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Blocks & GCD

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rsebbe

Introduction talk about Blocks and Grand Central Dispatch, concurrent programming techniques for Mac and iOS

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^{ } blocks and Raphael Sebbe creaceed.com cocoaheads.be @rsebbe
Ways to Get More • We can’t get more GHz out of silicon • Parallelism is the solution • Nowadays, multi-core systems is the rule • On it’s way to mobile devices (iPad2) • Specialized hardware, like GPU, too • Already there on mobile
GPU Core SIMD Multi- Cloud Farms/ Core Image OpenCL GLSL Data-driven Accelerate Threads Grand Central Dispatch Load Cron Balancing Task-driven OpenMP
Overview • Blocks & GCD are 2 different beasts • New language feature • Efficiently define tasks, and execute them • They complement each other nicely
Blocks • Concept of closure from functional programming Lisp, Caml, Haskell, • Blocks Javascript, etc. • are an extension of C language • store a piece of executable code • capture the execution context • are objects (Obj-C or C-based API)
Definition of a Block // Simple one void (^myblock)() = ^{ ! printf(“Hellon”); }; myblock(); // With one arg void (^myblock)(int) = ^(int arg){ ! printf(“Hello arg=%dn”, arg); }; myblock(1); // With a return value int (^myblock)(int) = ^(int arg){ ! printf(“Hello arg=%dn”, arg); ! return arg + 1; ~ anonymous }; function pointer int r = myblock(1);
Typedef’ing a Block // Typedef of a block type typedef void (^MyBlockType)(NSURL *assetURL, NSError *error); MyBlockType myBlock = ^(NSURL *assetURL, NSError *error) { ! // Some code }; // Syntax of untyped block as Obj-C method argument - (void)addPlayersToMatch:(GKMatch *)match matchRequest:(GKMatchRequest *)matchRequest completionHandler:(void (^)(NSError *))handler
Execution Context (1/3) • Definition of a block captures the execution context • Variable of standard C types are copied int a = 3; // Context is captured at it’s current state void (^myblock)() = ^{ ! printf(“Hello a=%dn”, a); }; a = 5; myblock(); // outputs “Hello a=3”
Execution Context (2/3) • Captured objects are retained id dict = [[NSDictionary alloc] init]; // Referenced objects are retained void (^myblock)() = ^{ ! NSLog(“Hello dict=%@n”, dict); }; [dict release]; myblock(); // outputs “Hello dict=....” Remark: For referenced ivar, it’s self that gets retained. Possible retain cycles.
Execution Context (3/3) • Shared (R/W) variables with __block __block int a = 3; // Context is captured at it’s current state void (^myblock)() = ^{ ! a += 1; ! printf(“Hello a=%dn”, a); }; a = 5; Remark: myblock(); // outputs “Hello a=6” __block objects (id, etc.) are not retained
Block Storage • When defined, blocks are stack objects, only valid in the definition scope • Must be copied to heap if used outside of creation context if (x) { if (x) { ! block = ^{ printf("truen");}; ! block = ^{ printf("truen");}; } else { ! block = [block copy]; ! block = ^{ printf("falsen");}; } else { } ! block = ^{ printf("falsen");}; block(); // unsafe!!! ! block = [block copy];Remark: } block(); // safe!!! If your method [block release]; takes a block as arg., always copy it.
Blocks Advantages • Idea: invoke this “block of code” later. • Callback API are naturally expressed • Good replacement to your many didEnd: methods • Task queueing / parallelism much easier • See GCD
Grand Central Dispatch • Need for parallelism that scales • Easy to program for wide adoption • Parallelism examples • iTunes - Coverflow, downloads... • iPhoto - import, uploads, preview • Twitter Profile Pictures, etc.
Grand Central Dispatch • Multi-core needs threads • Say you have a 4-core machine, you need 4 threads for maximum throughput. • Well... No. • This is a pre-emptive world
Threads (1/2) • Optimal number of threads for your app • Number of (virtual) cores • Load from other apps • Thread state (low speed I/O) • This continuously changes • Consequences: unused or overused cores
Threads (2/2) • 3 problems with threads: • Too low-level, you shouldn’t waste your time at this (when to create/destroy them) • You generally don’t have easy access to global system info • Thread programming is hard (sync primitives, etc.)
Blocks & GCD
Blocks & GCD
GCD Concept • Tasks in Queues Out In In Out Core Q’s are lightweight, have Core as many as you want
Why Q’s • Goal: Best use of cores • Let the system create/destroy threads for you • Write less code: wide adoption • Better overall user experience (responsiveness) • Technical: Less contention • Fewer locks • Scales much better with • number of tasks • number of cores
Queue Types • Serial queues: one task at a time Serial Q’s are • User queues themselves sync primitives • Main queue • Concurrent queues: multiple tasks • 3 priority levels Only the global one on 10.6 / iOS4
Queues dispatch_queue_t queue; // Main (serial) queue queue = dispatch_get_main_queue(); // User (serial) queue queue = dispatch_queue_create(“com.mycompany.qname”, ! NULL); // Global (concurrent) queue, with priority queue = dispatch_get_global_queue ! (DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
Adding Tasks // Synchronous, blocks until code is executed dispatch_sync(queue, ^{ ! // task code }); dispatch_apply(count, queue, ^(size_t i){ ! // process ith }); // Asynchronous, returns immediately to caller dispatch_async(queue, ^{ ! // task code API exists for }); function pointers dispatch_after(when, queue, ^{ too. ! // task code });
NSOperationQueue Core Core
Adding Operations // Synchronous, blocks until code is executed NSOperationQueue *queue = [[NSOQ alloc] init]; ! NSBlockOperation *op1 = [NSBlockOperation ! blockOperationWithBlock:^{ ! ! // some code! ! ! }]; NSBlockOperation *op2 = [NSBlockOperation ! blockOperationWithBlock:^{ Predates ! ! // some code! ! }]; GCD, has some ! odd APIs. Now built [op2 addDependency:op1]; ! on top of it. [queue addOperation:op1]; [queue addOperation:op2]; [queue addOperationWithBlock:^{ ! // some code! ! You can set }]; the max concurrent ops.
APIs using Blocks (1/2) // Collections (NSDictionary / NSArray / NSSet) - (void)enumerateKeysAndObjectsWithOptions: (NSEnumerationOptions)opts usingBlock:(void (^)(id key, id obj, BOOL *stop))block - (NSSet *)keysOfEntriesPassingTest:(BOOL (^)(id key, id obj, BOOL *stop))predicate Patterns for collections: - (NSArray *)keysSortedByValueUsingComparator: (NSComparator)cmptr enumeration, mutation, tests, comparison // NSRegularExpression - (void)enumerateMatchesInString:(NSString *)string options:(NSMatchingOptions)options range:(NSRange) range usingBlock:(void (^)(NSTextCheckingResult *result, NSMatchingFlags flags, BOOL *stop))block
APIs using Blocks (2/2) // ALAssetsLibrary - (void)enumerateGroupsWithTypes:(ALAssetsGroupType) types usingBlock:(ALAssetsLib...ationResultsBlock) enumerationBlock failureBlock: (ALAssetsLib...ssFailureBlock)failureBlock // Game Kit Patterns for accessors: - (void)loadScoresWithCompletionHandler:(void (^) callback / handler (NSArray *scores, NSError *error))completionHandler // AVFoundation, AppKit, many other, etc.
Xcode Demo’s • Extending UIAlertView using blocks • Delving inside Elasty video stabilization method
Blocks & GCD
Threads • Threads are not dead yet, they are just under your feet. (see crashlogs) • Some uses are well suited at threads • Real time needs • OpenGL • Core Image • ...
What’s Next? • Check WWDC videos [210, 308, 322] • Dispatch group (dispatch_group_*) • Source (dispatch_source_*) • Semaphores (dispatch_semaphore_*) • ARC interactions • File / Memory I/O • Target queues
Thank you! (Q&A?)

More Related Content

Blocks & GCD

  • 1. ^{ } blocks and Raphael Sebbe creaceed.com cocoaheads.be @rsebbe
  • 2. Ways to Get More • We can’t get more GHz out of silicon • Parallelism is the solution • Nowadays, multi-core systems is the rule • On it’s way to mobile devices (iPad2) • Specialized hardware, like GPU, too • Already there on mobile
  • 3. GPU Core SIMD Multi- Cloud Farms/ Core Image OpenCL GLSL Data-driven Accelerate Threads Grand Central Dispatch Load Cron Balancing Task-driven OpenMP
  • 4. Overview • Blocks & GCD are 2 different beasts • New language feature • Efficiently define tasks, and execute them • They complement each other nicely
  • 5. Blocks • Concept of closure from functional programming Lisp, Caml, Haskell, • Blocks Javascript, etc. • are an extension of C language • store a piece of executable code • capture the execution context • are objects (Obj-C or C-based API)
  • 6. Definition of a Block // Simple one void (^myblock)() = ^{ ! printf(“Hellon”); }; myblock(); // With one arg void (^myblock)(int) = ^(int arg){ ! printf(“Hello arg=%dn”, arg); }; myblock(1); // With a return value int (^myblock)(int) = ^(int arg){ ! printf(“Hello arg=%dn”, arg); ! return arg + 1; ~ anonymous }; function pointer int r = myblock(1);
  • 7. Typedef’ing a Block // Typedef of a block type typedef void (^MyBlockType)(NSURL *assetURL, NSError *error); MyBlockType myBlock = ^(NSURL *assetURL, NSError *error) { ! // Some code }; // Syntax of untyped block as Obj-C method argument - (void)addPlayersToMatch:(GKMatch *)match matchRequest:(GKMatchRequest *)matchRequest completionHandler:(void (^)(NSError *))handler
  • 8. Execution Context (1/3) • Definition of a block captures the execution context • Variable of standard C types are copied int a = 3; // Context is captured at it’s current state void (^myblock)() = ^{ ! printf(“Hello a=%dn”, a); }; a = 5; myblock(); // outputs “Hello a=3”
  • 9. Execution Context (2/3) • Captured objects are retained id dict = [[NSDictionary alloc] init]; // Referenced objects are retained void (^myblock)() = ^{ ! NSLog(“Hello dict=%@n”, dict); }; [dict release]; myblock(); // outputs “Hello dict=....” Remark: For referenced ivar, it’s self that gets retained. Possible retain cycles.
  • 10. Execution Context (3/3) • Shared (R/W) variables with __block __block int a = 3; // Context is captured at it’s current state void (^myblock)() = ^{ ! a += 1; ! printf(“Hello a=%dn”, a); }; a = 5; Remark: myblock(); // outputs “Hello a=6” __block objects (id, etc.) are not retained
  • 11. Block Storage • When defined, blocks are stack objects, only valid in the definition scope • Must be copied to heap if used outside of creation context if (x) { if (x) { ! block = ^{ printf("truen");}; ! block = ^{ printf("truen");}; } else { ! block = [block copy]; ! block = ^{ printf("falsen");}; } else { } ! block = ^{ printf("falsen");}; block(); // unsafe!!! ! block = [block copy];Remark: } block(); // safe!!! If your method [block release]; takes a block as arg., always copy it.
  • 12. Blocks Advantages • Idea: invoke this “block of code” later. • Callback API are naturally expressed • Good replacement to your many didEnd: methods • Task queueing / parallelism much easier • See GCD
  • 13. Grand Central Dispatch • Need for parallelism that scales • Easy to program for wide adoption • Parallelism examples • iTunes - Coverflow, downloads... • iPhoto - import, uploads, preview • Twitter Profile Pictures, etc.
  • 14. Grand Central Dispatch • Multi-core needs threads • Say you have a 4-core machine, you need 4 threads for maximum throughput. • Well... No. • This is a pre-emptive world
  • 15. Threads (1/2) • Optimal number of threads for your app • Number of (virtual) cores • Load from other apps • Thread state (low speed I/O) • This continuously changes • Consequences: unused or overused cores
  • 16. Threads (2/2) • 3 problems with threads: • Too low-level, you shouldn’t waste your time at this (when to create/destroy them) • You generally don’t have easy access to global system info • Thread programming is hard (sync primitives, etc.)
  • 19. GCD Concept • Tasks in Queues Out In In Out Core Q’s are lightweight, have Core as many as you want
  • 20. Why Q’s • Goal: Best use of cores • Let the system create/destroy threads for you • Write less code: wide adoption • Better overall user experience (responsiveness) • Technical: Less contention • Fewer locks • Scales much better with • number of tasks • number of cores
  • 21. Queue Types • Serial queues: one task at a time Serial Q’s are • User queues themselves sync primitives • Main queue • Concurrent queues: multiple tasks • 3 priority levels Only the global one on 10.6 / iOS4
  • 22. Queues dispatch_queue_t queue; // Main (serial) queue queue = dispatch_get_main_queue(); // User (serial) queue queue = dispatch_queue_create(“com.mycompany.qname”, ! NULL); // Global (concurrent) queue, with priority queue = dispatch_get_global_queue ! (DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
  • 23. Adding Tasks // Synchronous, blocks until code is executed dispatch_sync(queue, ^{ ! // task code }); dispatch_apply(count, queue, ^(size_t i){ ! // process ith }); // Asynchronous, returns immediately to caller dispatch_async(queue, ^{ ! // task code API exists for }); function pointers dispatch_after(when, queue, ^{ too. ! // task code });
  • 25. Adding Operations // Synchronous, blocks until code is executed NSOperationQueue *queue = [[NSOQ alloc] init]; ! NSBlockOperation *op1 = [NSBlockOperation ! blockOperationWithBlock:^{ ! ! // some code! ! ! }]; NSBlockOperation *op2 = [NSBlockOperation ! blockOperationWithBlock:^{ Predates ! ! // some code! ! }]; GCD, has some ! odd APIs. Now built [op2 addDependency:op1]; ! on top of it. [queue addOperation:op1]; [queue addOperation:op2]; [queue addOperationWithBlock:^{ ! // some code! ! You can set }]; the max concurrent ops.
  • 26. APIs using Blocks (1/2) // Collections (NSDictionary / NSArray / NSSet) - (void)enumerateKeysAndObjectsWithOptions: (NSEnumerationOptions)opts usingBlock:(void (^)(id key, id obj, BOOL *stop))block - (NSSet *)keysOfEntriesPassingTest:(BOOL (^)(id key, id obj, BOOL *stop))predicate Patterns for collections: - (NSArray *)keysSortedByValueUsingComparator: (NSComparator)cmptr enumeration, mutation, tests, comparison // NSRegularExpression - (void)enumerateMatchesInString:(NSString *)string options:(NSMatchingOptions)options range:(NSRange) range usingBlock:(void (^)(NSTextCheckingResult *result, NSMatchingFlags flags, BOOL *stop))block
  • 27. APIs using Blocks (2/2) // ALAssetsLibrary - (void)enumerateGroupsWithTypes:(ALAssetsGroupType) types usingBlock:(ALAssetsLib...ationResultsBlock) enumerationBlock failureBlock: (ALAssetsLib...ssFailureBlock)failureBlock // Game Kit Patterns for accessors: - (void)loadScoresWithCompletionHandler:(void (^) callback / handler (NSArray *scores, NSError *error))completionHandler // AVFoundation, AppKit, many other, etc.
  • 28. Xcode Demo’s • Extending UIAlertView using blocks • Delving inside Elasty video stabilization method
  • 30. Threads • Threads are not dead yet, they are just under your feet. (see crashlogs) • Some uses are well suited at threads • Real time needs • OpenGL • Core Image • ...
  • 31. What’s Next? • Check WWDC videos [210, 308, 322] • Dispatch group (dispatch_group_*) • Source (dispatch_source_*) • Semaphores (dispatch_semaphore_*) • ARC interactions • File / Memory I/O • Target queues