Libdispatch. Как сделать приложение отзывчивым

. iOS- . .

• Pthreads, POSIX. . C. .
• Background selectors. , . performSelector
  
  
  
  (, performSelectorOnMainThread:withObject:waitUntilDone:
  
  
  
  ).
• NSThread. . .
• Grand Central Dispatch. , — , . — libdispatch.
• NSOperation. GCD. , NSInvocationOperation



NSBlockOperation
  
  
  
  .

GCD.

Libdispatch — Apple, . GCD Mac OS X 10.6. libdispatch, GCD, Apache 10 2009. Unix, FreeBSD Linux. . libdispatch .

. , . : pthreads, background selectors, NSThread. — libdispatch . NSThread pthreads. .

GCD

« iOS»

, Obj-C ( iOS 10). — , , AVFoundation WebKit. , , Apple. , trustedTimestamp CLLocation.

, , PhysicsKit. , timeline — . , iOS ( , ). . , : lib + . , libobjc libxpc. , github .

, libdispatch. , . . , : debugDescription



, description



, hash



superclass



. Apple.

libdispatch. . (, libdispatch), . , . cmake xcodeconfig, .

:

• Swift ( dispatch ),
• Xcode, .

— , . , , , El Capitan, .

.

, libdispatch. , — queue_internal.h.

DISPATCH_STRUCT_HEADER



— . OS_OBJECT_HEADER



( OS_OBJECT_HEADER



— vtable ), , . (target queue) — .

DISPATCH_QUEUE_HEADER



DISPATCH_QUEUE_CACHELINE_PADDING



. , , . DISPATCH_QUEUE_HEADER



, «» ( ), , .

continuation. DISPATCH_CONTINUATION_HEADER



. , , , , , .

, libdispatch libpqw POSIX Thread API.

libpqw

, GCD pthread — libpwq, Apple. . 2011 , 0.9 2014 .

<pthread.h>



, . , : , , . , pthread_workqueue_additem_np



, , .

, . . , ( — , ), . . — , , , .

, libdispatch libpwq, pthreads. , ( Mac OS X Snow Leopard 2009 ).

dispatch_async

- libdispatch.

DispatchQueue.main.async {
    // some asynchronous code...
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

. swift' . , CDispatch — GCD Swift .

public class DispatchQueue : DispatchObject {
    ...
}

public extension DispatchQueue {

    ...

    public class var main: DispatchQueue {
        return DispatchQueue(queue: _swift_dispatch_get_main_queue())
    }

    ...

    @available(OSX 10.10, iOS 8.0, *)
    public func async(execute workItem: DispatchWorkItem) {
        CDispatch.dispatch_async(self.__wrapped, workItem._block)
    }

    ...
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

, . GCD dispatch_async.

(pthread_create) (libpwq) :

• dispatch_async
  
  
  
  
• _dispatch_continuation_async
  
  
  
  
• _dispatch_continuation_async2
  
  
  
  
• _dispatch_async_f2
  
  
  
  
• _dispatch_continuation_push
  
  
  
  
• dx_push
  
  
  
  
• _dispatch_queue_push
  
  
  
  
• _dispatch_queue_push_inline
  
  
  
  
• dx_wakeup
  
  
  
  
• _dispatch_queue_class_wakeup
  
  
  
  
• _dispatch_queue_class_wakeup_with_override
  
  
  
  
• _dispatch_queue_class_wakeup_with_override_slow
  
  
  
  
• _dispatch_root_queue_push_override_stealer
  
  
  
  
• _dispatch_root_queue_push_inline
  
  
  
  
• _dispatch_global_queue_poke
  
  
  
  
• _dispatch_global_queue_poke_slow
  
  
  
  
• pthread_create



pthread_workqueue_additem_np
  
  
  
  

. dispatch_async



:

void
dispatch_async(dispatch_queue_t dq, dispatch_block_t work)
{
    dispatch_continuation_t dc = _dispatch_continuation_alloc();
    uintptr_t dc_flags = DISPATCH_OBJ_CONSUME_BIT;

    _dispatch_continuation_init(dc, dq, work, 0, 0, dc_flags);
    _dispatch_continuation_async(dq, dc);
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

? -, — continuation. , _t



_s



. , - (, typedef struct dispatch_queue_s *dispatch_queue_t;



). -, , . , , 0.

, (_dispatch_continuation_async



_dispatch_continuation_async2



). (noinline



) (inline



) , , . — continuation . _dispatch_continuation_push



. , .

, _dispatch_async_f2



, QoS continuation — . _dispatch_continuation_push



, dx_push



. , _dispatch_queue_push_inline



. .

, — , . . , . .

_dispatch_queue_push_inline



. (, , libdispatch) :

• atomic_load_explicit
  
  
  
  . — <stdatomic.h>
  
  
  
  ;
• __builtin_expect()



__builtin_unreachable()
  
  
  
  , __builtin
  
  
  
  - — branch prediction.

— . _dispatch_async_f_redirect



continuation . — .

dx_wakeup



— _dispatch_queue_class_wakeup



. , . , , , dx_push



.

_dispatch_queue_class_wakeup_with_override



, _dispatch_queue_class_wakeup_with_override_slow



. slow



— .

, , . . pthread_create



pthread_workqueue_additem_np



. , libpwq, , .

, , — .

Swift

, swift-, libdispatch. , Swift. libdispatch c swift' . , , — API GCD.

, libdispatch Swift — Wrapper.swift



. .

, . libdispatch, dispatch_group_t



dispatch_queue_t



, - __wrapped



. - libdispatch __wrapped



.

:

public class DispatchQueue : DispatchObject {

    //     libdispatch
    internal let __wrapped:dispatch_queue_t

    ...

    final internal override func wrapped() -> dispatch_object_t {
        return unsafeBitCast(__wrapped, to: dispatch_object_t.self)
    }

    ...

    public func sync(execute workItem: ()->()) {
        //     
        dispatch_sync(self.__wrapped, workItem)
    }

    ...
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

, , . , , . , Private.swift



libdispatch. . swift



' (, libdispatch).

@available(*, unavailable, renamed:"DispatchQueue.async(self:group:qos:flags:execute:)")
public func dispatch_group_async(_ group: DispatchGroup, _ queue: DispatchQueue, _ block: @escaping () -> Void)
{
    fatalError()
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

— ,

, libdispatch. . libdispatch POSIX Thread API — API .

libdispatch (libpwq), .

— ? . - , .

. - pthread iOS, .

• Posix
• pthreads
• Swift GCD & Queue
• GCD Internals