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Automatic Reference Counting

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The document discusses Automatic Reference Counting (ARC) in Objective-C, introducing its functionalities, advantages, and the underlying system requirements necessary for its implementation. It explains how ARC simplifies memory management by assigning responsibilities to the compiler, eliminating the need for manual retain and release calls. Key rules and best practices for using ARC effectively are outlined, alongside potential pitfalls and examples of usage.

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Automatic Reference Counting Giuseppe Arici - @giuseppearici Superpartes Innovation Campus MOBILE DEVELOPER CONFERENCE 24 – 25 MAGGIO 2012 1
Giuseppe ARiCi  Apple iOS & Mac OS X Addicted Developer Superpartes Innovation Campus & H-Farm  Group Founder & Cocoa Preacher m # Pragma Mark ― pragmamark.org co pe y ;) ci. sep bo ari giu Fan  A Social & Lazy Connected Node → ot a [ tt | in | fb | * ] / giuseppe.arici ard n I'm  Mail Forwarder & Spammer vC giuseppe.arici@gmail.com 2
ARC 3
Attenzione: è un medicinale, leggere attentamente il foglio illustrativo. Consultare il medico se il disturbo si presenta ripetutamente. I medicinali vanno assunti con cautela, per un breve periodo di tempo, non superando le dosi consigliate e solo per le indicazioni riportate nel foglio illustrativo. In caso di dubbio rivolgersi al medico o al farmacista. Tenere il medicinale fuori dalla portata e dalla vista dei bambini. Da usare solo in età adulta. 4
OS X & iOS 5
Session Timeline • Da dove nasce ? • Che cos’è ? ARC • Come funziona ? • Quando utilizzarlo ? 6
Memory Management 100 UIImage <0x12345678> @”Hello WhyMCA !” NSData <0a1b2c3d4e...> UIVIew Backing Store 7
Memory Management 100 UIImage <0x12345678> UIVIew Backing Store 8
malloc / free UIImage <0x12345678> UIVIew Backing Store 9
Manual Reference Counting UIImage <0x12345678> 1 UIVIew Backing Store 3 10
Manual Reference Counting UIImage <0x12345678> 1 UIVIew Backing Store 2 11
Manual Reference Counting 0 UIVIew Backing Store 2 12
Garbage Collection 100 13
Garbage Collection 200 Generation 1 Generation 2 Generation 3 14
Garbage Collection 300 15
Garbage Collection 300 16
Garbage Collection 300 17
Garbage Collection 300 Heap Compaction 18
Garbage Collection 400 High Performance Concurrent Garbage Collector 19
Garbage Collection 400 X read barrier 20
C GC vs 21
Puntatori che giocano a Nascondino 22
Apple l’ha fatto comunque ! 23
Limiti del GC Objective-C 24
Limiti del GC Objective-C NSData *data = [obj someData]; const char *bytes = [data bytes]; NSUInteger length = [data length]; for (NSUInteger i = 0; i < length; ++i) { printf(“%x”, bytes[i]); } 25
Limiti del GC Objective-C 26
Limiti del GC Objective-C 27
iPhone GC vs 28
iPhone Android vs 29
30
Automatic Reference Counting “Automatic Reference Counting (ARC) in Objective-C makes memory management the job of the compiler. By enabling ARC with the new Apple LLVM compiler, you will never need to type retain or release again, dramatically simplifying the development process, while reducing crashes and memory leaks. The compiler has a complete understanding of your objects, and releases each object the instant it is no longer used, so apps run as fast as ever, with predictable, smooth performance.” (Apple, “iOS 5 for developers” – http://developer.apple.com/technologies/ios5) 31
Static Analyzer 32
Analysis + Auto-Fix + Goodies 33
Requisiti di Sistema • iOS 5 • OS X 10.7 • iOS 4 - no runtime ! • OS X 10.6 (64-bit) - no runtime ! 34
Requisiti di Runtime • Clang (LLVM compiler) 3.0 or later • Obj-C Runtime: objc4 493.9 or later • For iOS4 or OS X 10.6: libarclite_iphoneos.a or libarclite_macosx.a is linked to make ARC work 35
Conversione ad ARC 36
Conversione ad ARC 37
Conversione ad ARC NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; // code here [pool release]; @autoreleasepool { // code here } NSString *s = [(id)CFStringCreateWithX(…) autorelease]; NSString *s = CFBridgingRelease(CFStringCreateWithX(…)); 38
Conversione Automatica From the command line via the -ccc-arcmt-check and -ccc-arcmt-modify arguments. 39
Conversione Automatica 40
Nessuno è perfetto ! Eccezioni Toll-Free Bridging Strani Autorelease Pools Singleton Structs { case } Giochetti con i Puntatori Blocks 41
Conversione Parziale 42
Come Funziona ARC ARC è costituito da due componenti principali: • ARC front end compiler • ARC optimizer. 43
ARC front end compiler • Inserisce le release correttamente • ivar (strong): inserisce le release nel dealloc • Nel dealloc chiama il [super dealloc] • Genera errori sui qualificatori inconsistenti 44
ARC optimizer • Rimuove le retain e le release superflue • Ottimizza le prestazioni, minimizzando le sequenza multiple di retain e [auto]release 45
Come Funziona ARC Foo *obj = [[Foo alloc] init]; [foo bar]; 46
Come Funziona ARC Foo *obj = [[Foo alloc] init]; [foo bar]; [foo release]; 47
Come Funziona ARC Foo *obj = [[Foo alloc] init]; [foo bar]; objc_release(foo); 48
Come Funziona ARC retain retain release release release retain retain retain retain release release release retain retain release release retain release 49
Come Funziona ARC Foo *obj = [self foo]; [foo bar]; [self setFoo: newFoo]; [foo baz]; 50
Come Funziona ARC Foo *obj = objc_retain([self foo]); [foo bar]; [self setFoo: newFoo]; [foo baz]; objc_release(foo); 51
Come Funziona ARC - (Foo *) foo { return _foo; } 52
Come Funziona ARC - (Foo *) foo { return [[_foo retain] autorelease]; } 53
Come Funziona ARC - (Foo *) foo { return objc_retainAutoreleaseReturnValue(_foo); } 54
Come Funziona ARC retain autorelease retain release 55
Come Funziona ARC retain autorelease retain release 56
Come Funziona ARC objc_retainAutoreleaseReturnValue metodo chiamato Autorelease Pool metodo chiamante objc_retainAutorelease dReturnValue 57
Come Funziona ARC retain release 58
Riferimenti Ciclici 1 1 59
Riferimenti Ciclici 0 1 60
zeroing weak references Riferimento “debole” non sicuro nil X release Riferimento __weak X nil 61
Instruments 62
Memory Management Rules • You have ownership of any objects you create. • You can take ownership of an object using retain. • When no longer needed, you must relinquish ownership of an object you own. • You must not relinquish ownership of an object you don’t own. 63
Memory Management Rules Azione su oggetto Objective-C Metodo Objective-C Create and have ownership of it alloc/new/copy/mutableCopy Take ownership of it retain Relinquish it release Dispose of it dealloc 64
Qualificatori di Ownership In ARC le variabili id o di tipo oggetto devono avere uno di questi 4 qualificatori di ownership: • __strong (default) • __weak • __unsafe_unretained • __autoreleasing 65
__strong /* ARC */ id obj = [[NSObject alloc] init]; { //id __strong obj = [[NSObject alloc] init]; id __strong obj = [[NSObject alloc] init]; id obj = [[NSObject alloc] init]; } /* non-ARC */ { id obj = [[NSObject alloc] init]; [obj release]; } • Default for ‘id’ and object types ! 66
__weak { id __strong obj0 = [[NSObject alloc] init]; id __weak obj1 = obj0; } • No ownership of the object • When discarded, the variable is assigned to nil • In OS X, not all classes support weak 67
__unsafe_unretained id __unsafe_unretained obj = [[NSObject alloc] init]; warning: assigning retained obj to unsafe_unretained variable; obj will be released after assignment [-Warc-unsafe-retained-assign] • UNSAFE • __weak only for iOS5 / OSX Lion (or later). • You must take care of the variables manually 68
__autoreleasing @autoreleasepool { id __strong obj = [NSMutableArray array]; } - (BOOL) performOperationWithError:(NSError **)error; - (BOOL) performOperationWithError:(NSError * __autoreleasing *)error; • Pointer to ‘id’ or object types is qualified with __autoreleasing as default 69
Initialization id __strong obj0; id __weak obj1; id __autoreleasing obj2; id __strong obj0 = nil; id __weak obj1 = nil; id __autoreleasing obj2 = nil; Any variables that are qualified with __strong, __weak, and __autoreleasing, are initialized with nil. 70
Coding With ARC Rules • Che cosa è vietato fare • Che cosa è obbligatorio fare • Che cosa si può fare, con attenzione ! 71
Rule 1 1. Forget about using retain, release, retainCount, and autorelease [obj release]; error: ARC forbids explicit message send of 'release' [obj release]; 72
Rule 2 2. Forget about using NSAllocateObject and NSDeallocateObject NSAllocateObject (Class c, NSUInteger extraBytes, NSZone *zone) error: 'NSAllocateObject' is unavailable: not available in automatic reference counting mode 73
Rule 3 3. Follow the naming rule for methods related to object creation • if begins { alloc, new, copy, mutableCopy }, the caller has ownership • if begins { init }: - (id) init ... • instance method • return ‘id’ or types of [super|sub]class • not registered in autoreleasepool: the caller has ownership • special case: “initialize” 74
Rule 3 (Positive False) -(NSString*) copyRightString; -(NSString*) copyRightString NS_RETURNS_NON_RETAINED; • NS_RETURNS_NON_RETAINED • NS_RETURNS_RETAINED 75
Rule 4 4. Forget about calling super dealloc explicitly - (void) dealloc { free(buffer); [super dealloc]; } error: ARC forbids explicit message send of 'dealloc' [super dealloc]; not available in automatic reference counting mode 76
Rule 5 5. Use @autoreleasepool Instead of NSAutoreleasePool NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; ... [pool drain]; error: 'NSAutoreleasePool' is unavailable: not available in automatic reference counting mode NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; ^ 77
Rule 6 6. Forget About Using Zone (NSZone) • If __OBJC2__ is defined, the runtime just ignores zones 78
Rule 7 7. Object Type Variables Cannot Be Members of struct or union in C struct Data { NSMutableArray *array; }; error: ARC forbids Objective-C objs in structs or unions NSMutableArray *array; 79
Rule 8 8. 'id' and 'void *' Have to Be Cast Explicitly id obj = [[NSObject alloc] init]; void *p = obj; error: implicit conversion of a non-Objective-C pointer type 'void *' to 'id' is disallowed with ARC Special kind of casts: • (__bridge cast) • (__bridge_retained cast) • (__bridge_transfer cast) 80
Rule 8 (__bridge cast) id obj = [[NSObject alloc] init]; void *p = (__bridge void *)obj; id o = (__bridge id)p; 81
Rule 8 (__bridge_retained cast) /* ARC */ id obj = [[NSObject alloc] init]; void *p = (__bridge_retained void *)obj; /* non-ARC */ id obj = [[NSObject alloc] init]; void *p = obj; [(id)p retain]; 82
Rule 8 (__bridge_transfer cast) /* ARC */ id obj = (__bridge_transfer id)p; /* non-ARC */ id obj = (id)p; [obj retain]; [(id)p release]; 83
Rule 8 (Toll-Free Bridging) CFTypeRef CFBridgingRetain(id X) { return (__bridge_retained CFTypeRef)X; } id CFBridgingRelease(CFTypeRef X) { return (__bridge_transfer id)X; } • Convert an object between Obj-C and Core Foundation. 84
Rule 8 (Toll-Free Bridging) (__bridge cast) passaggio diretto del blocco puntato nessun trasferimento di ownership CFBridgingRetain() trasferimento di ownership da ARC a Core Foundation CFBridgingRelease() trasferimento di ownership da Core Foundation ad ARC 85
Rule 9 9. Switch cases must be wrapped in scope with { } switch (someInt) { case 0: { NSLog(@"Case 0"); } break; } 86
Rule 10 10.Exceptions use must be exceptional • For performance purpose, ARC code leaks on exceptions • In Obj-C ARC is not exception-safe for normal releases by default • -fobjc-arc-exceptions / -fno-objc-arc-exceptions: enable / disable • In Obj-C++ -fobjc-arc-exceptions is enabled by default 87
Rule 11 11.IBOutlet must be declared as weak • Outlets should be defined as declared properties • Outlets should generally be weak • Top-level objects (or, in iOS, a storyboard scene) should be strong Resource Programming Guide 88
Rule 12 12.Singletons should be redefined with dispatch_async static MyClass *sharedManager = nil; - (id)retain + (MyClass*)sharedManager { { return self; @synchronized(self) { } if (sharedManager == nil) { - (NSUInteger)retainCount sharedManager = { [[super allocWithZone:NULL] init]; return NSUIntegerMax; //cannot be released } } } - (oneway void)release return sharedManager; { } //do nothing + (id)allocWithZone:(NSZone *)zone } { - (id)autorelease return [[self sharedManager] retain]; { } return self; - (id)copyWithZone:(NSZone *)zone } { return self; } Resource Programming Guide 89
Rule 12 (new singleton) + (id)sharedInstance { static dispatch_once_t predicate; dispatch_once(&predicate, ^{ sharedInstance = [[MyClass alloc] init]; }); return sharedInstance; } Single threaded results Multi threaded results ----------------------- ----------------------- @synchronized: 3.3829 seconds @synchronized: 33.5171 seconds dispatch_once: 0.9891 seconds dispatch_once: 1.6648 seconds http://bjhomer.blogspot.it/2011/09/synchronized-vs-dispatchonce.html 90
Rule 13 13.Be careful with performSelector #pragma clang diagnostic push #pragma clang diagnostic ignored "-Warc-performSelector-leaks" [self performSelector:self.mySel]; #pragma clang diagnostic pop 91
Constructs With ARC • Property • Array • Block 92
Property Property modifier Ownership qualifier assign __unsafe_unretained copy __strong (note: new copied object is assigned) retain __strong strong __strong unsafe_unretained __unsafe_unretained weak __weak 93
Property • Same ownership rules as variables • copy: copies the object by using copyWithZone • manually declared ivar has to have the same ownership qualifier as the property 94
Property @property (nonatomic, readonly) MyObj* obj; @synthesize obj; error: ARC forbids synthesizing a property of an Objective-C object with unspecified ownership or storage attribute • ARC forbids synthesizing a readonly property without specifying an ownership qualifier 95
Array id objs0[10]; id __weak objs1[20]; NSMutableArray *objs2 = [NSMutableArray alloc] init]; • 'C' static arrays qualified with {__strong, __weak, __autoreleasing} are initialized with nil • For dynamic arrays, should be used containers in the Foundation Framework 96
Block void (^block)(void) = ^{ [self doSomethingImportant]; }; ivar = block; // ARC inserts [block copy] 97
Block return ^{ [self doSomethingGreat]; }; 98
Block [array addObject: ^{ … }]; - (id)method { return ^{ … }; } 99
Block [[[[[[[[[[block copy] copy] copy] copy] copy] copy] copy] copy] copy] copy] 100
Block __block variable in MRC: mutable & “debole” in ARC: mutable & strong ! __weak variable 101
Block & Riferimenti Ciclici i MyObj { id ivar; void (^block)(void); } ^{ [self something]; [ivar something]; }; 102
Block & Riferimenti Ciclici __weak MyClass *weakSelf = self; ^{ [weakSelf something]; }; 103
Block & Riferimenti Ciclici __weak MyClass *weakSelf = self; ^{ if (weakSelf) { [weakSelf->ivar something]; } }; 104
Block & Riferimenti Ciclici __weak MyClass *weakSelf = self; ^{ if (weakSelf) { nil !? [weakSelf->ivar something]; } }; 105
Block & Riferimenti Ciclici __weak MyClass *weakSelf = self; ^{ MyClass *localSelf = weakSelf; if (localSelf) { [localSelf->ivar something]; } }; 106
ARC MACRO // define some LLVM3 macros if the code is compiled with a different compiler (ie LLVMGCC42) #ifndef __has_feature #define __has_feature(x) 0 #endif #ifndef __has_extension #define __has_extension __has_feature // Compatibility with pre-3.0 compilers. #endif #if __has_feature(objc_arc) && __clang_major__ >= 3 #define ARC_ENABLED 1 #endif // __has_feature(objc_arc) Check @ compile-time if ARC is enabled 107
Conclusioni • La Garbage collection é una gran bella idea ! • L'Objective-C è un gran bel linguaggio ! MA • L'Objective-C + La Garbage Collection non sono una gran bella accoppiata ! 108
Conclusioni ARC non è la Garbage Collection … anche se ci arriva vicino ;) 109
Conclusioni • Le zeroing weak references sono indispensabili ! • Utilizziamo __weak per evitare i riferimenti ciclici ! • Utilizziamo __unsafe_unretained se e solo se non possiamo usare __weak ! • Utilizziamo Instruments Leaks per individuare i riferimenti ciclici ! 110
Conclusioni • ARC sa gestire soltanto gli oggetti Objective-C ! • Utilizziamo sempre __bridge, CFBridgingRetain, CFBridgingRelease per interagire con Core Foundation ! 111
Conclusioni • ARC è una figata ! • Iniziamo subito ad usarlo ! • Convertiamo il nostro codice ! • Risparmiamo tempo ! • Risparmiamo denaro ! • Viviamo meglio ! 112
ARC Reference: documentation and articles Clang: Automatic Reference Counting documentation • Apple: Transitioning to ARC Release Notes • Apple: Xcode New Features User Guide: Automatic Reference Counting • Mike Ash: Friday Q&A about Automatic Reference Counting • StackOverflow: How does automatic reference counting work? • StackOverflow: What are the advantages and disadvantages of using ARC? • StackOverflow: What is the difference between ARC and garbage collection? • informIT: Automatic Reference Counting in Objective-C, Part 1 • informIT: Automatic Reference Counting in Objective-C, Part 2 The Details • Mike Ash: Zeroing weak references without ARC • Github: Support for zeroing weak references on iOS 4 / OS X 10.6 • Assembler - A look under ARC’s hood • MKBlog - Migrating your code to Objective-C ARC • Ray Wenderlich - Beginning ARC in iOS 5 Tutorial 113
ARC Reference: videos • WWDC 2011 – Session 322 – Introduction to Automatic Reference Counting • WWDC 2011 – Session 322 – Objective-C Advancements in depth • WWDC 2011 – Session 308 – Blocks and Grand Central Dispatch in Practice • NSScreenCasts Episode 9 - Automatic Reference Counting 114
ARC Reference: books • Pro Multithreading and Memory Management for iOS and OS X with ARC, Grand Central Dispatch, and Blocks – By Kazuki Sakamoto, Apress • Programming iOS 5: Fundamentals of iPhone, iPad, and iPod touch Development Second Edition – By Matt Neuburg, O’Reilly 115
Sorry !? Automatic Reference Counting Specifications 116
Questions ? giuseppe.arici@gmail.com 117
Thanks ! giuseppearici.com pragmamark.org 118

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Automatic Reference Counting

  • 1. Automatic Reference Counting Giuseppe Arici - @giuseppearici Superpartes Innovation Campus MOBILE DEVELOPER CONFERENCE 24 – 25 MAGGIO 2012 1
  • 2. Giuseppe ARiCi  Apple iOS & Mac OS X Addicted Developer Superpartes Innovation Campus & H-Farm  Group Founder & Cocoa Preacher m # Pragma Mark ― pragmamark.org co pe y ;) ci. sep bo ari giu Fan  A Social & Lazy Connected Node → ot a [ tt | in | fb | * ] / giuseppe.arici ard n I'm  Mail Forwarder & Spammer vC giuseppe.arici@gmail.com 2
  • 4. Attenzione: è un medicinale, leggere attentamente il foglio illustrativo. Consultare il medico se il disturbo si presenta ripetutamente. I medicinali vanno assunti con cautela, per un breve periodo di tempo, non superando le dosi consigliate e solo per le indicazioni riportate nel foglio illustrativo. In caso di dubbio rivolgersi al medico o al farmacista. Tenere il medicinale fuori dalla portata e dalla vista dei bambini. Da usare solo in età adulta. 4
  • 5. OS X & iOS 5
  • 6. Session Timeline • Da dove nasce ? • Che cos’è ? ARC • Come funziona ? • Quando utilizzarlo ? 6
  • 7. Memory Management 100 UIImage <0x12345678> @”Hello WhyMCA !” NSData <0a1b2c3d4e...> UIVIew Backing Store 7
  • 8. Memory Management 100 UIImage <0x12345678> UIVIew Backing Store 8
  • 9. malloc / free UIImage <0x12345678> UIVIew Backing Store 9
  • 10. Manual Reference Counting UIImage <0x12345678> 1 UIVIew Backing Store 3 10
  • 11. Manual Reference Counting UIImage <0x12345678> 1 UIVIew Backing Store 2 11
  • 12. Manual Reference Counting 0 UIVIew Backing Store 2 12
  • 14. Garbage Collection 200 Generation 1 Generation 2 Generation 3 14
  • 18. Garbage Collection 300 Heap Compaction 18
  • 19. Garbage Collection 400 High Performance Concurrent Garbage Collector 19
  • 20. Garbage Collection 400 X read barrier 20
  • 21. C GC vs 21
  • 22. Puntatori che giocano a Nascondino 22
  • 23. Apple l’ha fatto comunque ! 23
  • 24. Limiti del GC Objective-C 24
  • 25. Limiti del GC Objective-C NSData *data = [obj someData]; const char *bytes = [data bytes]; NSUInteger length = [data length]; for (NSUInteger i = 0; i < length; ++i) { printf(“%x”, bytes[i]); } 25
  • 26. Limiti del GC Objective-C 26
  • 27. Limiti del GC Objective-C 27
  • 28. iPhone GC vs 28
  • 29. iPhone Android vs 29
  • 30. 30
  • 31. Automatic Reference Counting “Automatic Reference Counting (ARC) in Objective-C makes memory management the job of the compiler. By enabling ARC with the new Apple LLVM compiler, you will never need to type retain or release again, dramatically simplifying the development process, while reducing crashes and memory leaks. The compiler has a complete understanding of your objects, and releases each object the instant it is no longer used, so apps run as fast as ever, with predictable, smooth performance.” (Apple, “iOS 5 for developers” – http://developer.apple.com/technologies/ios5) 31
  • 33. Analysis + Auto-Fix + Goodies 33
  • 34. Requisiti di Sistema • iOS 5 • OS X 10.7 • iOS 4 - no runtime ! • OS X 10.6 (64-bit) - no runtime ! 34
  • 35. Requisiti di Runtime • Clang (LLVM compiler) 3.0 or later • Obj-C Runtime: objc4 493.9 or later • For iOS4 or OS X 10.6: libarclite_iphoneos.a or libarclite_macosx.a is linked to make ARC work 35
  • 38. Conversione ad ARC NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; // code here [pool release]; @autoreleasepool { // code here } NSString *s = [(id)CFStringCreateWithX(…) autorelease]; NSString *s = CFBridgingRelease(CFStringCreateWithX(…)); 38
  • 39. Conversione Automatica From the command line via the -ccc-arcmt-check and -ccc-arcmt-modify arguments. 39
  • 41. Nessuno è perfetto ! Eccezioni Toll-Free Bridging Strani Autorelease Pools Singleton Structs { case } Giochetti con i Puntatori Blocks 41
  • 43. Come Funziona ARC ARC è costituito da due componenti principali: • ARC front end compiler • ARC optimizer. 43
  • 44. ARC front end compiler • Inserisce le release correttamente • ivar (strong): inserisce le release nel dealloc • Nel dealloc chiama il [super dealloc] • Genera errori sui qualificatori inconsistenti 44
  • 45. ARC optimizer • Rimuove le retain e le release superflue • Ottimizza le prestazioni, minimizzando le sequenza multiple di retain e [auto]release 45
  • 46. Come Funziona ARC Foo *obj = [[Foo alloc] init]; [foo bar]; 46
  • 47. Come Funziona ARC Foo *obj = [[Foo alloc] init]; [foo bar]; [foo release]; 47
  • 48. Come Funziona ARC Foo *obj = [[Foo alloc] init]; [foo bar]; objc_release(foo); 48
  • 49. Come Funziona ARC retain retain release release release retain retain retain retain release release release retain retain release release retain release 49
  • 50. Come Funziona ARC Foo *obj = [self foo]; [foo bar]; [self setFoo: newFoo]; [foo baz]; 50
  • 51. Come Funziona ARC Foo *obj = objc_retain([self foo]); [foo bar]; [self setFoo: newFoo]; [foo baz]; objc_release(foo); 51
  • 52. Come Funziona ARC - (Foo *) foo { return _foo; } 52
  • 53. Come Funziona ARC - (Foo *) foo { return [[_foo retain] autorelease]; } 53
  • 54. Come Funziona ARC - (Foo *) foo { return objc_retainAutoreleaseReturnValue(_foo); } 54
  • 55. Come Funziona ARC retain autorelease retain release 55
  • 56. Come Funziona ARC retain autorelease retain release 56
  • 57. Come Funziona ARC objc_retainAutoreleaseReturnValue metodo chiamato Autorelease Pool metodo chiamante objc_retainAutorelease dReturnValue 57
  • 58. Come Funziona ARC retain release 58
  • 61. zeroing weak references Riferimento “debole” non sicuro nil X release Riferimento __weak X nil 61
  • 63. Memory Management Rules • You have ownership of any objects you create. • You can take ownership of an object using retain. • When no longer needed, you must relinquish ownership of an object you own. • You must not relinquish ownership of an object you don’t own. 63
  • 64. Memory Management Rules Azione su oggetto Objective-C Metodo Objective-C Create and have ownership of it alloc/new/copy/mutableCopy Take ownership of it retain Relinquish it release Dispose of it dealloc 64
  • 65. Qualificatori di Ownership In ARC le variabili id o di tipo oggetto devono avere uno di questi 4 qualificatori di ownership: • __strong (default) • __weak • __unsafe_unretained • __autoreleasing 65
  • 66. __strong /* ARC */ id obj = [[NSObject alloc] init]; { //id __strong obj = [[NSObject alloc] init]; id __strong obj = [[NSObject alloc] init]; id obj = [[NSObject alloc] init]; } /* non-ARC */ { id obj = [[NSObject alloc] init]; [obj release]; } • Default for ‘id’ and object types ! 66
  • 67. __weak { id __strong obj0 = [[NSObject alloc] init]; id __weak obj1 = obj0; } • No ownership of the object • When discarded, the variable is assigned to nil • In OS X, not all classes support weak 67
  • 68. __unsafe_unretained id __unsafe_unretained obj = [[NSObject alloc] init]; warning: assigning retained obj to unsafe_unretained variable; obj will be released after assignment [-Warc-unsafe-retained-assign] • UNSAFE • __weak only for iOS5 / OSX Lion (or later). • You must take care of the variables manually 68
  • 69. __autoreleasing @autoreleasepool { id __strong obj = [NSMutableArray array]; } - (BOOL) performOperationWithError:(NSError **)error; - (BOOL) performOperationWithError:(NSError * __autoreleasing *)error; • Pointer to ‘id’ or object types is qualified with __autoreleasing as default 69
  • 70. Initialization id __strong obj0; id __weak obj1; id __autoreleasing obj2; id __strong obj0 = nil; id __weak obj1 = nil; id __autoreleasing obj2 = nil; Any variables that are qualified with __strong, __weak, and __autoreleasing, are initialized with nil. 70
  • 71. Coding With ARC Rules • Che cosa è vietato fare • Che cosa è obbligatorio fare • Che cosa si può fare, con attenzione ! 71
  • 72. Rule 1 1. Forget about using retain, release, retainCount, and autorelease [obj release]; error: ARC forbids explicit message send of 'release' [obj release]; 72
  • 73. Rule 2 2. Forget about using NSAllocateObject and NSDeallocateObject NSAllocateObject (Class c, NSUInteger extraBytes, NSZone *zone) error: 'NSAllocateObject' is unavailable: not available in automatic reference counting mode 73
  • 74. Rule 3 3. Follow the naming rule for methods related to object creation • if begins { alloc, new, copy, mutableCopy }, the caller has ownership • if begins { init }: - (id) init ... • instance method • return ‘id’ or types of [super|sub]class • not registered in autoreleasepool: the caller has ownership • special case: “initialize” 74
  • 75. Rule 3 (Positive False) -(NSString*) copyRightString; -(NSString*) copyRightString NS_RETURNS_NON_RETAINED; • NS_RETURNS_NON_RETAINED • NS_RETURNS_RETAINED 75
  • 76. Rule 4 4. Forget about calling super dealloc explicitly - (void) dealloc { free(buffer); [super dealloc]; } error: ARC forbids explicit message send of 'dealloc' [super dealloc]; not available in automatic reference counting mode 76
  • 77. Rule 5 5. Use @autoreleasepool Instead of NSAutoreleasePool NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; ... [pool drain]; error: 'NSAutoreleasePool' is unavailable: not available in automatic reference counting mode NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; ^ 77
  • 78. Rule 6 6. Forget About Using Zone (NSZone) • If __OBJC2__ is defined, the runtime just ignores zones 78
  • 79. Rule 7 7. Object Type Variables Cannot Be Members of struct or union in C struct Data { NSMutableArray *array; }; error: ARC forbids Objective-C objs in structs or unions NSMutableArray *array; 79
  • 80. Rule 8 8. 'id' and 'void *' Have to Be Cast Explicitly id obj = [[NSObject alloc] init]; void *p = obj; error: implicit conversion of a non-Objective-C pointer type 'void *' to 'id' is disallowed with ARC Special kind of casts: • (__bridge cast) • (__bridge_retained cast) • (__bridge_transfer cast) 80
  • 81. Rule 8 (__bridge cast) id obj = [[NSObject alloc] init]; void *p = (__bridge void *)obj; id o = (__bridge id)p; 81
  • 82. Rule 8 (__bridge_retained cast) /* ARC */ id obj = [[NSObject alloc] init]; void *p = (__bridge_retained void *)obj; /* non-ARC */ id obj = [[NSObject alloc] init]; void *p = obj; [(id)p retain]; 82
  • 83. Rule 8 (__bridge_transfer cast) /* ARC */ id obj = (__bridge_transfer id)p; /* non-ARC */ id obj = (id)p; [obj retain]; [(id)p release]; 83
  • 84. Rule 8 (Toll-Free Bridging) CFTypeRef CFBridgingRetain(id X) { return (__bridge_retained CFTypeRef)X; } id CFBridgingRelease(CFTypeRef X) { return (__bridge_transfer id)X; } • Convert an object between Obj-C and Core Foundation. 84
  • 85. Rule 8 (Toll-Free Bridging) (__bridge cast) passaggio diretto del blocco puntato nessun trasferimento di ownership CFBridgingRetain() trasferimento di ownership da ARC a Core Foundation CFBridgingRelease() trasferimento di ownership da Core Foundation ad ARC 85
  • 86. Rule 9 9. Switch cases must be wrapped in scope with { } switch (someInt) { case 0: { NSLog(@"Case 0"); } break; } 86
  • 87. Rule 10 10.Exceptions use must be exceptional • For performance purpose, ARC code leaks on exceptions • In Obj-C ARC is not exception-safe for normal releases by default • -fobjc-arc-exceptions / -fno-objc-arc-exceptions: enable / disable • In Obj-C++ -fobjc-arc-exceptions is enabled by default 87
  • 88. Rule 11 11.IBOutlet must be declared as weak • Outlets should be defined as declared properties • Outlets should generally be weak • Top-level objects (or, in iOS, a storyboard scene) should be strong Resource Programming Guide 88
  • 89. Rule 12 12.Singletons should be redefined with dispatch_async static MyClass *sharedManager = nil; - (id)retain + (MyClass*)sharedManager { { return self; @synchronized(self) { } if (sharedManager == nil) { - (NSUInteger)retainCount sharedManager = { [[super allocWithZone:NULL] init]; return NSUIntegerMax; //cannot be released } } } - (oneway void)release return sharedManager; { } //do nothing + (id)allocWithZone:(NSZone *)zone } { - (id)autorelease return [[self sharedManager] retain]; { } return self; - (id)copyWithZone:(NSZone *)zone } { return self; } Resource Programming Guide 89
  • 90. Rule 12 (new singleton) + (id)sharedInstance { static dispatch_once_t predicate; dispatch_once(&predicate, ^{ sharedInstance = [[MyClass alloc] init]; }); return sharedInstance; } Single threaded results Multi threaded results ----------------------- ----------------------- @synchronized: 3.3829 seconds @synchronized: 33.5171 seconds dispatch_once: 0.9891 seconds dispatch_once: 1.6648 seconds http://bjhomer.blogspot.it/2011/09/synchronized-vs-dispatchonce.html 90
  • 91. Rule 13 13.Be careful with performSelector #pragma clang diagnostic push #pragma clang diagnostic ignored "-Warc-performSelector-leaks" [self performSelector:self.mySel]; #pragma clang diagnostic pop 91
  • 92. Constructs With ARC • Property • Array • Block 92
  • 93. Property Property modifier Ownership qualifier assign __unsafe_unretained copy __strong (note: new copied object is assigned) retain __strong strong __strong unsafe_unretained __unsafe_unretained weak __weak 93
  • 94. Property • Same ownership rules as variables • copy: copies the object by using copyWithZone • manually declared ivar has to have the same ownership qualifier as the property 94
  • 95. Property @property (nonatomic, readonly) MyObj* obj; @synthesize obj; error: ARC forbids synthesizing a property of an Objective-C object with unspecified ownership or storage attribute • ARC forbids synthesizing a readonly property without specifying an ownership qualifier 95
  • 96. Array id objs0[10]; id __weak objs1[20]; NSMutableArray *objs2 = [NSMutableArray alloc] init]; • 'C' static arrays qualified with {__strong, __weak, __autoreleasing} are initialized with nil • For dynamic arrays, should be used containers in the Foundation Framework 96
  • 97. Block void (^block)(void) = ^{ [self doSomethingImportant]; }; ivar = block; // ARC inserts [block copy] 97
  • 98. Block return ^{ [self doSomethingGreat]; }; 98
  • 99. Block [array addObject: ^{ … }]; - (id)method { return ^{ … }; } 99
  • 100. Block [[[[[[[[[[block copy] copy] copy] copy] copy] copy] copy] copy] copy] copy] 100
  • 101. Block __block variable in MRC: mutable & “debole” in ARC: mutable & strong ! __weak variable 101
  • 102. Block & Riferimenti Ciclici i MyObj { id ivar; void (^block)(void); } ^{ [self something]; [ivar something]; }; 102
  • 103. Block & Riferimenti Ciclici __weak MyClass *weakSelf = self; ^{ [weakSelf something]; }; 103
  • 104. Block & Riferimenti Ciclici __weak MyClass *weakSelf = self; ^{ if (weakSelf) { [weakSelf->ivar something]; } }; 104
  • 105. Block & Riferimenti Ciclici __weak MyClass *weakSelf = self; ^{ if (weakSelf) { nil !? [weakSelf->ivar something]; } }; 105
  • 106. Block & Riferimenti Ciclici __weak MyClass *weakSelf = self; ^{ MyClass *localSelf = weakSelf; if (localSelf) { [localSelf->ivar something]; } }; 106
  • 107. ARC MACRO // define some LLVM3 macros if the code is compiled with a different compiler (ie LLVMGCC42) #ifndef __has_feature #define __has_feature(x) 0 #endif #ifndef __has_extension #define __has_extension __has_feature // Compatibility with pre-3.0 compilers. #endif #if __has_feature(objc_arc) && __clang_major__ >= 3 #define ARC_ENABLED 1 #endif // __has_feature(objc_arc) Check @ compile-time if ARC is enabled 107
  • 108. Conclusioni • La Garbage collection é una gran bella idea ! • L'Objective-C è un gran bel linguaggio ! MA • L'Objective-C + La Garbage Collection non sono una gran bella accoppiata ! 108
  • 109. Conclusioni ARC non è la Garbage Collection … anche se ci arriva vicino ;) 109
  • 110. Conclusioni • Le zeroing weak references sono indispensabili ! • Utilizziamo __weak per evitare i riferimenti ciclici ! • Utilizziamo __unsafe_unretained se e solo se non possiamo usare __weak ! • Utilizziamo Instruments Leaks per individuare i riferimenti ciclici ! 110
  • 111. Conclusioni • ARC sa gestire soltanto gli oggetti Objective-C ! • Utilizziamo sempre __bridge, CFBridgingRetain, CFBridgingRelease per interagire con Core Foundation ! 111
  • 112. Conclusioni • ARC è una figata ! • Iniziamo subito ad usarlo ! • Convertiamo il nostro codice ! • Risparmiamo tempo ! • Risparmiamo denaro ! • Viviamo meglio ! 112
  • 113. ARC Reference: documentation and articles Clang: Automatic Reference Counting documentation • Apple: Transitioning to ARC Release Notes • Apple: Xcode New Features User Guide: Automatic Reference Counting • Mike Ash: Friday Q&A about Automatic Reference Counting • StackOverflow: How does automatic reference counting work? • StackOverflow: What are the advantages and disadvantages of using ARC? • StackOverflow: What is the difference between ARC and garbage collection? • informIT: Automatic Reference Counting in Objective-C, Part 1 • informIT: Automatic Reference Counting in Objective-C, Part 2 The Details • Mike Ash: Zeroing weak references without ARC • Github: Support for zeroing weak references on iOS 4 / OS X 10.6 • Assembler - A look under ARC’s hood • MKBlog - Migrating your code to Objective-C ARC • Ray Wenderlich - Beginning ARC in iOS 5 Tutorial 113
  • 114. ARC Reference: videos • WWDC 2011 – Session 322 – Introduction to Automatic Reference Counting • WWDC 2011 – Session 322 – Objective-C Advancements in depth • WWDC 2011 – Session 308 – Blocks and Grand Central Dispatch in Practice • NSScreenCasts Episode 9 - Automatic Reference Counting 114
  • 115. ARC Reference: books • Pro Multithreading and Memory Management for iOS and OS X with ARC, Grand Central Dispatch, and Blocks – By Kazuki Sakamoto, Apress • Programming iOS 5: Fundamentals of iPhone, iPad, and iPod touch Development Second Edition – By Matt Neuburg, O’Reilly 115
  • 116. Sorry !? Automatic Reference Counting Specifications 116
  • 117. Questions ? giuseppe.arici@gmail.com 117
  • 118. Thanks ! giuseppearici.com pragmamark.org 118