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.
Related
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.
I am doing a lot of GCD and asynchronous rendering and data retrieval work lately and I really need to nail the mental model about how asynchronous is done.
I want to focus on setNeedsDisplay and the NSURLConnectionDelegate suite of methods.
Is it correct to call setNeedsDisplay asynchronous? I often call it via dispatch_async(dispatch_get_main_queue(), ^{}) which confuses me.
The NSURLConnectionDelegate callbacks are described as asynchronous but are they not actually concurrently run on the main thread/runloop. I am a but fuzzy on the distinction here.
More generally in the modern iOS era of GCD what is the best practice for making GCD and these methods play nice together. I'm just looking for general guidelines here since I use them regularly and am just trying not to get myself in trouble.
Cheers,
Doug
No, you generally don't call setNeedsDisplay asynchronously. But if you're invoking this from a queue other than the main queue (which I would guess you are), then you should note that you never should do UI updates from background queues. You always run those from the main queue. So, this looks like the very typical pattern of dispatching a UI update from a background queue to the main queue.
NSURLConnection is described as asynchronous because when you invoke it, unless you used sendSynchronousRequest, your app immediately returns while the connection progresses. The fact that the delegate events are on the main queue is not incompatible with the notion that the connection, itself, is asynchronous. Personally, I would have thought it bad form if I can some delegate methods that were not being called from the same queue from which the process was initiated, unless that was fairly explicit via the interface.
To the question of your question's title, whether NSURLConnection uses GCD internally, versus another concurrency technology (NSOperationQueue, threads, etc.), that's an internal implementation issue that we, as application developers, don't generally worry about.
To your final, follow-up question regarding guidelines, I'd volunteer the general rule I alluded to above. Namely, all time consuming processes that would block your user interface should be dispatched to background queue, but any subsequent UI updates required by the background queue should be dispatched back to the main queue. That's the most general rule of thumb I can think of that encapsulates why we generally do concurrent programming and how to do so properly.
What is the differance between adding a operation which make a synchronous NSURLConnection request in NSOperationQueue ( or synchronous request from a thread ( not main thread)) AND making a asynchronous request from the main thread ?
Both will not block main thread so UI will remain responsive but is there any advantage of using one over other? I know in later method i can track request progress etc but assume that progress and other HTTP stuff is not important here.
They are very similar. The biggest problem with synchronous requests is that they can't easily be cancelled. Depending on your application, that could be a problem. Imagine you are downloading a big document and the user moves to another screen so you no longer need that information. In our case, I actually chose doing asynchronous NSURLConnections on a secondary NSThread, which may be overkill for some apps. It is more complicated, but it gives us the ability to both cancel requests and to decode the JSON/XML/image data on secondary threads so they don't impact main thread user interactivity.
Asynchronous requests are scheduled on the run loop and setup as a run loop source, triggering the code automatically only when there is data received from the network (as any socket source).
Synchronous requests running on a NSThread monopolizes a thread to monitor the incoming data, which is in general quite overkill.
You can always cancel an NSURLConnection even if it has been executed asynchronously, using the cancel method.
I bet using the new API that allows to send an asynchronous request on an NSOperationQueue (+sendAsynchronousRequest:queue:completionHandler:) uses GCD under the hood and dispatch_source_create, or something similar, so that it behave the same way as when an NSURLConnection is scheduled on the run loop, avoiding using an additional thread (watch the WWDC'12 videos that explains why threads are evil and their usage should be minimized), the difference only being that allows you to use a block to be informed upon completion instead of using the delegate mechanism.
Some years ago I created a class that embedded NSURLConnection asynchronous calls and delegate management into a nice block API (see OHURLLoader on my github) that makes it easier to use (feel free to take a look). I bet the new API that uses NSOperationQueues uses the same principle, still doing asynchronous requests on the runloop but allowing you to use blocks instead of having to implement a delegate.
The historical position was that there's an advantage in power consumption, and therefore battery life, in asynchronous requests — presumably including both the older delegate approach and the new block-based approach.
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/
My app downloads several resources from server, data and data descriptors. These downloads, triggered by user actions, can be performed simultaneously, let's say, up to 50 downloads at a time. All these asynchronous tasks end up creating objects in memory, (e.g. appending leaves to data structures, such as adding keys to mutable dictionaries or objects to arrays). My question is: can this cause stability issues? For instance, if several simultaneous tasks try to add keys to the same dictionary, am I supposed to handle the situation, placing some kind of locks? If I implement a for cycle which looks for graphical elements in an array, is it possible that other running tasks might change the array content 'during' the cycle? Any reference or major, general orientation about this multitasking, multithreading issues other than official documentation?
Depends how you are dealing with the downloads - if you are using NSURLConnection it handles the separate threading / concurrency for you and your code is reentrant thus you don't have to worry about simultaneous action.
If you are creating your own threads you potentially have issues.
EDIT:
Your code runs in a main thread (the main run loop), lets say you have an NSURLConnection that is also running then it will run in a separate thread. However your delegate code that deals with events that happen while the connection is in progress runs in your run loop, not in the other thread. This means your code can only ever execute one thing at a time. A connection succeeded method would not get called at the same time as any of your other code. If you had a for loop running then it would block your main thread until it has finished looping, in the meanwhile if the connection finished while the for loop is still running then your delegate code will not execute until after the loop has finished.
You may want to look into Grand Central Dispatch's (GCD) and barrier blocks. Barrier blocks will allow you to do what y oh want in the background and "lock" resources.
Check out the Apple documentation and Mike Ash's blog post here on GCD.
The basic gist is that you use a concurrent queue that you create to perform the reads and use a barrier block to block all access to that resource for writing. good stuff.
Good luck
Tim