NSOperation vs Grand Central Dispatch

Question

I m learning about concurrent programming for iOS  So far I ve read about NSOperation NSOperationQueue and GCD  What are the reasons for using NSOperationQueue over GCD and vice versa   Sounds like both GCD and NSOperationQueue abstract away the explicit creation of NSThreads from the user  However the relationship between the two approaches isn t clear to me so any feedback to appreciated

User · Answer

GCD is indeed lower-level than NSOperationQueue, its major advantage is that its implementation is very light-weight and focused on lock-free algorithms and performance.

NSOperationQueue does provide facilities that are not available in GCD, but they come at non-trivial cost, the implementation of NSOperationQueue is complex and heavy-weight, involves a lot of locking, and uses GCD internally only in a very minimal fashion.

If you need the facilities provided by NSOperationQueue by all means use it, but if GCD is sufficient for your needs, I would recommend using it directly for better performance, significantly lower CPU and power cost and more flexibility.

User · Answer

Well  NSOperations are simply an API built on top of Grand Central Dispatch  So when you   re using NSOperations  you   re really still using Grand Central Dispatch  It   s just that NSOperations give you some fancy features that you might like  You can make some operations dependent on other operations  reorder queues after you sumbit items  and other things like that  In fact  ImageGrabber is already using NSOperations and operation queues  ASIHTTPRequest uses them under the hood  and you can configure the operation queue it uses for different behavior if you   d like  So which should you use  Whichever makes sense for your app  For this app it   s pretty simple so we just used Grand Central Dispatch directly  no need for the fancy features of NSOperation  But if you need them for your app  feel free to use it

User · Answer

Both NSQueueOperations and GCD allow executing heavy computation task in the background on separate threads by freeing the UI Application Main Tread   Well  based previous post we see NSOperations has addDependency so that you can queue your operation one after another sequentially    But I also read about GCD serial Queues you can create run your operations in the queue using dispatch queue create  This will allow running a set of operations one after another in a sequential manner   NSQueueOperation Advantages over GCD    It allows to add dependency and allows you to remove dependency so for one transaction you can run sequential using dependency and for other transaction run concurrently while GCD  doesn t allow to run this way  It is easy to cancel an operation if it is in the queue it can be stopped if it is running  You can define the maximum number of concurrent operations  You can suspend operation which they are in Queue You can find how many pending operations are there in queue

User · Answer

In line with my answer to a related question  I m going to disagree with BJ and suggest you first look at GCD over NSOperation   NSOperationQueue  unless the latter provides something you need that GCD doesn t   Before GCD  I used a lot of NSOperations   NSOperationQueues within my applications for managing concurrency  However  since I started using GCD on a regular basis  I ve almost entirely replaced NSOperations and NSOperationQueues with blocks and dispatch queues  This has come from how I ve used both technologies in practice  and from the profiling I ve performed on them   First  there is a nontrivial amount of overhead when using NSOperations and NSOperationQueues  These are Cocoa objects  and they need to be allocated and deallocated  In an iOS application that I wrote which renders a 3-D scene at 60 FPS  I was using NSOperations to encapsulate each rendered frame  When I profiled this  the creation and teardown of these NSOperations was accounting for a significant portion of the CPU cycles in the running application  and was slowing things down  I replaced these with simple blocks and a GCD serial queue  and that overhead disappeared  leading to noticeably better rendering performance  This wasn t the only place where I noticed overhead from using NSOperations  and I ve seen this on both Mac and iOS   Second  there s an elegance to block-based dispatch code that is hard to match when using NSOperations  It s so incredibly convenient to wrap a few lines of code in a block and dispatch it to be performed on a serial or concurrent queue  where creating a custom NSOperation or NSInvocationOperation to do this requires a lot more supporting code  I know that you can use an NSBlockOperation  but you might as well be dispatching something to GCD then  Wrapping this code in blocks inline with related processing in your application leads in my opinion to better code organization than having separate methods or custom NSOperations which encapsulate these tasks   NSOperations and NSOperationQueues still have very good uses  GCD has no real concept of dependencies  where NSOperationQueues can set up pretty complex dependency graphs  I use NSOperationQueues for this in a handful of cases   Overall  while I usually advocate for using the highest level of abstraction that accomplishes the task  this is one case where I argue for the lower-level API of GCD  Among the iOS and Mac developers I ve talked with about this  the vast majority choose to use GCD over NSOperations unless they are targeting OS versions without support for it  those before iOS 4 0 and Snow Leopard

User · Answer

GCD is a low-level C-based API that enables very simple use of a task-based concurrency model  NSOperation and NSOperationQueue are Objective-C classes that do a similar thing  NSOperation was introduced first  but as of 10 5 and iOS 2  NSOperationQueue and friends are internally implemented using GCD   In general  you should use the highest level of abstraction that suits your needs  This means that you should usually use NSOperationQueue instead of GCD  unless you need to do something that NSOperationQueue doesn t support   Note that NSOperationQueue isn t a  dumbed-down  version of GCD  in fact  there are many things that you can do very simply with NSOperationQueue that take a lot of work with pure GCD   Examples  bandwidth-constrained queues that only run N operations at a time   establishing dependencies between operations  Both very simple with NSOperation  very difficult with GCD   Apple s done the hard work of leveraging GCD to create a very nice object-friendly API with NSOperation  Take advantage of their work unless you have a reason not to   Caveat  On the other hand  if you really just need to send off a block  and don t need any of the additional functionality that NSOperationQueue provides  there s nothing wrong with using GCD  Just be sure it s the right tool for the job

User · Answer

GCD is a low-level C-based API  NSOperation and NSOperationQueue are Objective-C classes  NSOperationQueue is objective C wrapper over GCD  If you are using NSOperation  then you are implicitly using Grand Central Dispatch   GCD advantage over NSOperation  i  implementation For GCD implementation is very light-weight NSOperationQueue is complex and heavy-weight  NSOperation advantages over GCD   i  Control On Operation you can Pause  Cancel  Resume an NSOperation  ii  Dependencies you can set up a dependency between two NSOperations operation will not started until all of its dependencies return true for finished   iii  State of Operation can monitor the state of an operation or operation queue  ready  executing or finished  iv  Max Number of Operation you can specify the maximum number of queued operations that can run simultaneously  When to Go for GCD or NSOperation when you want more control over queue  all above mentioned  use NSOperation  and for simple cases where you want less overhead   you just want to do some work  into the background  with very little additional work  use GCD  ref  https   cocoacasts com choosing-between-nsoperation-and-grand-central-dispatch  http   iosinfopot blogspot in 2015 08 nsthread-vs-gcd-vs-nsoperationqueue html http   nshipster com nsoperation

User · Answer

GCD is very easy to use - if you want to do something in the background  all you need to do is write the code and dispatch it on a background queue  Doing the same thing with NSOperation is a lot of additional work    The advantage of NSOperation is that  a  you have a real object that you can send messages to  and  b  that you can cancel an NSOperation  That s not trivial  You need to subclass NSOperation  you have to write your code correctly so that cancellation and correctly finishing a task both work correctly  So for simple things you use GCD  and for more complicated things you create a subclass of NSOperation   There are subclasses NSInvocationOperation and NSBlockOperation  but everything they do is easier done with GCD  so there is no good reason to use them

User · Answer

Another reason to prefer NSOperation over GCD is the cancelation mechanism of NSOperation  For example  an App like 500px that shows dozens of photos  use NSOperation we can cancel requests of invisible image cells when we scroll table view or collection view  this can greatly improve App performance and reduce memory footprint  GCD can t easily support this   Also with NSOperation  KVO can be possible   Here is an article from Eschaton which is worth reading

[ios] NSOperation vs Grand Central Dispatch

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