Im trying to convert the following to objective-c code.
This is the current thread I have in C and works fine
//calling EnrollThread method on a thread in C
pthread_t thread_id;
pthread_create( &thread_id, NULL, EnrollThread, pParams );
//What the EnrollThread method structure looks like in C
void* EnrollThread( void *arg )
What my method structure looks like now that I've changed it to objective-c
-(void)enrollThreadWithParams:(LPBIOPERPARAMS)params;
Now I'm not sure how to call this objective-c method with the pthread_create call.
I've tried something like this:
pthread_create( &thread_id, NULL, [refToSelf enrollThreadWithParams:pParams], pParams );
But I believe I have it wrong. Can anyone enlighten me on why this does not work and what it is I need to do to fix it so that I can create my thread in the background? My UI is getting locked until the method finishes what it's doing.
I was thinking of also using dispatch_sync but I haven't tried that.
In objective C you don't really use pthread_create, although you can still use it, but the thread entry point needs to be a C function, so I'm not sure if this would be the best approach.
There are many options, as you can read in the Threading and Concurrency documents.
performSelectorInBackground method of NSObject (and subclasses)
dispatch_async (not dispatch_sync as you mentioned)
NSOperation and NSOperationQueue
NSThread class
I would suggest giving it a shot to the first one, since it is the easiest, and very straightforward, also the second one is very easy because you don't have to create external objects, you just place inline the code to be executed in parallel.
The go to reference for concurrent programming is the Concurrency Programming Guide which walks you through dispatch queues (known as Grand Central Dispatch, GCD) and operation queues. Both are incredibly easy to use and offer their own respective advantages.
In their simplest forms, both of these are pretty easy to use. As others have pointed out, the process for creating a dispatch queue and then dispatching something to that queue is:
dispatch_queue_t queue = dispatch_queue_create("com.domain.app", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
// something to do in the background
});
The operation queue equivalent is:
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
[queue addOperationWithBlock:^{
// something to do in the background
}];
Personally, I prefer operation queues where:
I need controlled/limited concurrency (i.e. I'm going to dispatch a bunch of things to that queue and I want them to run concurrent with respect to not only the main queue, but also with respect to each other, but I don't want more than a few of those running simultaneously). A good example would be when doing concurrent network requests, where you want them running concurrently (because you get huge performance benefit) but you generally don't want more than four of them running at any given time). With an operation queue, one can specify maxConcurrentOperationCount whereas this tougher to do with GCD.
I need fine level of control over dependencies. For example, I'm going to start operations A, B, C, D, and E, but B is dependent on A (i.e. B shouldn't start before A finishes), D is dependent upon C, and E is dependent upon both B and D finishing.
I need to enjoy concurrency on tasks that, themselves, run asynchronously. Operations offer a fine degree of control over what determines when the operation is to be declared as isFinished with the use of NSOperation subclass that uses "concurrent operations". A common example is the network operation which, if you use the delegate-based implementation, runs asynchronously, but you still want to use operations to control the flow of one to the next. The very nice networking library, AFNetworking, for example, uses operations extensively, for this reason.
On the other hand, GCD is great for simple one-off asynchronous tasks (because you can avail yourself of built-in "global queues", freeing yourself from making your own queue), serial queues for synchronizing access to some shared resource, dispatch sources like timers, signaling between threads with semaphores, etc. GCD is generally where people get started with concurrent programming in Cocoa and Cocoa Touch.
Bottom line, I personally use operation queues for application-level asynchronous operations (network queues, image processing queues, etc.), where the degree of concurrency becomes and important issue). I tend to use GCD for lower-level stuff or quick and simple stuff. GCD (with dispatch_async) is a great place to start as you dip your toe into the ocean of concurrent programming, so go for it.
There are two things I'd encourage you to be aware of, regardless of which of these two technologies you use:
First, remember that (in iOS at least) you always want to do user interface tasks on the main queue. So the common patterns are:
dispatch_async(queue, ^{
// do something slow here
// when done, update the UI and model objects on the main queue
dispatch_async(dispatch_get_main_queue(), ^{
// UI and model updates can go here
});
});
or
[queue addOperationWithBlock:^{
// do something slow here
// when done, update the UI and model objects on the main queue
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
// do UI and model updates here
}];
}];
The other important issue to consider is synchronization and "thread-safety". (See the Synchronization section of the Threading Programming Guide.) You want to make sure that you don't, for example, have the main thread populating some table view while, at the same time, some background queue is changing the data used by that table view at the same time. You want to make sure that while any given thread is using some model object or other shared resource, that another thread isn't mutating it, leaving it in some inconsistent state.
There's too much to cover in the world of concurrent programming. The WWDC videos (including 2011 and 2012) offer some great background on GCD and asynchronous programming patterns, so make sure you avail yourself of that great resource.
If you already have working code, there is no reason to abandon pthreads. You should be able to use it just fine.
If, however, you want an alternative, but you want to keep your existing pthread entry point, you can do this easily enough...
dispatch_queue_t queue = dispatch_queue_create("EnrollThread", DISPATCH_QUEUE_SERIAL);
dispatch_async(queue, ^{
EnrollThread(parms);
});
I've used GCD before with dispatch_async for background-threading units of work like parsing data from a network request (in, say, JSON or XML), and it's WONDERFUL. But what if I have a background task that's going to run for the length of the application? Is dispatch_async still a good fit for this case, or is there a better way to implement it?
Sure, but create your own dispatch queue for it. (If you use a global queue, you may tie up that queue--or one of its threads--forever.)
dispatch_queue_t dispatchQueue = dispatch_queue_create("com.mycompany.myapp.ForeverOperation", DISPATCH_QUEUE_SERIAL);
dispatch_async(dispatchQueue, ^{
// worker routine
});
More traditionally, you would create an explicit thread for this, and if it will run forever, that might make more "sense". But the results are basically the same.
NSThread * myWorkerThread = [[NSThread alloc] initWithTarget:...
[myWorkerThread start];
If you need to communicate with other threads/queues, as always, standard synchronization techniques may be required.
This really has nothing to do with dispatch_async, which is merely one approach to doing something in a background thread. If you want to do something in a background thread, do it in a background thread! Just be aware that doing this constantly can be a drag on your app, since you've only got so much processing time and only so many processors; you may end up having to study that in Instruments. You might want to break up your task into little bits and do it in short chunks every so often. Both GCD and NSOperation can help with that.
I was testing performSelector:onThread:withObject:waitUntilDone: for inter-thread communication as recommended by Matt Ghallagher's tutorial. Turns out that it is extremely slow, and pretty much no good for real time audio applications. Any suggestions?
On the main thread:
AnyClass myObject;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// do asynchronous work
// environment objects and variables will be available
[myObject doSomething];
});
It really does not get any simpler than that...
This is the way I am trying to manage my thread
-(void)ExecuteThread {
#autoreleasepool {
bInsideDifferentThread = YES;
//some code...
bInsideDifferentThread = NO;
}
[NSThread exit];
}
-(void)ThreadCallerEvent {
NSThread *myThread = [[NSThread alloc] initWithTarget:self selector:#selector(ExecuteThread) object:nil];
if (!bInsideThread)
[myThread start];
else
{
[myThread cancel];
}
}
I do it this way becuase I don't want the thread to be started until it has finished working. The problem is that this is generating leaks from a non released memory allocated at [NSThread init]
Any ideas of how to fix this problem?
I ran a fragment similar to yours and wasn't able to detect leaks; but the context is almost certainly different. I'm really not sure what ARC is doing with myThread in your example when it goes out of scope. A typical pattern for using NSThread would be:
[NSThread detachNewThreadSelector:#selector(executeThread)
toTarget:self
withObject:nil];
in which case you're not responsible for dealing directly with the thread you are detaching. (Note: I changed your method name to use camel case which is the preferred method and variable naming convention in Cocoa.)
All of the above said, managing threads are no longer the preferred way of achieving concurrent design. It's perfectly acceptable; but Apple is encouraging developers to migrate to GCD. Far better to think in terms of units of work that need to be performed concurrently.
Without understanding your needs more deeply in this case, it's hard to know what advantages, if any, working directly with threads might offer you; but I would consider looking at concurrent queues/GCD more closely. Perhaps you could simply use something like:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// do your background work
});
and achieve both a clearer design for concurrency and avoid whatever memory management issues you are now seeing.
I'm writing a multi threaded application for iOS.
I am new to Objective-C, so I haven't played around with threads in iPhone before.
Normally when using Java, I create a thread, and send "self" as object to thread. And from the thread I can therefor call the main thread.
How is this done in Objective C?
How can I call the main thread from a thread? I have been trying with NSNotificationCenter, but I get a sigbart error :/
This is how the thread is started:
NSArray *extraParams = [NSArray arrayWithObjects:savedUserName, serverInfo, nil]; // Parameters to pass to thread object
NSThread *myThread = [[NSThread alloc] initWithTarget:statusGetter // New thread with statusGetter
selector:#selector(getStatusFromServer:) // run method in statusGetter
object:extraParams]; // parameters passed as arraylist
[myThread start]; // Thread started
activityContainerView.hidden = NO;
[activityIndicator startAnimating];
Any help would be great!
you accomplish this by adding a message to the main thread's run loop.
Foundation provides some conveniences for this, notably -[NSObject performSelectorOnMainThread:withObject:waitUntilDone:] and NSInvocation.
using the former, you can simply write something like:
[self performSelectorOnMainThread:#selector(updateUI) withObject:nil waitUntilDone:NO]
a notification may be dispatched from a secondary thread (often the calling thread).
You can either use performSelectorOnMainThread:withObject:waitUntilDone: or, if you're targeting iOS 4 and later, Grand Central Dispatch, which doesn't require you to implement a method just to synchronize with the main thread:
dispatch_async(dispatch_get_main_queue(), ^ {
// Do stuff on the main thread here...
});
This often makes your code easier to read.
Though this is not a direct answer to your question I would highly recommend you take a look at Grand Central Dispatch. It generally gives better performance than trying to use threads directly.
As Justin pointed out, you can always perform a function in the main thread by calling performSelectorOnMainThread, if you really needed too.