Karol Kozimor talking at Cracow.mobi 2011

1. simply mobile

2. About Me
Used to be a Linux kernel developer
Currently developing hardware accessories for iOS
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3. What this talk isn’t about
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4. What this talk isn’t about
Hardware accessories
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5. What this talk isn’t about
Hardware accessories
App Store overnight success
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6. What this talk isn’t about
Hardware accessories
App Store overnight success
POSIX threads
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7. 150% thread content
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8. Plan
1. Why we love and hate threads
2. Grand Central Dispatch
3. NSThread class
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9. Threading
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10. Threading
Not only for multi-core chips
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11. Threading
Not only for multi-core chips
Responsiveness is the king
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12. Threading
Not only for multi-core chips
Responsiveness is the king
“The snappy”
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13. Threading
Not only for multi-core chips
Responsiveness is the king
“The snappy”
Asynchronicity
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14. Half-measures
Asynchronous I/O
CoreData
Notifications and KVO
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15. General solution
Threads
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16. Downsides
Overhead
Especially on embedded systems
#1 issue: threads are difficult
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17. Really?
Thread management
IPC, messaging, synchronization
Locking
Reentrancy
???
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18. Solution
Grand Central Dispatch
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19. Grand Central Dispatch
Language features
Runtime libraries
System enhancements
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20. Together
Automate thread management
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22. Language features: blocks
Small, self-contained pieces of code
Anonymous, inline collection of code
Available: Snow Leopard, iOS 4.0
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23. Blocks features
Typed arguments
Return value
Access (and modify) variables
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24. Mainly useful as
Callbacks
Self-contained units of work
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25. Syntax
BOOL (^block)(void);
BOOL areKittensCute = YES;
block = ^{ return areKittensCute; };
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26. Example
- (void)blink:(UIColor *)color
{
self.view.backgroundColor = color;
[UIView animateWithDuration:1.0
animations:^{self.view.backgroundColor = [UIColor
darkGrayColor];}];
}
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28. Grand Central API
GCD provides FIFO queues for blocks
Three types:
Main
Concurrent
Serial
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29. Concurrent queues
Blocks dequeued in FIFO order
… but run concurrently if resources allow
Finish in any order
Created automatically
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30. Serial queues
Useful for serialization of tasks
Created manually
dispatch_queue_create(“com.yourname.queue”, NULL);
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31. Really?
Thread management
IPC, messaging, synchronization
Locking
Reentrancy
???
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32. When queues are not enough
Dispatch sources
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33. Dispatch source API
C-level API
Can monitor for events (signals, data) and dispatch
handlers to appropriate queues
Low-level, useful for OS X but maybe not for iOS
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34. Back to square one
NSThread class
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35. The real thing
Cost of threads:
Creation time
Memory footprint (512 kB)
Design overhead
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36. Conclusion — Don’t use
threads!
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37. … unless you have no other
choice
High level event monitoring
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38. Thread lifecycle
1. Initialize
2. Start
1. Create autorelease pool, setup sources
2. Do work
3. Cancel
4. Drain pool
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39. Thread init
newThread = [[NSThread alloc] initWithTarget:self
selector:@selector(newThreadEntryPoint:) object:nil];
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40. Entry point
- (void)newThreadEntryPoint:(id)param
{
…
}
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41. Thread start
[newThread start];
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42. Housekeeping
- (void)newThreadEntryPoint:(id)param
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc]
init];
…
}
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43. Get to work
Two options:
Execute a predefined workset
Monitor for events
Run loop
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44. Event sources
NSInputStream *catnipStream;
[catnipStream setDelegate:self];
[catnipStream scheduleInRunLoop:[NSRunLoop currentRunLoop]
forMode:NSDefaultRunLoopMode];
[catnipStream open];
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45. Running the loop
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode
beforeDate:[NSDate distantFuture]];
} while (!done);
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46. The whole package
- (void)runThread
{
newThread = [[NSThread alloc] initWithTarget:self selector:@selector(newThreadEntryPoint:)
object:nil];
[newThread start];
}
- (void)newThreadEntryPoint:(id)param
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
[catnipStream setDelegate:self];
[catnipStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
[catnipStream open];
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
} while (!done);
}
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47. Reuse & recycle
What if I need on-demand threads?
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48. Run loop maintenance
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode
beforeDate:[NSDate distantFuture]];
} while (!done);
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49. Run loop maintenance
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode
beforeDate:[NSDate distantFuture]];
} while ([[NSThread currentThread] isCancelled] == NO);
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50. Run loop maintenance
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode
beforeDate:[NSDate dateWithTimeIntervalSinceNow:5]];
} while ([[NSThread currentThread] isCancelled] == NO);
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51. Clean up!
[catnipStream close];
[catnipStream removeFromRunLoop:[NSRunLoop currentRunLoop]
forMode:NSDefaultRunLoopMode];
[catnipStream setDelegate:nil];
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52. A lot of code?
- (void)runThread
{
newThread = [[NSThread alloc] initWithTarget:self selector:@selector(newThreadEntryPoint:)
object:nil];
[newThread start];
}
- (void)newThreadEntryPoint:(id)param
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
[catnipStream setDelegate:self];
[catnipStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
[catnipStream open];
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate
dateWithTimeIntervalSinceNow:5]];
} while ([[NSThread currentThread] isCancelled] == NO);
[catnipStream close];
[catnipStream removeFromRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
[catnipStream setDelegate:nil];
[pool drain];
}
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53. More maintenance
Consider draining the autorelease pool from within
the loop if the thread will be running for a long time
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate
dateWithTimeIntervalSinceNow:5]];
[pool drain];
pool = [[NSAutoreleasePool alloc] init];
} while ([[NSThread currentThread] isCancelled] == NO);
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54. How to quit?
[newThread cancel];
Safe to use from any thread
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55. Conclusions
iOS applications are heavily threaded internally
Main thread, web rendering thread etc.
GCD is also implemented via threads
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56. Caveats
Key-value observers are always executed in the
current thread context
What happens when KVO from secondary threads is
used to handle or update UI outlets?
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58. Solution
performSelectorOnMainThread:(SEL)aSelector
withObject:(id)arg waitUntilDone:(BOOL)wait
(on secondary thread)
[self
performSelectorOnMainThread:@selector(updateField
s:) withObject:data waitUntilDone:YES];
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59. Why?
Responsiveness!
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60. Questions?
Karol Kozimor
karol.kozimor@iosyn.com
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61. More info
Concurrency Programming Guide
Operation Queues
Threading Programming Guide
WWDC 2011: Blocks and Grand Central Dispatch in
Practice
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