Managing CPU intensive threads on iOS - iphone

Question

I’m an experienced C/C++ programmer coming up to speed on Objective C on the iPhone. I have done a lot of searching, but haven’t found a satisfactory answer on what must be a common question; I apologize if this is answered elsewhere, pointers would be appreciated.
My app is very CPU intensive. The UI has a simple display that shows progress and a start/stop button. What is the best way to allocate the most possible CPU cycles to getting the work done, while still ensuring that the display is updated regularly and the start/stop button is responsive? I have read that you should not do work in the main thread, but beyond that I haven’t found many suggestions. In light of this I have implemented my work in an NSOperation queue. I have also put the screen refresh in its own queue. I have also liberally sprinkled the code with NSThread sleepForTimeIntervals. I have experimented with different sleep times from .001 to 1 ([NSThread sleepForTimeIntervals .1] for instance). In spite of this the screen display is sluggish at best (10s of seconds) and pressing the stop button highlights the button but nothing happens again for 10s of seconds.
1.) Are NSOperation Queues a reasonable choice? If not, what else?
2.) How do I minimize the sleeping? (Obviously I want the work to get as many cycles as possible/reasonable, and I’m not sure that my sleeps are doing anything at all to all the UI to update.)
3.) Is there a better technique to keep the UI up to date? For instance, can I use NSTimer or some other method to send a message to the UI telling it to update and/or check the status of the buttons?
Thank you for your support.

Answer 1

1.) Are NSOperation Queues a reasonable choice? If not, what else?
NSOperationQueue sounds like it would be reasonable.
of course, you have choice: pthreads, libdispatch (aka GCD), c++ thread libraries built on top of pthreads, etc, etc , etc. if you don't spawn much/many, then it just comes down to the model you favor.
2.) How do I minimize the sleeping? (Obviously I want the work to get as many cycles as possible/reasonable, and I’m not sure that my sleeps are doing anything at all to all the UI to update.)
don't sleep =) you can use a timer for your ui elements or an explicit callback or notification to notify dependencies. if the dependencies peform ui updates, then you will likely add the message to the main thread's message queue.
3.) Is there a better technique to keep the UI up to date? For instance, can I use NSTimer or some other method to send a message to the UI telling it to update and/or check the status of the buttons?
that really depends on what you are doing. if you merely want to update a progress bar, then you can write the value from the secondary thread and read the value from the main thread. then use a timer on the main run loop to periodically message your object to update its display (based on the current value). for something like an unstaged progress indicator this may be good.
another alternative is more useful for events or stages: it would involve posting updates (e.g. notifications or callbacks to a delegate) from the secondary thread as progress is made (more info under #2).
Update
I wasn't sure this was appropriate in the iOS model, but it sounds like it is.
yes, that's fine - there are many appraches you can take. which is 'best' depends on the context.
My current understanding is to launch the UI in one thread (not the main!),
you really don't explicitly launch the UI; the main thread is (generally) driven by pushing events and messages onto the main thread. the main thread uses a run loop and processes the queued messages/events at each iteration of the run loop. you can also schedule these messages in the future (more on that in a bit). having said that, all your messages to UIKit and AppKit (if you target osx) objects should be on the main thread (as a generalization which you will eventually learn there are exceptions to this). if you have a specific implementation which is completely separated from messaging UIKit objects' methods and that program is thread safe, then you can actually perform those messages from any thread because it does not affect the state of the UIKit implementation. simplest example:
#interface MONView : UIView
#end
#implementation MONView
// ...
- (NSString *)iconImageName { return #"tortoise.png"; } // pure and threadsafe
#end
launch my worker thread, use a timer to generate a signal to the UI to take a look at a progress value and update the progress bar appropriately. For the purposes of this particular application your second to last paragraph is ample and I don't need to go to the lengths of the last paragraph (at least for now). Thank you.
to do this, you can use an approach similar to this:
#interface MONView : UIView
{
NSTimer * timer;
MONAsyncWorker * worker; // << this would be your NSOperation subclass, if you use NSOperation.
}
#end
#implementation MONView
// callback for the operation 'worker' when it completes or is cancelled.
- (void)workerWillExit
{
assert([NSThread isMainThread]); // call on main
// end recurring updates
[self.timer invalidate];
self.timer = nil;
// grab what we need from the worker
self.worker = nil;
// update ui
}
// timer callback
- (void)timerUpdateCallback
{
assert([NSThread isMainThread]); // call on main
assert(self.worker);
double progress = self.worker.progress;
[self updateProgressBar:progress];
}
// controller entry to initiate an operation
- (void)beginDownload:(NSURL *)url
{
assert([NSThread isMainThread]); // call on main
assert(nil == worker); // call only once in view's lifetime
// create worker
worker = [[MONAsyncWorker alloc] initWithURL:url];
[self.operationQueue addOperation:worker];
// configure timer
const NSTimeInterval displayUpdateFrequencyInSeconds = 0.200;
timer = [[NSTimer scheduledTimerWithTimeInterval:displayUpdateFrequencyInSeconds target:self selector:#selector(timerUpdateCallback) userInfo:nil repeats:YES] retain];
}
#end
note that this is a very primitive demonstration. it's also more common to put the timer, update handling, and operation in the view's controller, not the view.

Answer 2

Are you doing your UI updates on the main thread? This is very important because UIKit is not thread-safe and using it from a secondary thread can lead to sluggish behavior (or crashes for that matter). You usually should not need to use sleep in your background threads/queues for the UI to remain responsive (unless your UI itself is very CPU-intensive but that doesn't seem to be the case here).
You can check any of your methods that update the UI if they are running on the main thread with something like
NSAssert([NSThread isMainThread], #"UI update not running on main thread");
An easy and lightweight way to synchronize UI updates with the main thread is to use Grand Central Dispatch:
dispatch_async(dispatch_get_main_queue(), ^ {
//do your UI updates here...
});

Answer 3

Here you are my answers to your questions.
1) Since you are an experienced C programmer, you will feel comfortable with Grand Central Dispatch (GCD), a C based API for concurrency.
2) With GCD, you do not need to sleep at all. Simply dispatch asynchronously the work you need to do in a queue using the maximum priority (DISPATCH_QUEUE_PRIORITY_HIGH).
3) When you need to update the UI, simply dispatch on the main queue ( within the same block doing the work, using dispatch_get_main_queue() ) the UI update as needed.
Take a look at the relevant GCD documentation here.

Answer 4

I'd have a model object that does the CPU tasks, which has a delegate callback for when the output changes, and a view controller. In viewDidLoad you set the view controller as the delegate of your model. The model, therefore, can use threads and sends messages back on the main queue, when the calculated data has been updated. Unless your case is specifically complex, is just use Grand Central Dispatch and dispatch_async the intensive task onto another thread.
Certainly, you should not be calling sleepForTimeInterval anywhere to achieve what you want.