Mobile Application Development

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Denis Harvey
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1 Object Lifecycle Mobile Application Development Creating objects Memory management Destroying objects Basic ios Development 11-Nov-11 Mobile App Development 1 11/11/11 2 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 Create = Allocate + Initialize Person *person = nil; person = [[Person alloc] init]; 11/11/11 3 Implementing Your own -init Method #import 2id)init{ //allowsuperclasstoinitializeitsstatefirst ifself=[superinit]){ age=0; name=@ Bob ; //dootherinitialization... 11/11/11 4

2 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 2id)init{ return[selfinitwithname:@ NoName ]; 2id)initWithName:NSString*)name{ return[selfinitwithname:nameage:0]; 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? 11/11/ /11/11 6 Memory Management Allocation Destruction C Malloc Free Objective-C Alloc dealloc Calls must be balanced, otherwise, your program may leak or crash 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 of 1 Why we release in methods objects created with alloc -retain increments retain count -release decrements retain count When retain count reaches 0, object is destroyed -dealloc method invoked automatically 11/11/ /11/11 8

3 Balanced Calls Person *person = nil; person = [[Person alloc] init]; [person setname:@ Jimmy Jones ]; [person setage:32]; [person castballot]; [person dosomethingelse]; 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 // When we re done with person, release it [person release]; // person will be destroyed here 11/11/ /11/11 10 Messaging deallocated objects Person *person = [[Person alloc] init]; //... [person release]; // Object is deallocated person = nil; [person dosomething]; // No effect Implementing a -dealloc method 2 void)dealloc{ //Doanycleanupthat snecessary //... //whenwe redone,callsupertocleanusup 11/11/ /11/11 12

4 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 2[superdealloc] You only deal with alloc, copy, retain, release Object Ownership { //instancevariables NSString*name;//Personclass owns thename intage; //methoddeclarations 2NSString*)name; 2void)setName:NSString*)value; 2int)age; 2void)setAge:int)age; 2BOOL)canLegallyVote; 11/11/ /11/11 14 Object Ownership 2NSString*)name{returnname; void)setname:nsstring*)newname{ ifname!=newname){ [namerelease]; //ifwedon tdothis,wewouldlosethe //referencetotheobjectpointedtobynameafter //theassignment,whichwouldcauseamemoryleak name=[newnameretain]; //PersonistakingownershipofnewName //name 11/11/11 15 Releasing Instance Variables #import 2void)dealloc{ //Doanycleanupthat snecessary [name&release];& & //whenwe redone,callsupertocleanus up 11/11/11 16

5 Returning a Newly Created Object Attempt #1 2NSString*)fullName{ NSString*result; result= [[NSStringalloc]initWithFormat:@ %@%@, firstname,lastname]; returnresult; Not ideal: caller must remember to call release on the NSString after using it Returning a Newly Created Object Attempt #2 2NSString*)fullName{ NSString*result; result= [[NSStringalloc]initWithFormat:@ %@%@, firstname,lastname]; [resultrelease]; returnresult; Wrong: result is released too early! Method would return bogus value 11/11/ /11/11 18 Returning a Newly Created Object Attempt #3 2NSString*)fullName{ NSString*result; result= [[NSStringalloc]initWithFormat:@ %@%@, firstname,lastname]; [resultautorelease]; returnresult; Just right: result is released, but not right away Caller gets valid object and could retain if needed Autoreleasing Objects Calling -autorelease flags an object to be sent release at some point in the future Lets 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 11/11/ /11/11 20

6 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 // So don t- we re cool! This is a convention - follow it in methods you define! 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 Autorelease is not garbage collection Objective-C on iphone OS does not have garbage collection 11/11/ /11/11 22 Autorelease Pools in pictures) Pool$ [object$release];$ [object$release];$ [object$release];$ Pool$released$ Objects$autoreleased$ Here$go$into$pool$ Pool$created$ [object$ autorelease];$ 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$the$name$to$remain$valid!$ [name$retain];$ //$...$ //$Eventually,$we ll$release$it$maybe$in$our$hdealloc?)$ [name$release];$ 11/11/ /11/11 24

7 Memory Management Revisited What is going to happen if you were to run the code below assuming other parts are implemented perfectly? 2void)viewDidLoad{ stuff=arr; [arrrelease]; 25 Memory Management Revisited How about this piece of code? { 2void)viewDidLoad{ NSArray*arr=[[NSArrayalloc] self.stuff=arr; [arrrelease]; 26 Memory Management Revisited First version would crash as soon as the app is started, while the second version would work just fine The difference? Whether or not self is used: self.stuff = arr; This is quite puzzling. Shouldn t we be able to access the class instance variable directly? Yes, we do. We can certainly manipulate instance variables within the class without having to use: self. What is the problem then? Memory Management Revisited The problem is related to memory management Why version 1 crashed? stuff = arr; this assignment does not retain the array arr) aside from the implicit retain via alloc+init) When the next statement is executed: [arr release]; the retain count of the array arr) drops to 0, and therefore, it is deallocated => stuff would point to nothing, which caused the crash Why version 2 works? self.stuff = arr; in this assignment, we are actually calling the setter of the property, which would increase the retain count by 1 due to the property nonatomic, retain) NSArray *stuff; 27 28

8 Property nonatomic, retain) NSArray *stuff; retain means that the retain count for the property) will be increased after the assignment self.stuff = arr; // stuff s retain count will be increased by 1 Furthermore, the previous object which stuff pointed to) is sent a release message This is why we can do this in viewdidunload: self.namefield = nil; However, the release message is sent to instance variables directly in dealloc: method [namefield release]; The self Keyword self is to be used within a method definition to refer to the object that performs the method You normally send a message to self However, the dot notation for declared properties introduced in Objective-C 2.0) added confusion, especially for C++/Java programmers What does self.stuff mean? self.stuff = arr; // equivalent to [self setstuff: arr]; arr = self.stuff; // equivalent to arr = [self stuff]; Automatic Reference Counting Still not garbage collection They want to keep GC out; don t want the possibility of unexpected delays in run This is just the compiler inserting appropriate reference counting code automatically Based on static analysis of code Clang static analyser It is still important to understand the rules of retain/release ARC vs GC ARC is more deterministic Follows the same rigid rules of retain/release, just inserted automatically by the compiler GC can result in sawtooth memory consumption patterns No UI stutter with ARC 11-Nov-11 Mobile App Development Nov-11 Mobile App Development 32

9 Concurrency With only one thread, long-running operations can interfere with user interaction System can t update the screen App looks like it s hung Multiple threads allow multiple computations without locking up your entire application Blocking operation can happen on the background thread Threads on iphone Based on POSIX threads API /usr/include/pthread.h Higher-level wrappers in Foundation framework NSThread Object-oriented, easy to use 11/11/ /11/11 34 NSThread Run loop automatically instantiated for each thread Run loop is the main event loop that the app runs App spends most of its time in the run loop Each NSThread needs to create its own autorelease pool One is not created for you Includes methods for inter-thread messaging NSThread Creating a new thread Attach a selector to the new thread The selector method does the actual work Once the work is complete, the selector on the background thread sends a message back to the main thread to notify completion All interaction with the user must happen on the main thread 11/11/ /11/11 36

10 Using NSThread 2void)someAction:id)sender{ //Fireupnewthread withtarget:selfobject:somedata]; 2 void)dowork:id)somedata{ NSAutoreleasePool*pool=[[NSAutoreleasePoolalloc]init]; [somedatadolotsofwork]; //Sendmessagebacktomainthread [selfperformselectoronmainthread:@selectoralldone:) withobject:[somedataresult]waituntildone:no]; [poolrelease]; Dangers of Multiple Threads Can add considerable amount of complexity to app design Threads share memory; they have access to the same data structures Need coordination to decide which thread has access to a particular data structure at a particular time Synchronization and Locking 11/11/ /11/11 38 UIKit and Threadsafety Threadsafety A single data object accessed in two different threads may lead to a race condition UIKit classes are not threadsafe Objects must be created and messaged from the main thread Notable exception: UIImage can be created in a background thread But since UIImageView is not threadsafe, you can t set it in a background thread 11/11/11 39 Locks Protect critical sections of code Thread-safety NSLock 2void)init{ mylock=[[nslockalloc]init]; 2void)someMethod{ [mylocklock]; //Executecriticalsectioncode [mylockunlock]; 11/11/11 40

11 Conditions NSCondition is useful for producer/ consumer model //Ontheproducerthread 2void)produceData{ [conditionlock]; //Producenewdata newdataexists=yes; [conditionsignal]; [conditionunlock]; //Ontheconsumerthread 2void)consumeData{ [conditionlock]; while!newdataexists){ [conditionwait]; //Consumethenewdata newdataexists=no; [conditionunlock]; Danger of Locks Very easy to make mistakes All it takes is one poorly behaved client Accessing shared data outside of a lock Deadlocks Priority inversion Main thread is always supposed to have highest priority 11/11/ /11/11 42 Threading Pitfalls Subtle, nondeterministic bugs may be introduced Notoriously difficult to replicate Code may become more difficult to maintain Poorly managed threads and synchronization may actually result in slower code Alternatives to Explicit Threading Asynchronous nonblocking) functions Call a function, and pass a pointer to a callback function Specify a target/action or a delegate for callback Timers One-shot or recurring Specify a callback method Managed by the run loop NSOperation: higher level construct 11/11/ /11/11 44

12 NSOperation Manages thread creation and lifecycle Encapsulate a unit of work in an object System can schedule the operation when it is appropriate Specify priorities and dependencies Creating an NSOperation Subclass Define a custom init method 2id)initWithSomeObject:id)someObject{ self=[superinit]; ifself){ self.someobject=someobject; returnself; Override main method 2void)main{ [someobjectdolotsoftimeconsumingwork]; 11/11/ /11/11 46 NSOperationQueue Operations are typically scheduled by adding to a queue Choose a maximum number of concurrent operations Runtime worries about creating an appropriate number of threads Queue runs operations based on priority and dependencies NSInvocationOperation Concrete subclass of NSOperation For lightweight tasks 2void)someAction:id)sender{ NSInvocationOperation*operation= [[NSInvocationOperationalloc]initWithTarget:self selector:@selectordowork:) object:someobject]; [queueaddoperation:operation]; [operationrelease]; 11/11/ /11/11 48

Design Phase. Create a class Person. Determine the superclass. NSObject (in this case)

Design Phase. Create a class Person. Determine the superclass. NSObject (in this case) Design Phase Create a class Person Determine the superclass NSObject (in this case) 8 Design Phase Create a class Person Determine the superclass NSObject (in this case) What properties should it have?