Because all classes inherit from the root class, the methods aren t restricted to any part of the inheritance hierarchy

31 Informal Protocols Being informal, protocols declared in categories don t receive much language support. There s no type checking at compile 6me nor a check at run6me to see whether an object conforms to the protocol. To get these benefits, you must use a formal protocol. An informal protocol may be useful when all the methods are op6onal, such as for a delegate, but (on Mac OS X v10.5 and later) it is typically be?er to use a formal protocol with op6onal methods.

32 Delega6on Design Pa?ern The delega6on is a commonly used pa?ern in object- oriented programming. It is a situa6on where an object, instead of performing a tasks itself, delegates that task to another, helper object. The helper object is called the delegate. The delega6ng object keeps a reference to the other object the delegate and at the appropriate 6me sends a message to it. The message informs the delegate of an event that the delega6ng object is about to handle or has just handled. The delegate may respond to the message by upda6ng the appearance or state of itself or other objects in the applica6on, and in some cases it can return a value that affects how an impending event is handled.

33 Delegate in Cocoa The delega6ng object is typically a framework object, and the delegate is typically a custom controller object. The delega6ng object holds a weak reference to its delegate. NSApplica6on asks its delegate if it should open an Un5tled window when the applica6on launches NSWindow objects ask their delegates if they should allow a window to be closed

34 Delegate Example For example, suppose you have an NSWindow. And you'd like to implement its delegate's windowdidmove: MyClass - (void)windowdidmove:(nsno6fica6on*) no6fica6on { Then you could create an instance of MyClass and assign it as the window's delegate: MyClass *mydelegate = [[MyClass alloc] init]; [window setdelegate: mydelegate]; On the NSWindow side, it probably has code similar to this to see if the delegate responds to the windowdidmove: message using respondstoselector: and send it if appropriate. if([[self delegate] respondstoselector:@selector (windowdidmove:)]) { [[self delegate] windowdidmove:no6fica6on]; h?p://stackoverflow.com/ques6ons/626898/how- do- i- create- delegates- in- objec6ve- c

35 (In)Formal Protocol for NSObject(NSWindowNo6fica6ons) - (void)windowdidmove:(nsno6fica6on *)no6fica6on; //... other - (void)windowdidmove:(nsno6fica6on *)no6fica6on; //... other MyDelegate < NSWindowNo6fica6ons >

36 Selectors SEL sel NSString *lower = (([str NSLog (@"Responds to lowercasestring: %@", lower); if ([str respondstoselector:sel]) //(lower NSLog(@"lowercaseString is: %@", [str lowercasestring]);

37 Selectors In Objec6ve- C, selector has two meanings. It can be used to refer simply to the name of a method when it s used in a source- code message to an object. It also, though, refers to the unique iden6fier that replaces the name when the source code is compiled. Compiled selectors are of type SEL. All methods with the same name have the same selector. You can use a selector to invoke a method on an object this provides the basis for the implementa6on of the target- ac6on design pa?ern in Cocoa.

38 Methods and direc6ve lets you refer to the compiled selector SEL setwidthheight; setwidthheight Convert a character string to a selector at run6me: setwidthheight = NSSelectorFromString(aBuffer); Conversion in the opposite direc6on: NSString *method; method = NSStringFromSelector(setWidthHeight);

39 Key- Value Coding Key- value coding is a mechanism for accessing an object s proper6es indirectly, using strings to iden6fy proper6es, rather than through invoca6on of an accessor method or accessing them directly through instance variables. In essence, key- value coding defines the pa?erns and method signatures that your applica6on s accessor methods implement.

40 Predicate Basics A predicate is a logical operator that returns a Boolean value (true or false). comparison predicate compound predicate Cocoa supports a wide range of types of predicate, including the following: Simple comparisons, such as grade == 7 or firstname like 'Mark' Case or diacri6c insensi6ve lookups, such as name contains[cd] 'citroen' Logical opera6ons, such as (firstname beginswith 'M') AND (lastname like 'Adderley')