1.
Practical Concurrent
Programming
Chris Eidhof

2.
Merhabā

3.
Why is concurrency hard?

4.
When to use concurrency?
Always use the main thread

5.
When to use concurrency?
— Networking
— Expensive stuff

6.
Decision workflow
1. Measure
2. Change
3. Measure again

7.
How to draw things in the
background

8.
Recipe
1. Take drawRect: code
2. Put it in a background thread
3. Update the main thread

9.
The drawRect: code
- (void)drawRect:(CGRect)rect
{
CGContextRef ctx = UIGraphicsGetCurrentContext();
// expensive
// drawing
// code
}

10.
Moving it to a different thread
[queue addOperationWithBlock:^{
UIGraphicsBeginImageContextWithOptions(size, NO, 0);
// expensive
// drawing
// code
UIImage *i = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
// TODO: update the main thread
}];

11.
Updating the main thread
[queue addOperationWithBlock:^{
UIGraphicsBeginImageContextWithOptions(size, NO, 0);
// expensive
// drawing
// code
UIImage *i = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
self.imageView.image = i;
}];
}];

12.
Deckset filters

13.
[self.queue addOperationWithBlock:^{
CIImage *ciImage = [CIImage imageWithContentsOfURL:sourceURL];
CIFilter *depthOfFieldFilter = [CIFilter filterWithName:@"CIDepthOfField"];
...
CIImage *finalImage = [alphaFilter valueForKey:kCIOutputImageKey];
CGContextRef cgContext = CGBitmapContextCreate(NULL, size.width, size.height, 8,
size.width * 4, colorSpace, kCGImageAlphaPremultipliedLast);
CIContext *context = [CIContext contextWithCGContext:cgContext options:nil];
CGImageRef outputImage = [context createCGImage:finalImage fromRect:ciImage.extent];
...
CGImageDestinationAddImage(destination, outputImage, nil);
CGImageDestinationFinalize(destination);
}];

14.
... the shared resource was the GPU!

15.
Solution
NSDictionary *options = @{kCIContextUseSoftwareRenderer: @YES};
CIContext *context = [CIContext contextWithCGContext:cgContext
options:options];
... and ...
self.queue.maxConcurrentOperationCount = 1;

16.
Solution, part 2
... we removed the complicated filter

17.
How to not load things from the network
dispatch_async(backgroundQueue, ^{
NSData *data = [NSData dataWithContentsOfURL:url];
NSArray *graphItems = [NSJSONSerialization
JSONObjectWithData:data options:0 error:&error];
UIImage* image = [self drawGraph:graphItems];
dispatch_async(dispatch_get_main_queue(), ^{
self.imageView.image = image;
});
});

18.
Grand Central Dispatch

19.
Threading is hard
GCD moves it to the system-
level

20.
GCD Thread Pool
Main
Thread
High
Priority
Queue
Serial
Queue
Parallel
Queue
Serial
Queue
Main
Queue
Serial
Queue
Concurrent
Queue
Serial
Queue
Default
Priority
Queue
Low
Priority
Queue
Background
Priority
Queue
Custom Queues
GCD Queues
Threads

21.
— Simpler
— Faster
— Thread-pool management
— Memory-efficient
— Async means: no deadlock!

22.
How to load things from the network
Use NSURLSession

23.
Operation Queues

24.
dispatch_async(backgroundQueue, ^{
NSArray *graphData = [self generateDataPointsForGraph];
UIImage* image = [self drawGraph:graphData];
dispatch_async(dispatch_get_main_queue(), ^{
self.imageView.image = image;
});
});

25.
Step 1: Use NSOperation
NSOperationQueue* drawingQueue = [[NSOperationQueue alloc] init];
[drawingQueue addOperationWithBlock:^{
NSArray *graphData = [self generateDataPointsForGraph];
UIImage* image = [self drawGraph:graphData];
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
self.imageView.image = image;
}];
}];
How to cancel this?

26.
Step 2: Use NSBlockOperation
NSBlockOperation* drawingOperation =
[NSBlockOperation blockOperationWithBlock:^{
NSArray *graphData = [self generateDataPointsForGraph];
UIImage* image = [self drawGraph:graphData];
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
self.imageView.image = image;
}];
}];
[drawingQueue addOperation:drawingOperation];

27.
Step 3: Pull out the completion handler
NSOperation* drawingOperation = [NSBlockOperation blockOperationWithBlock:^{
NSArray *graphData = [self generateDataPointsForGraph];
self.image = [self drawGraph:graphData];
}];
drawingOperation.completionBlock = ^{
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
self.imageView.image = self.image;
}];
};

28.
Step 4: Custom NSOperation subclass
NSData *data = [self generateDataPointsForGraph];
NSOperation* drawingOperation =
[DrawingOperation drawingOperationWithData:data];
drawingOperation.completionBlock = ^{
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
self.imageView.image = image;
}];
};

29.
How to efficiently import
data into Core Data

30.
Efficiently Importing Data
— Implement Find-or-Create Efficiently
— Reduce Peak Memory Footprint
https://developer.apple.com/library/ios/
documentation/Cocoa/Conceptual/CoreData/
Articles/cdImporting.html

31.
Importing employees
for (WebserviceEmployee *webserviceEmployee) {
NSNumber *identifier = webserviceEmployee.identifier;
Employee *employee = [self findEmployeeWithIdentifier:identifier];
if (employee == nil) {
employee = [self createEmployeWithIdentifier:identifier];
}
// Update data
}

32.
Importing more efficiently
NSMutableArray *identifiers = [NSMutableArray array];
for (WebserviceEmployee *webserviceEmployee) {
NSNumber *identifier = webserviceEmployee.identifier;
[identifiers addObject:identifier];
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
fetchRequest.predicate = [NSPredicate
predicateWithFormat:@"(employeeID IN %@)", identifiers];

33.
Import in batches
Use a separate context

34.
Normal Core Data Stack

35.
NSManagedObjectContextNSManagedObjectContext NSManagedObjectContext
SQLite
NSPersistentStore
NSPersistentStoreCoordinator
Double MOC Stack

36.
Small imports
NSManagedObjectContext* context =
[[NSManagedObjectContext alloc]
initWithConcurrencyType:NSPrivateQueueConcurrencyType];
context.persistentStoreCoordinator = self.ptStoreCoordinator;
[self.context performBlock:^{
[self import];
}];

37.
[[NSNotificationCenter defaultCenter]
addObserverForName:NSManagedObjectContextDidSaveNotification
object:nil
queue:nil
usingBlock:^(NSNotification* note)
{
NSManagedObjectContext *moc = self.mainMOC;
if (note.object != moc) {
[moc performBlock:^(){
[moc mergeChangesFromContextDidSaveNotification:note];
}];
};
}];

38.
NSManagedObjectContextNSManagedObjectContext NSManagedObjectContext
!
SQLite
NSPersistentStore
NSPersistentStoreCoordinator
!
!
!
Double MOC Stack

39.
You might want to consider using a different
concurrency style, and this time you have two
persistent store coordinators, two almost
completely separate Core Data stacks.
Source: http://asciiwwdc.com/2013/sessions/
211

40.
SQLite
!
NSPersistentStore
NSPersistentStoreCoordinator
NSManagedObjectContext
NSPersistentStore
NSPersistentStoreCoordinator
NSManagedObjectContext
!
!
!
NSPersistentStore
NSPersistentStoreCoordinator
NSManagedObjectContext
NSPersistentStore
NSPersistentStoreCoordinator
NSManagedObjectContext
!
!
!

41.
SQLite
!
NSPersistentStore
NSPersistentStoreCoordinator
NSManagedObjectContext
NSPersistentStore
NSPersistentStoreCoordinator
NSManagedObjectContext
!
!
!
NSPersistentStore
NSPersistentStoreCoordinator
NSManagedObjectContext
NSPersistentStore
NSPersistentStoreCoordinator
NSManagedObjectContext
!
!
!
NSManagedObjectContextNSManagedObjectContext NSManagedObjectContext
!
SQLite
NSPersistentStore
NSPersistentStoreCoordinator
!
!
!

42.
Importing Recap
Didn't turn out to be a problem: we shipped
the sqlite file for the initial import.

43.
How to make objects play well
when concurrent

44.
@interface Account : NSObject
@property (nonatomic) double balance;
- (void)transfer:(double)euros to:(Account*)other;
@end

45.
- (void)transfer:(double)euros to:(Account*)other
{
self.balance = self.balance - euros;
other.balance = other.balance + euros;
}
What happens if two methods call this method
at the same time? From different threads?

46.
The same code, how the compiler sees it
- (void)transfer:(double)euros to:(Account*)other
{
double currentBalance = self.balance;
self.balance = currentBalance - euros;
double otherBalance = other.balance;
other.balance = otherBalance + euros;
}

47.
a b [a.transfer:20 to:b] [a.transfer:30 to:b]
100 0
currentBalance = 100
currentBalance = 100
100 0
a.balance = 100 - 20
80 0
b.balance = b.balance + 20
80 20
a.balance = 100 - 30
70 20

48.
a b [a.transfer:20 to:b] [a.transfer:30 to:b]
100 0
currentBalance = 100
currentBalance = 100
100 0
a.balance = 100 - 20
80 0
b.balance = b.balance + 20
80 20
a.balance = 100 - 30
70 20

49.
- (void)transfer:(double)euros to:(Account*)other
{
@synchronized(self) {
self.balance = self.balance - euros;
other.balance = other.balance + euros;
}
}

50.
- (void)transfer:(double)euros to:(Account*)other
{
@synchronized(self) {
@synchronized(other) {
self.balance = self.balance - euros;
other.balance = other.balance + euros;
}
}
}
Problem: deadlock.

51.
- (void)transfer:(double)euros to:(Account*)other
{
@synchronized(self.class) {
self.balance = self.balance - euros;
other.balance = other.balance + euros;
}
}

52.
Solution: move concurrency
to a different level

53.
Do it the GCD way
Account* account = [Account new];
Account* other = [Account new];
dispatch_queue_t accountOperations =
dispatch_queue_create("accounting", DISPATCH_QUEUE_SERIAL);
dispatch_async(accountOperations, ^{
[account transfer:200 to:other];
});
dispatch_async will never block.

54.
There are two ways of constructing a software
design: One way is to make it so simple that
there are obviously no deficiencies, and the
other way is to make it so complicated that
there are no obvious deficiencies. The first
method is far more difficult.
— Tony Hoare

55.
Thanks
— @chriseidhof
— http://www.objc.io
— http://www.uikonf.com
— http://www.decksetapp.com

56.
Resources
— Concurrency Programming Guide
— Threading Programming Guide
— NSOperationQueue class reference
— http://www.objc.io/issue-2/
— http://www.opensource.apple.com/source/
objc4/objc4-551.1/runtime/objc-sync.mm
— http://googlemac.blogspot.de/2006/10/
synchronized-swimming.html
— WWDC12 #211: Concurrent User Interfaces on
iOS

57.
Icons are from the Noun Project:
— Coffee Maker by Maureen Placente
— Coffee by Julia Soderberg
— Railroad Crossing by Edward Boatman
— Database by Stefan Parnarov
— Drawing by Daniel Shannon
— Hammer by Alex AS
— Lock by P.J. Onori
— Photoshop by Joe Harrison
— Register by Wilson Joseph