In a job interview, I was asked why I should use blocks and GCD instead of NSURLConnection in order to download files asyncronously. After some research I haven't found a good reason to do that. I have multiple apps where I use just NSURLConnection just fine for multiple simultaneous downloads. Is their question attempting to ascertain whether I'm conforming to whatever is trendy (GCD, blocks) or is there any actual, substantial advantage to doing async fetches in this way?
Thanks.
In iOS 7, you generally should not use block-based methods to download files asynchronously. In order to support background transfers, you must use NSURLSession with delegate methods, and cannot use the block-based methods. Beyond that, I'm not sure what is meant here by "instead of NSURLConnection" in any case.
If they meant sendAsynchronousRequest:queue:completionHandler: (which is NSURLConnection), it's convenient, but much less flexible and powerful than the delegate-based NSURLConnection, so the only answer I would have is "because sometimes it's more convenient, and keeps the code closer together, when you don't need much flexibility."
Unless what they actually mean is the part of GCD that really does this: Dispatch I/O. There are reasons to use that directly (particularly if you're using non-HTTP protocols, or if you're managing an HTTP server rather than a client), but they're rare, and not usually for "downloading files asynchronously." The higher level APIs are preferred in most cases.
If you were doing many, many connections, transferring a ton of data over a very fast network connection, I can perhaps see how NSURLConnection's use the runloop for I/O handling and callbacks might become problematic, if you're scheduling these NSURLConnections on the main runloop. That said, you could just as easily spool up a lower priority background thread with its own runloop to keep those operations off the main thread.
If you didn't need all the extra machinery of NSURLConnection (caching, authentication, etc), dispatch_io* is almost certainly a lower overhead mechanism for handling raw network I/O, but you would really be giving up quite a bit of functionality for what I expect, practically speaking, to be a very marginal performance improvement.
I'm not sure. Mainly because NSURLConnection's sendAsynchronousRequest method has a completion handler built into it which uses a block.
Maybe a trick question? To me, it seems like the interviewer just wanted to see if you could conclude that they both can serve the same function.
Blocks and GCD aren't specifically for downloads, but they can make downloading easier. You would have to use them in conjunction with something that did the downloading (like NSURLConnection).
The advantage of using GCD with an NSURLConnection is that you can package it together nicely and don't have to rely on spread-out delegate methods. You can limit the number of connections easily too as well as pausing and stopping connections.
My "go to" set up for complex networking is to use an NSOperationQueue and NSOperation subclasses to do the work.
Each operation then uses a NSURLConnection and its delegate methods to download the data and then process it.
In a way this is already using GCD through the NSOperationQueue but I can't see a reason to use any other method combining blocks and GCD etc...
Did they give you a "correct" answer?
Related
I have a streaming iOS app that captures video to Wowza servers.
It's a beast, and it's really finicky.
I'm grabbing configuration settings from a php script that shoots out JSON.
Now that I've implemented that, I've run into some strange threading issues. My app connects to the host, says its streaming, but never actually sends packets.
Getting rid of the remote configuration NSURLConnection (which I've made sure is properly formatted) delegate fixes the problem. So I'm thinking either some data is getting misconstrued across threads or something like that.
What will help me is knowing:
Are NSURLConnection delegate methods called on the main thread?
Will nonatomic data be vulnerable in a delegate method?
When dealing with a complex threaded app, what are the best practices for grabbing data from the web?
Have you looked at AFNetworking?
http://www.raywenderlich.com/30445/afnetworking-crash-course
https://github.com/AFNetworking/AFNetworking
It's quite robust and helps immensely with the threading, and there are several good tutorials.
Are NSURLConnection delegate methods called on the main thread?
Yes, on request completion it gives a call back on the main thread if you started it on the main thread.
Will nonatomic data be vulnerable in a delegate method?
Generally collection values (like array) are vulnerable with multiple threads; the rest shouldn't create anything other than a race problem.
When dealing with a complex threaded app, what are the best practices for grabbing data from the web?
I feel it's better to use GCD for handling your threads, and asynchronous retrieval using NSURLConnection should be helpful. There are few network libraries available to do the boilerplate code for you, such as AFNetworking, and ASIHTTPRequest (although that is a bit old).
Are NSURLConnection delegate methods called on the main thread?
Delegate methods can be executed on a NSOperationQueue or a thread. If you not explicitly schedule the connection, it will use the thread where it receives the start message. This can be the main thread, but it can also any other secondary thread which shall also have a run loop.
You can set the thread (indirectly) with method
- (void)scheduleInRunLoop:(NSRunLoop *)aRunLoop forMode:(NSString *)mode
which sets the run loop which you retrieved from the current thread. A run loop is associated to a thread in a 1:1 relation. That is, in order to set a certain thread where the delegate methods shall be executed, you need to execute on this thread, retrieve the Run Loop from the current thread and send scheduleInRunLoop:forMode: to the connection.
Setting up a dedicated secondary thread requires, that this thread will have a Run Loop. Ensuring this is not always straight forward and requires a "hack".
Alternatively, you can use method
- (void)setDelegateQueue:(NSOperationQueue *)queue
in order to set the queue where the delegate methods will be executed. Which thread will be actually used for executing the delegates is then undetermined.
You must not use both methods - so schedule on a thread OR a queue. Please consult the documentation for more information.
Will nonatomic data be vulnerable in a delegate method?
You should always synchronize access to shared resources - even for integers. On certain multiprocessor systems it is not even guaranteed that accesses to a shared integer is safe. You will have to use memory barriers on both threads in order to guarantee that.
You might utilize serial queues (either NSOperationQueue or dispatch queue) to guarantee safe access to shared resources.
When dealing with a complex threaded app, what are the best practices for grabbing data from the web?
Utilize queues, as mentioned, then you don't have to deal with threads. "Grabbing data" is not only a threading problem ;)
If you prefer a more specific answer you would need to describe your problem in more detail.
To answer your first question: The delegate methods are called on the thread that started the asynchronous load operation for the associated NSURLConnection object.
I'm developing an application which must extract new messages for user from the server. I have implemented this as a timer which fires every 2 seconds and call a selector which is a method that implement HTTP request and server respond processing. So I think it is quite a bad idea to do this in a main thread.I have no experience with multithreading in iOS. So I want to know what fits well for parallelizing of this task: GCD or NSThread?
You should work as far up the API stack as possible. That way you can concentrate on programming functionality and not managing threads. GCD uses threads that the system has gotten going anyway and its much more efficient than managing your own code. Its even better to aim to encapsulate your networking into NSOperations that can then be put on an NSOperationQueue and these will be executed on one or more background threads, whatever the system deems is a good number for the current power status and other things like that.
The benefit of NSOperation over a pure GCD approach is that operations can be cancelled. Once a block is committed to GCD it has to run no matter what.
If you want to encapsulate your HTTP requests in NSOperations you might be interested to know that someone has already done this for you. AFNetworking is one of the most widely regarded iOS networking stacks and uses NSOperations as its base to build on and as such is very easily multi threaded.
A good idea would be to try and encapsulate your parsing code into an NSOperation and then as your network request operations return you can create parsing operation instances and put them on another queue for processing in the background.
I am confused on where to use which multithreading tool in iOS for hitting services and changing UI based on service data,
firstly I got accustomed to using NSURLConnection and its delegates, used didreceiveresponse, didreceivedata etc delegates to achieve the task
secondly I learned and used GCD to hit services and update the UI from within the block code
Now I am learning to use performSelectorInBackground() to do work in background thread
Clearly confused on which tool to use where?
NSURLConnection with delegate calls is "old school" way of receiving data from remote server. Also it's not really comfortable to use with few NSURLConnection instances in a single class (UIViewController or what not). Now it's better to use sendAsynchronousRequest.. method with completion handler. You can also define on which operation queue (main for UI, or other, background one) the completion handler will be run.
GCD is good for different tasks, not only fetching remote resources with initWithContentsOfURL: methods. You can also control what type of queues will receive your blocks (concurrent, serial ,etc.)
performSelectorInBackground: is also "old school" way of performing a method in background thread. If you weren't using ARC, you'd need to setup separate autorelease pool to avoid memory leaks. It also has a limitation of not allowing to accept arbitrary number of parameters to given selector. In this case it's recommended to use dispatch_async.
There are also NSOperationQueue with NSOperation and its subclasses (NSInvocationOperation & NSBlockOperation), where you can run tasks in the background as well as get notifications on main thread about finished tasks. IMHO They are more flexible than GCD in a way that you can create your own subclasses of operations as well as define dependencies between them.
The most important thing is, that you never change UI anyway in another thread except the main thread.
I think, that all points you mentioned use the same technique in the background: GDC. But I'm not sure of that.
Anyway it doesn't matter which tool you should use in terms of threading.
It's rather a question of your purpose. If you wan't to fetch an small image or just few data you can use contentsOfURLin a performSelectorInBackground() or a GDC dispatch block.
If it's about more data and more information like progress or error handling you should stick with *NSURLConnection`.
I suggest using GCD in all cases. Other techniques are still around but mainly for backward compatibility.
GCD is better for 3 reasons (at least):
It's extremely easy to use and the code remains very readable because of the use of blocks
It is lower level than things like NSOperation so it is much faster when you need high performance multi threading
It's lightweight and non-intrusive so your code doesn't have to change substantially when you want to add thread management in the middle of a method.
Can someone explain the relationship between asynchronous NSURL requests and GCD and NSOperationQueues?
I am not sure when to use each.
Right now, I have been "getting away" with asynchronous NSURL requests when I need to fetch/upload data to the server. But it has been suggested that I should use GCD. My problem is I do not know in what real life examples GCD would be better. Does anyone have any common use cases for me? And if I use GCD to store a queue of 10 asynchronous NSURL GET requests, for example, how would this benefit me? Does it even make sense to have an asynchronous NSURL request inside grand central dispatch queue or an NSOperationQueue?
Thank you!
What is particularly confusing in your case is that we're mixing an HTTP request queue (where requests would be sent one after the other) and an operation queue (where random computational work is executed one task after the other).
A standard NSURLConnection instance calls its delegate on the main thread, which is OK if you're not doing complex work on the data or on your UI at that time. But say you need to download a big file and write it chunk by chunk as a file on disk, all while scrolling down a table view. Now your scrolling might get choppy when you write data on the disk, blocking the main thread.
That's where GCD or its higher level abstraction NSOperationQueue comes into play. To solve this issue, you need to offload your data write calls off of the main thread. You can do that by specifying an NSOperationQueue instance on your NSURLConnection via setDelegateQueue:. This will ensure your delegate, and therefore your write calls, will be called in a background thread. You can also leave the delegate calls on the main thread, and wrap your expensive write calls inside a block which you will then execute off the main thread with dispatch_async. Now NSOperationQueue basically wraps a dispatch queue, and you're avoiding an extra thread switch by using it over a raw dispatch queue so I would recommend the NSOperationQueue solution (which also happens to looks simpler).
AFNetworking is a great library, and it solves this issue in a third way: it fires off an NSThread which is dedicated to NSURLConnection delegate calls. That's the pre-GCD way of offloading work off the main thread. Although it works, GCD offers a more efficient and good-citizen way of presenting your background work to the system.
Last, if you're looking for an HTTP request queue, Cocoa doesn't provide it. You will have to either build a scheduler yourself, or as you already figured it out use AFNetworking which again is a great choice.
If you're interested in this topic, GCD has a lot more to offer than just hosting NSURLConnection delegate calls, and I would recommend you read Apple's great Concurrency Programming Guide or watch the excellent WWDC 2011 videos on GCD (Blocks and Grand Central Dispatch in Practice and Mastering Grand Central Dispatch).
This is off the topic but if you use AFNetworking Lib you dont have to do anything you mentioned above.
"AFNetworking is a delightful networking library for iOS and Mac OS X. It's built on top of NSURLConnection, NSOperation, and other familiar Foundation technologies. It has a modular architecture with well-designed, feature-rich APIs that are a joy to use. "
Answer to your question, please read this
http://www.cocoaintheshell.com/2011/04/nsurlconnection-synchronous-asynchronous/
There are various questions around this topic, and lots of advice saying NOT to use sendSynchronousRequest within dispatch_async, because it blocks the thread, and GCD will spawn lots of new worker threads to service all the synchronous URL requests.
Nobody seems to have a definitive answer as to what iOS 5, [NSURLConnection sendAsynchronousRequest:queue:completionHandler:] does behind the scenes.
One post I read states that it 'might' optimise, and it 'might' use the run loop - but certainly won't create a new thread for each request.
When I pause my debugger when using sendAsynchronousRequest:queue:completionHandler, the stack trace looks like this:
..now it appears that sendAsynchronousRequest:queue:completionHandler, is actually calling sendSynchronousRequest, and I still have tons of threads created when I use the async method instead of the sync method.
Yes, there are other benefits to using the async call, which I don't want to discuss in this post.
All I'm interested in is performance / thread / system usage, and if i'm worse off using the sync call inside dispatch_async instead of using the async call.
I don't need advice on using ios4 async calls either, this is purely for educational purposes.
Does anyone have any insightful answers to this?
Thanks
This is actually open source. http://libdispatch.macosforge.org/
It is very unlikely that you will be able to manage the worker threads more efficiently than Apple's implementation. In this context "asynchronous" doesn't mean select/poll, it just means that the call will return immediately. So it is not surprising that the implementation is spawning threads.
As the previous answer stated, both GCD (libdispatch) and the libblocksruntime are open source. Internally, iOS/OS X manage a global pool of pthreads, plus any app-specific pools you create in your user code. Since there's a 1:N mapping between OS threads and dispatch tasks, you don't have to (and shouldn't) worry about thread creation and disposal under the hood. To that end, a GCD task doesn't use any time in an app's run loop after invocation as it's punted to a background thread.
Most kinds of NSURL operations are I/O-bound; there's no amount of concurrency voodo that can disguise this, but if Apple's own async implementation uses its synchronous counterpart in the background, it probably suggests it's quite highly optimized already. Contrary to what the previous answer said, libdispatch does use scalable I/O internally (kqueue on OS X/iOS/BSD), and if you hand-rolled something yourself you'd have to deal with file-descriptor readiness yourself to yield similar performance.
While possible, the return on time invested is probably marginal. Stick with Apple's implementation and stop worrying about threads!