According to the documentation of CLLocationManagerDelegate
The methods of your delegate object are called from the thread in which you started the corresponding location services. That thread must itself have an active run loop, like the one found in your application’s main thread.
I am not clear as to whether this means that to receive location manager updates on a background thread, we must instantiate the location manager on that background thread or simply call the startUpdatingLocation() method on that thread.
In any event, this explains an issue when a CLLocationManagerDelegate does not receive any events from a CLLocationManager which was started on a background thread:
That thread must itself have an active run loop
If I understand run loop functioning correctly, all NSThreads are instantiated with a run loop, but the run loop will only be running if you assign some work to the thread. Therefore, to have a CLLocationManager send events correctly on a background thread, we need to set the thread's run loop to loop permanently so that it can process the CLLocationManager's calls as they arrive.
A reasonable solution to making sure the run loop is running is suggested in this question but the author implies that this is a processor expensive way of doing it.
Also, according to the threading documentation,
Threading has a real cost to your program (and the system) in terms of memory use and performance
I appreciate that we are all using lots of threading anyway, by using Grand Central Dispatch, but Grand Central Dispatch probably mitigates a lot of this in its internal thread management.
So my first question is, is it worthwhile setting up a background thread with a continuously running run loop, in order to have location events dealt with on a background thread, or will this involve an unreasonable extra amount of processing when compared to leaving the manager on the main thread?
Secondly, if it is worthwhile, is there a good way to do this using Grand Central Dispatch. As I understand the documentation, Grand Central Dispatch manages its own threads and we have no means of knowing which thread a given block will be executed on. I presume we could simply execute the usual run loop code to make the run loop of whichever thread our CLLocationManager instantiation is run on loop continuously, but might this not then affect other tasks independently assigned to Grand Central Dispatch?
This is a somewhat opinion-based question, but I have a pretty strong opinion on it :D
No.
Just deliver the events to the main queue, and dispatch any work to a background queue if it's non-trivial. Anything else is a lot of complexity for little benefit. CLLocationManager pre-dates GCD, so this was useful information in the days when we occasionally managed run loops by hand and dispatching from one thread to another was a pain. GCD gets rid of most of that, and is absolutely the tool you should use for this. Just let GCD handle it with dispatch_async.
You absolutely should not set up your own NSThread for this kind of thing. They're still necessary at times for interacting with C++, but generally if GCD can handle something, you should let it, and avoid NSThread as much as possible.
Related
I have a background thread that is doing a bunch of work - loading the application. The main thread is displaying progress on a UIProgressView.
The background thread is being spawned with performSelectorInBackground (though, I'm not wed to this method if a different approach makes this problem easier to solve)
[self performSelectorInBackground:#selector(loadAppInBackground) withObject:self];
On a couple occasions a bug has caused the background thread to crash (different bugs as the app evolves) which results in the progress bar stopping, but the user getting no clear indication that anything is wrong.
I'd like to detect this situation and fail more gracefully than simply hanging until the user gives up on waiting.
Because the duration of the load process can vary greatly, simply timing out isn't an ideal option.
What's the best way for the foreground thread to detect that the background thread has failed? Since the foreground thread is busy dealing with the UI, would it require a second background thread to monitor the first? That seems ugly.
Is there some thread-to-thread communication mechanism that could be used to "ping" the background process? Better yet, a low level system mechanism of checking the status of other threads?
The debugger knows about all the threads that are running... and seems to know their status. I'm wondering if there's a call available to my app to do the same.
If the background task runs in some sort of regular cycle (eg, there's a big loop where much of the work gets done), it can set a flag every so often to indicate that it's still alive.
One way to do this is to have background thread store [NSDate timeIntervalSinceReferenceDate] somewhere, and, in your main thread, occasionally (perhaps on a timer) compare that to the current time. If the difference is greater than some reasonable limit you can guess that the background thread has died.
Another way is to have the background thread simply set a Boolean, and have the main thread interrogate that and clear it on some regular basis. If the Boolean doesn't get set again between main thread interrogations you can infer that it has died.
The first technique has the advantage that you can "tune" the "reasonable limit" to tolerate code (in either thread) that is somewhat irregular in it's timing. The second approach generally requires timings that are more predictable.
Of course with either approach you want to somehow avoid "blowing the whistle" if the background thread has just finished up and you simply haven't recognized that yet.
A common technique is to have an extra thread to check for life signs of the thread in question - a so called heartbeat thread. The heartbeat thread polls the thread by checking if it responds in a timely manner, if not, deems the thread dead and terminates it.
A simple heartbeat thread implementation would be to check a counter that is incremented regularly by the other thread, if the counter is not incremented within a certain time it is regarded as dead and then an appropriate action could be taken like restarting thread or killing app. Another more common way is if the hb thread sends messages to the thread and checks for a response with a timeout.
It seems like there is no mechanism in objective c to check the status of a background thread directly. Any of the answers provided are decent options... either timing out, or having the thread create some sort of evidence of its continued existence.
I was hoping for something a little more simple, reliable, and real-time.
I'm going to experiment with catching an exception in the thread and perhaps producing a notification like "BackgroundThreadException" that the foreground thread could listen for and react to.
I'm working on an iPhone Game where the player tilts the iPhone to make the character move, but unfortunately all of the timers I'm using to animate the scenario are slowing down my game. I've been told to use NSThreads, but I really don't know anything about them. My question is, What are the differences between using NSThreads and NSTimers? Or what are the advantages of using NSThreads? How are they useful?
Timers are used for asynchronous, not concurrent, execution of code.
A timer waits until a certain time interval has elapsed and then fires, sending a specified message to a target object. For example, you could create an NSTimer object that sends a message to a window, telling it to update itself after a certain time interval.
Threads are for concurrent execution of code.
An NSThread object controls a thread of execution. Use this class when you want to have an Objective-C method run in its own thread of execution. Threads are especially useful when you need to perform a lengthy task, but don’t want it to block the execution of the rest of the application. In particular, you can use threads to avoid blocking the main thread of the application, which handles user interface and event-related actions. Threads can also be used to divide a large job into several smaller jobs, which can lead to performance increases on multi-core computers.
See also:
How do I use NSTimer?
Timer Programming Topics
Threaded Programming Guide
Timers are objects that simply call methods at a later time. Threads are additional "streams" of stuff to be processed, in psuedo-parallel. It's like comparing apples to oranges—they're not really related. The only relation I can see is if you want to do some processing in the background, but you shouldn't be doing UI calls in background threads, Why are you using timers to make your character move? Knowing that, we might be able to supply an alternate solution.
I want to make a class which as long as an instance of it is alive, keeps a thread (worker) going and when someone calls a method on it - performTaskWithData:(NSData*)data - then it should process this data in its worker thread.
If additional data is sent while an operation is taking place, then this new data/operation should be queued until the previous processing is done.
I need each instance of this helper class to hold one single worker thread (i.e. the same thread should handle all the processing).
How should I go about doing this?
NSRunLoop? Synchronize access to data block being passed?
Starting in iOS4, Grand Central Dispatch provides by far the simplest and most powerful interface to multithreaded programming.
If you're a registered developer, go watch some of the WWDC videos from 2010 about it. It's intimidating at first, but it's actually really simple and good.
You can do this directly with NSThreads and run loops. However, I would consider using NSOperationQueues, one per instance of your class and set the maximum concurrency of the queue to 1. Your performTaskWithData: would simply add a new instance of a subclass of NSOperation to the queue and that's it.
Is there any way I can kill a thread spawn through:
[NSThread detachNewThreadSelector:#selector(serverFetchInThread) toTarget:self withObject:nil];
The scenario I am working on is that in my main thread I am letting user enter data in my search bar and what ever user is typing I need to send to server for searching in a separate thread. Now, if user changes his selection by deleting old data and entering new data I do not want the previous thread to waste its time, kill it and spawn a new thread with new data.
Be there any other better way to handle this situation, please guide me.
No, there is no way to kill a thread from another thread. And for good reason as there is no way to do so in a fashion where the targeted thread is killed without risk of crashing the app.
To directly answer your question; you need to have some kind of a flag that indicates to the thread that it should stop doing whatever it is doing and exit.
However, a couple of questions are raised by your question:
First, why are you using threads and not using GCD? Concurrency via GCD or NSOperation is the generally recommended way to solve such problems.
Secondly, if you are talking to a server, are you using HTTP (most of the time, that is the case)? If so, why not directly use the asynchronous features of NSURL and friends?
Have a good look at using NSOperationQueue.
You can subclass NSOperation it to wrap up your server communications, and even make that queue serial (maximum operations = 1).
If a server operation is not yet finished and user has generated more input, you can cancel the existing one, and add the new one.
Due to the effect of the NSOperation wrapping your connection, you can just use the simple synchronous version and keep the connection handling very straightforward.
Also worth mentioning is compatibility. I would prefer to use GCD and blocks, but for compatibility, NSOperationQueue is required.
For performance reasons, I instantiate a dedicated NSThread to process incoming messages that are streamed from a network server. I use an NSOperation for the purpose of instantiating the connection and receiving incoming data through the NSURLConnection delegates, but as soon as new data comes in and gets parsed, I offload the processing of the message to the dedicated NSThread. The idea is to let one thread focus on receiving incoming messages and let the other thread just do the processing.
What's the proper way to shut down the NSThread when the applicationDidEnterBackground comes in?
Also, how should I restart the NSThread when applicationWillEnterForeground comes in?
Other than the main thread, it seems the state of other background threads is not maintained between going to sleep and restarting.
By the way, I'm all for using NSOperations for most tasks that have a measurable amount of work -- ie, accessing a resource over the network, performing a calculation, etc. However, in this case, I need to process messages on the fly on a long-living dedicated thread that is always there by calling performSelector:onThread:withObject:waitUntilDone: and passing it the target thread. It seems NSOperation isn't a good fit for this.
I would appreciate your input.
"For performance reasons"?
If processing doesn't take much time, run everything (including NSURLConnection) on the main thread. Concurrency bugs are a major pain.
If you want things to run serially, you can emulate a "single thread" with an NSOperationQueue with maxConcurrentOperations = 1. I'm pretty sure that NSOperationQueue uses thread pools (and on 4.0, GCD, which probably uses thread pools), which means you don't need to keep a thread running all the time.
Apart from that, your process is automatically suspended and resumed by the system, so you don't need to kill off threads.
I'm not sure what you mean by "the state of other background threads".