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PropertiesThe goal of the @property directive is to make it easy to create and congureproperties by automatically generating these accessor methods. It allows youto specify the behavior of a public property on a semantic level, and it takescare of the implementation details for you.

The @property Directive

// Car.himport !"oundation/"oundation.h#

@interface Car $ %&'b(ect

@property )''* running+

@end

// Car.mimport Car.h

@implementation Car

@synthesi-e running running+ // 'ptional for 0code 1.12

@end

3 4)''*5running 6 return running+73 4void5set8unning$4)''*5new9alue 6 running new9alue+7

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:fter declaring the property with the @property directive, you can callthese methods as if they were included in your class;s interface andimplementation les. honda ??Car alloc init+honda.running

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@end

The readonly attribute is an easy way to make a property read3only. It

omits the setter method and prevents assignment via dot3notation, butthe getter is unaDected

8emember that @property also generates an instance variable for us,which is why we can access running without declaring it anywhere 4wecan;t use self.running here because the property is read3only5.

The nonatomic Attribute

:tomicity has to do with how properties behave in a threadedenvironment. Ehen you have more than one thread, it;s possible for thesetter and the getter to be called at the same time. This means that thegetter/setter can be interrupted by another operation, possibly resultingin corrupted data.

:tomic properties lock the underlying ob(ect to prevent this fromhappening, guaranteeing that the get or set operation is working with acomplete value. =owever, it;s important to understand that this is onlyone aspect of thread3safetyFusing atomic properties does not necessarilymean that your code is thread3safe.

Groperties declared with @property are atomic by default, and this doesincur some overhead. &o, if you;re not in a multi3threaded environment 4oryou;re implementing your own thread3safety5, you;ll want to override thisbehavior with the nonatomic attribute, like so$

@property 4nonatomic5 %&&tring >model+ :ccessors for atomic properties must bothbe either generated or user3

dened. 'nly non3atomic properties let you mi3and3match synthesi-edaccessors with custom ones

Memory Management

'b(ective3C uses a more eHcient alternative called ob(ect ownership.Ehen you start interacting with an ob(ect, you;re said to ownthat ob(ect,which means that it;s guaranteed to eist as long as you;re using it. Ehenyou;re done with it, you relinuish ownership, andFif the ob(ect has noother ownersFthe operating system destroys the ob(ect and frees up the

underlying memory.

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The strong Attribute

// Gerson.himport !"oundation/"oundation.h#

@interface Gerson $ %&'b(ect

@property 4nonatomic5 %&&tring >name+

@end

// Gerson.mimport Gerson.h

@implementation Gerson

3 4%&&tring >5description 6 return self.name+7

@end

// Car.himport !"oundation/"oundation.h#import Gerson.h

@interface Car $ %&'b(ect

@property 4nonatomic5 %&&tring >model+@property 4nonatomic, strong5 Gerson >driver+

@end

// main.mimport !"oundation/"oundation.h#import Car.h

import Gerson.h

int main4int argc, const char > argv?5 6 @autoreleasepool 6 Gerson >(ohn ??Gerson alloc init+ (ohn.name @John+

Car >honda ??Car alloc init+

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honda.model @=onda Civic+ honda.driver (ohn+

%&*og4@B@ is driving the B@, honda.driver, honda.model5+ 7

return K+7

The strong attribute creates an owning relationship to whatever ob(ect isassigned to the property. This is the implicit behavior for all ob(ectproperties, which is a safe default because it makes sure the value eistsas long as it;s assigned to the property.

&ince driver is a strong relationship, the honda ob(ect takes ownership of(ohn. This ensures that it will be valid as long as honda needs it.

The weak Attribute

// Gerson.himport !"oundation/"oundation.h#

@class Car+

@interface Gerson $ %&'b(ect

@property 4nonatomic5 %&&tring >name+@property 4nonatomic, strong5 Car >car+

@end

The @class Car line is a forward declaration of the Car class. It;s liketelling the compiler, LTrust me, the Car class eists, so don;t try to nd itright now.M Ee have to do this instead of our usual import statementbecause Car also imports Gerson.h, and we would have an endless loop ofimports. 4Compilers don;t like endless loops.5

=owever, strong references pose a problem if, for eample, we need areference from driver back to the Car ob(ect he;s driving. "irst, let;s add acar property to Gerson.h$

%et, add the following line to main.m right after the honda.driverassignment$

honda.driver (ohn+

(ohn.car honda+ // :dd this line

Ee now have an owning relationship from honda to (ohn and anotherowning relationship from (ohn to honda. This means that both ob(ects will

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alwaysbe owned by another ob(ect, so the memory management systemwon;t be able to destroy them even if they;re no longer needed.

This is called a retain cycle, which is a form of memory leak, and memoryleaks are bad. "ortunately, it;s very easy to this problemF(ust tell oneof the properties to maintain a weak reference to the other ob(ect. InGerson.h, change the car declaration to the following

The weak attribute creates a non3owning relationship to car. This allows(ohn to have a reference to honda while avoiding a retain cycle. )ut, thisalso means that there is a possibility that honda will be destroyed while

(ohn still has a reference to it. &hould this happen, the weak attribute willconveniently set car to nil in order to avoid a dangling pointer

: common use case for the weak attribute is parent3child data structures.)y convention, the parent ob(ect should maintain a strong reference with

it;s children, and the children should store a weak reference back to theparent

The copy Attribute

// Car.h@property 4nonatomic, copy5 %&&tring >model+

// main.mimport !"oundation/"oundation.h#import Car.h

int main4int argc, const char > argv?5 6 @autoreleasepool 6 Car >honda ??Car alloc init+ %&Nutable&tring >model ?%&Nutable&tring stringEith&tring$@=ondaCivic+ honda.model model+

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%&*og4@B@, honda.model5+ ?model set&tring$@%issa 9ersa+ %&*og4@B@, honda.model5+ // &till =onda Civic 7 return K+

7

The copy attribute is an alternative to strong. Instead of taking ownershipof the eisting ob(ect, it creates a copy of whatever you assign to theproperty, then takes ownership of that. 'nly ob(ects that conform to the%&Copying protocolcan use this attribute.

Groperties that represent values 4opposed to connections or relationships5are good candidates for copying. "or eample, developers usually copy%&&tring properties instead of strongly reference them$

%&Nutable&tringis a subclass of %&&tring that can be edited in3place. Ifthe model property didn;t create a copy of the original instance, we wouldbe able to see the altered string 4%issan 9ersa5 in the second%&*og45output.

https://developer.apple.com/library/ios/documentation/cocoa/Reference/Foundation/Protocols/NSCopying_Protocol/Reference/Reference.html#//apple_ref/occ/intf/NSCopyinghttp://rypress.com/tutorials/objective-c/data-types/nsstring.html#nsmutablestringhttp://rypress.com/tutorials/objective-c/data-types/nsstring.html#nsmutablestringhttps://developer.apple.com/library/ios/documentation/cocoa/Reference/Foundation/Protocols/NSCopying_Protocol/Reference/Reference.html#//apple_ref/occ/intf/NSCopying