2/20/2016 EEC492/693/793 - iPhone Application Development 12/20/2016 EEC492/693/793 - iPhone...

05/05/23EEC492/693/793 - iPhone Application

Development 105/05/23EEC492/693/793 - iPhone Application

Development 1

EEC-492/693/793EEC-492/693/793iPhone Application iPhone Application DevelopmentDevelopment

Lecture 22Lecture 22Concurrency and Multithreading Concurrency and Multithreading (slides based on CS193P, Stanford)(slides based on CS193P, Stanford)

Wenbing Zhao & Nigamanth SridharWenbing Zhao & Nigamanth Sridhar

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


Development 2

Threads on iPhone

Based on POSIX threads API /usr/include/pthread.h

Higher-level wrappers in Foundation framework NSThread Object-oriented, easy to use


Development 3

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


Development 4

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


Development 5

Using NSThread


Development 6

- (void)someAction:(id)sender { // Fire up new thread [NSThread detachNewThreadSelector:@selector(doWork:) withTarget:self object:someData];}

- (void)doWork:(id)someData { NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; [someData doLotsOfWork];

// Send message back to main thread [self performSelectorOnMainThread:@selector(allDone:) withObject:[someData result] waitUntilDone:NO]; [pool release];}

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


Development 7

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


Development 8

Locks

Protect critical sections of code Thread-safety NSLock


Development 9

- (void)init { myLock = [[NSLock alloc] init];}- (void)someMethod { [myLock lock]; // Execute critical section code [myLock unlock];}

Conditions

NSCondition is useful for producer/consumer model


Development 10

// On the producer thread- (void) produceData { [condition lock]; // Produce new data newDataExists = YES; [condition signal]; [condition unlock];}

// On the consumer thread- (void) consumeData { [condition lock]; while(!newDataExists) { [condition wait]; } // Consume the new data newDataExists = NO; [condition unlock];}

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


Development 11

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


Development 12

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


Development 13

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


Development 14

Creating an NSOperation Subclass Define a custom init method

Override main method


Development 15

- (id)initWithSomeObject:(id)someObject { self = [super init]; if (self) { self.someObject = someObject; } return self;}

- (void)main { [someObject doLotsOfTimeConsumingWork];}

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


Development 16

NSInvocationOperation

Concrete subclass of NSOperation For lightweight tasks


Development 17

- (void)someAction:(id)sender { NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(doWork:) object:someObject]; [queue addOperation:operation]; [operation release];}

2/20/2016 EEC492/693/793 - iPhone Application Development 12/20/2016 EEC492/693/793 - iPhone...

Documents

Transcript of 2/20/2016 EEC492/693/793 - iPhone Application Development 12/20/2016 EEC492/693/793 - iPhone...