Object Life Cycle and Memory Management

Transcription

1 Object Life Cycle and Memory Management

2 Custom Classes

3 Design Phase Create a class Person Determine the superclass NSObject (in this case) What properties should it have? Name, age, whether they can vote What actions can it perform? Cast a ballot

4 Defining a class A public header and a private implementation

5 Class interface declared in header file #import Person: NSObject { // instance variables NSString *name; int age; // method declarations - (NSString *)name; - (void)setname:(nsstring *)value; - (int)age; - (void)setage:(int)age; - (BOOL)canLegallyVote; -

6 Defining a class A public header and a private implementation

7 Implementing custom class Implement setter/getter methods Implement action methods

8 Class Implementation #import Person - (int)age { return age; - (void)setage:(int)value { age = value; //... and other

9 Calling your own methods #import Person - (BOOL)canLegallyVote { return ([self age] >= 18); - (void)castballot { if ([self canlegallyvote]) { // do voting stuff else { NSLog (@ I m not allowed to vote!

10 Superclass methods As we just saw, objects have an implicit variable named self Like this in Java and C++ Can also invoke superclass methods using super - (void)dosomething { // Call superclass implementation first [super dosomething]; // Then do our custom behavior int foo = bar; //...

11 Cross-File Dependencies A dependency is a relationship between two help limit the effects of dependency-caused recompilation. #import Car: NSObject{ Tire *tire[4]; Engine *engine;

12 Object Lifecycle

13 Object Lifecycle Creating objects Memory management Destroying objects

14 Object Layout

15 Object Creation Two step process allocate memory to store the object initialize object state + alloc Class method that knows how much memory is needed - init Instance method to set initial values, perform other setup

16 id anobject = [[SomeClass alloc] init]; if ( anobject ) [anobject someothermessage]; else... Create = Allocate + Initialize Person *person = nil; person = [[Person alloc] init]; Because an init... method might return an object other than the newly allocated receiver, or even return nil, it s important that programs use the value returned by the initialization method, not just that returned by alloc or allocwithzone:. The following code is very dangerous, since it ignores the return of init. id anobject = [SomeClass alloc]; [anobject init]; [anobject someothermessage]; If there s a chance that the init... method might return nil (see Handling Initialization Failure ), then you should check the return value before proceeding:

17 Implementing your own -init method #import Person - (id)init { // allow superclass to initialize its state first if (self = [super init]) { age = 0; name Bob ; // do other initialization... return

18 Another init example - (id)init { // Assign self to value returned by super's designated initializer // Designated initializer for NSObject is init self = [super init]; if (self) { creationdate = [[NSDate alloc] init]; return self;

19 Multiple init methods Classes may define multiple init methods - (id)init; - (id)initwithname:(nsstring *)name; - (id)initwithname:(nsstring *)name age:(int)age; Less specific ones typically call more specific with default values - (id)init { return [self initwithname:@ No Name ]; - (id)initwithname:(nsstring *)name { return [self initwithname:name age:0];

20 Finishing Up With an Object Person *person = nil; person = [[Person alloc] init]; [person setname:@ Jimmy Jones ]; [person setage:32]; [person castballot]; [person dosomethingelse]; // What do we do with person when we re done?

21 Memory Management Allocation Destruction C malloc free Objective-C alloc dealloc Calls must be balanced Otherwise your program may leak or crash However, you ll never call -dealloc directly One exception, we ll see in a bit...

22 Reference Counting Every object has a retain count Defined on NSObject As long as retain count is > 0, object is alive and valid +alloc and -copy create objects with retain count == 1 -retain increments retain count -release decrements retain count When retain count reaches 0, object is destroyed -dealloc method invoked automatically One-way street, once you re in -dealloc there s no turning back

23 Balanced Calls Person *person = nil; person = [[Person alloc] init]; [person setname:@ Jimmy Jones ]; [person setage:32]; [person castballot]; [person dosomethingelse]; // When we re done with person, release it [person release]; // person will be destroyed here

24 Reference counting in action Person *person = [[Person alloc] init]; Retain count begins at 1 with +alloc [person retain]; Retain count increases to 2 with retain [person release]; Retain count decreases to 1 with release [person release]; Retain count decreases to 0, -dealloc automatically called

25 Messaging deallocated objects Person *person = [[Person alloc] init]; //... [person release]; // Object is deallocated [person dosomething]; // Crash! person = nil; [person dosomething]; // No effect

26 Implementing a -dealloc method #import Person - (void)dealloc { // Do any cleanup that s necessary //... // when we re done, call super to clean us up [super

27 #import RetainTracker : // RetainTracker - (id) init { if (self = [super init]) { NSLog (@"init: Retain count of %d.", [self retaincount]); return (self); // init - (void) dealloc { NSLog (@"dealloc called. Bye Bye."); [super dealloc]; // // RetainTracker int main (int argc, const char * argv[]) { RetainTracker *tracker = [RetainTracker new]; // count: 1 [tracker retain]; // count: 2 NSLog (@"%d", [tracker retaincount]); [tracker retain]; // count: 3 NSLog (@"%d", [tracker retaincount]); [tracker release]; // count: 2 NSLog (@"%d", [tracker retaincount]); [tracker release]; // count: 1 NSLog (@"%d", [tracker retaincount]); [tracker retain]; // count 2 NSLog (@"%d", [tracker retaincount]); [tracker release]; // count 1 NSLog (@"%d", [tracker retaincount]); [tracker release]; // count: 0, dealloc it return (0); // main

28 Object Lifecycle Recap Objects begin with a retain count of 1 Increase and decrease with -retain and release When retain count reaches 0, object deallocated automatically You never call dealloc explicitly in your code Exception is calling -[super dealloc] You only deal with alloc, copy, retain, release [Class new]=[[class alloc] init];

29 Object Ownership #import Person : NSObject { // instance variables NSString *name; // Person class owns the name int age; // method declarations - (NSString *)name; - (void)setname:(nsstring *)value; - (int)age; - (void)setage:(int)age; - (BOOL)canLegallyVote; -

30 #import Person Object Ownership - (NSString *)name { return name; - (void)setname:(nsstring *)newname { if (name!= newname) { [name release]; name name = = [newname retain]; copy]; // name s // has retain retain count count has of been 1, we bumped own it up by

31 Releasing Instance Variables #import Person - (void)dealloc { // Do any cleanup that s necessary [name // when we re done, call super to clean us up [super dealloc];

32 Collections and Memory Management id anobject = [aset anyobject]; [aset removeobject:anobject];// if no object other than aset owned anobject, the next line causes a crash [anobject somemessage]; id anobject = [[aset anyobject] retain]; [aset removeobject:anobject]; [anobject somemessage];// remember to send anobject a release message when you have finished with it

33 Autorelease

34 Returning a newly created object - (NSString *)fullname { NSString *result; result = [[NSString alloc] initwithformat:@ %@ %@, firstname, lastname]; [result release]; [result autorelease]; return result; Wrong: result is leaked! Just right: Wrong: result result is released, is released but not too right early! away Caller gets Method valid returns object bogus and could value retain if needed

35 Autoreleasing Objects Calling -autorelease flags an object to be sent release at some point in the future Let s you fulfill your retain/release obligations while allowing an object some additional time to live Makes it much more convenient to manage memory Very useful in methods which return a newly created object

36 How does -autorelease work? Object is added to current autorelease pool Autorelease pools track objects scheduled to be released When the pool itself is released, it sends -release to all its objects UIKit automatically wraps a pool around every event dispatch

37 Autorelease Pools (in pictures)

38 Reference Counting Every object has a retain count Defined on NSObject As long as retain count is > 0, object is alive and valid +alloc and -copy create objects with retain count == 1 -retain increments retain count -release decrements retain count When retain count reaches 0, object is destroyed -dealloc method invoked automatically One-way street, once you re in -dealloc there s no turning back -autorelease?

39 Hanging Onto an Autoreleased Object Many methods return autoreleased objects Remember the naming conventions... They re hanging out in the pool and will get released later If you need to hold onto those objects you need to retain them Bumps up the retain count before the release happens name = [NSMutableString string]; // We want to name to remain valid! [name retain]; //... // Eventually, we ll release it (maybe in our -dealloc?) [name release];

40 #import RetainTracker : // RetainTracker - (id) init { // init if (self = [super init]) { NSLog (@"init: Retain count of %d.", [self retaincount]); return (self); - (void) dealloc { NSLog (@"dealloc called. Bye Bye."); [super dealloc]; int main (int argc, const char * argv[]) { NSAutoreleasePool *pool; pool = [[NSAutoreleasePool alloc] init]; RetainTracker *tracker; tracker = [RetainTracker new]; // count: 1 [tracker retain]; // count: 2 [tracker autorelease]; // count: still 2 [tracker release]; // count: 1 NSLog (@"releasing pool"); [pool release]; // gets nuked, sends release to tracker return (0); // // RetainTracker

41 Side Note: Garbage Collection Autorelease is not garbage collection Objective-C on iphone OS does not have garbage collection

42 Practical Memory Management (Method Names & Autorelease) Methods whose names includes alloc, copy, or new return a retained object that the caller needs to release NSMutableString *string = [[NSMutableString alloc] init]; // We are responsible for calling -release or -autorelease [string autorelease]; All other methods return autoreleased objects NSMutableString *string = [NSMutableString string]; // The method name doesn t indicate that we need to release it This is a convention- follow it in methods you define!

43 - (void)printhello { Examples NSString *string; string = [[NSString alloc] initwithstring:@"hello"]; NSLog(string); [string release]; - (void)printhello { NSString *string; string = [NSString stringwithformat:@"hello"]; NSLog(string); - (void)printwindowtitle { NSString *string; string = [mywindow title]; NSLog(string);

44 Keeing the Pool Clean int i; for (i=0; i< ; i++) { id object = [somearray objectindex: i]; NSString *desc=[object description]; // and do something with the description

45 Keeping the Pool Clean NSAutoreleasePool *pool; pool=[[nsautoreleasepool alloc] init]; int i; for (i=0; i< ; i++) { id object = [somearray objectindex: i]; NSString *desc=[object description]; // and do something with the description if (i%1000 ==0) { [pool release]; //[pool drain]; pool = [[NSAutoreleasePool alloc] init];

46 Collection and Object Ownership When you add an object to a collection such as an array, dictionary, or set, the collection takes ownership of it. The collection will relinquish ownership when the object is removed from the collection or when the collection is itself released. NSMutableArray *array; NSUInteger i; //... for (i = 0; i < 10; i++) { NSNumber *conveniencenumber = [NSNumber numberwithinteger:i]; [array addobject:conveniencenumber];

47 NSMutableArray *array; NSUInteger i; Memory Leak //... for (i = 0; i < 10; i++) { NSNumber *allocednumber = [[NSNumber alloc] initwithinteger: i]; [array addobject:allocednumber]; [allocednumber release];

48 Another Leak Example -(void) addnumbertoarray: (float)afloat { NSNumber *anumber = [[NSNumber alloc] initwithfloat:afloat]; [anarray addobject: anumber]; -(void) addnumbertoarray: (float)afloat { NSNumber *anumber = [[NSNumber alloc] initwithfloat:afloat]; [anarray addobject: anumber]; [anumber release]; -(void) addnumbertoarray:(float)afloat { NSNumber *anumber = [NSNumber numberwithfloat:afloat]; [anarray addobject: anumber];

49 Nested Autorelease Pool Cocoa always expects there to be an autorelease pool available. Autorelease pools are arranged in a stack (being nested ). When you create a new autorelease pool, it is added to the top of the stack. When pools are deallocated, they are removed from the stack. When an object is sent an autorelease message, it is added to the current topmost pool for the current thread. The ability to nest autorelease pools means that you can include them in any function or method. A main function may create an autorelease pool and call another function that creates another autorelease pool. A single method might have an autorelease pool for an outer loop, and another autorelease pool for an inner loop.

50 Nested Autorelease Pools What if you release an autorelease pool that is not the top of the stack? this causes all (unreleased) autorelease pools above it on the stack to be released, along with all their objects.