I noticed that the following banal call from my main thread
[self performSelectorOnMainThread:#selector(rollBar:)
withObject:nil
waitUntilDone:false];
was causing [NSThread isMultiThreaded] to report that my app
had become multithreaded. I thought that was only supposed
to happen when you detach a thread, something that queueing
a message within one thread shouldn't need to do.
Any insights?
This question not the same as this one.
Stop the press
My fault, rollBar: called [m_progress_bar incrementBy: 0.5];.
The the pretty, lickable, animating NSProgressIndicator is responsible
for making my app become multithreaded. Which is surprising.
I didn't know that.
Surprisingly, [m_progress_bar usesThreadedAnimation] always
returns NO, even though the bar animates when my app is hung.
Are you sure about this? I put the following code in my appDelegate's init method:
NSLog(#"Multi-threaded before? %#", [NSThread isMultiThreaded] ? #"Yes" : #"No");
[self performSelectorOnMainThread: #selector(setDelegate:) withObject: self waitUntilDone: NO];
NSLog(#"Multi-threaded after? %#", [NSThread isMultiThreaded] ? #"Yes" : #"No");
and got this result in the console:
2008-10-21 07:26:28.296 MyApplication[82963:20b] Multi-threaded before? No
2008-10-21 07:26:28.297 MyApplication[82963:20b] Multi-threaded after? No
I also added the second NSLog(...) statement to my applicationWillTerminate: method and it also told me it was not multithreaded.
Related
It sounds confusing but it looks like this
AVPlayer *capturedPlayer = _player;
dispatch_async(_subtitlesQueue, ^{
// Parse the requested subtitle track and create a subtitle time observer
subripString = [NSString stringWithContentsOfFile:filePath encoding:NSUTF8StringEncoding error:nil];
subripEntries = [SubRipParser parse:subripString];
if (!subripEntries.count)
return;
dispatch_async(dispatch_get_main_queue(), ^{
_subtitlesTimeObserver = [capturedPlayer addPeriodicTimeObserverForInterval:CMTimeMake(1, 5)
queue:_subtitlesQueue
usingBlock:^(CMTime time){}];
});
});
The above piece of code is called when a button is clicked. It crashes. I'm new to GCD and the whole queue thing so perhaps I'm misunderstanding, but shouldn't the above work?
If I change the call on the main queue to a synchronous then it works. The crash happens from the subtitleQueue on a call to AVPlayer's makePeriodicCall (or the like).
The async call also works if I add the periodic time observer to the main queue instead of custom serial queue. However, the docs say that adding on a different queue should be ok.
Question 2)
And while I'm here, I also have a question about the part that "captures" the AVPlayer. Is capturing the variable like that safe enough or do I have to use __weak and make sure it's not NULL within the block? My situation is such that the controller that contains the AVPlayer is a singleton, so it exists throughout the lifetime of the application. I think this makes not using the __weak modifier ok. Am I correct in thinking this?
Cheers, and thanks for any help!
EDIT:
The exception is a EXC_BAD_ACCESS code 2, so something which shouldn't be accessed is. It happens on a separate thread that is running the _subtitlesQueue. And it happens on a call to [AVPlayerPeriodicCaller _effectiveRateChanged]
I also printed out the values for the capturedPlayer and _subtitlesQueue (pointer values) before the outer dispatch_async is called on the _subtitlesQueue, before the inner dispatch_async is called on the main queue and inside the dispatch_async on the main queue before the addPeriodicTimeObserver is called. They are all the same.
EDIT2:
If I add a synchronized block around the periodic time observer creation on the subtitleQueue then things work...
#synchronized(_subtitlesQueue) {
_subtitlesTimeObserver = [capturedPlayer addPeriodicTimeObserverForInterval:CMTimeMake(1, 5)
queue:_subtitlesQueue
usingBlock:subtitleTimeObservedBlock];
}
All
There seems to be a bug that causes EXC_BAD_ACCESS in -[AVPlayerPeriodicCaller _effectiveRateChanged] when you add a periodic observer to a playing AVPlayer. The workaround that I'm using is:
BOOL playing = player.rate > 0.0f;
if (playing)
{
[player pause];
}
[player addPeriodicTimeObserverForTimeInterval:myTime queue:mySerialQueue usingBlock:myBlock];
if (playing)
{
[player play];
}
As you pointed out, another workaround is to pass NULL instead of a serial queue, since that has the effect of enqueueing the blocks on the main thread dispatch queue.
Simple question: what happens if I do this:
- (void)viewDidLoad
{
[self performSelectorInBackground:#selector(myBGMethod) withObject:nil];
}
-(void)myBGMethod
{
[self myOtherMethod];
}
-(void)myOtherMethod
{
NSLog(#"This is not Inception");
//some more code here
}
Will the NSLog() and other code in myOtherMethod be run on the main thread or in the background?
It'll be run in the background thread.
You can confirm this by calling NSLog inside all your methods. By default, NSLog prints the thread number along the process ID (pid).
It'll be run in the background. Once you make the call to myBGMethod in another thread, whatever it calls is made on that same thread unless it specifically requests another thread.
As a side note, depending on which version of iOS you want to support, you might want to learn more about Grand Central Dispatch. It makes multithreading a lot simpler.
If you are ever curious about what thread a particular line of code is executing on, you can put a breakpoint on that line and check the Debug Navigator pane in Xcode:
In this case, I put a breakpoint on NSLog(...)
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(#"HI!");
});
and you can see that we're in Thread 2 com.apple.root.default-priority
Currently i'm having trouble using two functions in my -(void)viewDidLoad, both of these functions uses NSUrlRequest to send HTTPPost to a webservice to recieve data.
It works fine untill [self bar] decides to kick in before [self foo] is completely finished. So, is there any smart way of checking if [self bar] is completely finished before starting [self foo]?
-(void)viewDidLoad{
[self foo]; // initiates a nsxmlparsercall to a webservice to get values.
[self bar]; // relies on the values recieved from [self foo] to make it's own call.
/* However, [self bar] always crashes before initiating it's request.
/* It crashes when the variables that are being sent with the poststring
/* are being set, as they are null.
/* Which means that the `[self foo]` doesnt get completed before starting [self bar];
}
I might be very off at this point, i've even considered overriding -(void)viewDidload and setting a bool to control when it's ok to fire the second function, but that seems like super poor coding..
Any suggestions and/or tips on how to point me in the right direction will be highly appreciated. Thanks in advance.
I best place to put your function will be one of the delegate methods of nsxmlparser that is
- (void)parserDidEndDocument:(NSXMLParser *)parser
{
[self bar];
}
This fine if you are parsing the response on a background thread and it doesn't matter if the function bar is called on main thread or background thread.
But if you want to call the bar function specifically on main thread then you can use this function
[self performSelectorOnMainThread:#SEL(bar) withObject:nil waitUntilDone:YES];
you mean in [self foo] function you want to parse some thing and when its completely parsed then you want to call [self bar]; function right?
okay then you can fire a notification when parsing gets completed. in by this notification you can call the method you want.
I have a problem with compatibility of my application with an iOS5 b7 and GM versions.
The issue occurs in the next lines of code:
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
} while (!done);
App crashes with signal EXC_BAD_ACCESS after some iterations.
The number of passed iterations is random (from 2 till 7).
Also everything works quite well on iOS4 and iOS3.
The same issue occurs in Apple's sample: XMLPerformance Sample.
What do you think about this?
October 12th thousands of users of my app will upgrade to iOS5 and I don't want my app to be with such a strange error in the AppStore.
4 hours passed and I've found the problem. I will describe how I've resolved the problem in XMLPerformance sample.
The problem was in NSAutoreleasePool. There is #property (nonatomic, assign) NSAutoreleasePool *downloadAndParsePool;. When the app starts to download Top300 Paid Apps RSS new thread is created using [NSThread detachNewThreadSelector:#selector(downloadAndParse:) toTarget:self withObject:url];. So in that thread we should keep local autorelease pool. It is done in next way:
- (void)downloadAndParse:(NSURL *)url {
self.downloadAndParsePool = [[NSAutoreleasePool alloc] init];
// initializing internet connection and libxml parser.
if (rssConnection != nil) {
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
} while (!done);
}
// Release resources used only in this thread.
[downloadAndParsePool release];
self.downloadAndParsePool = nil;
}
So in downloadAndParse: everything looks fine. Now let's look in one method that is called when an item from RSS is parsed:
- (void)finishedCurrentSong {
// sending new item to delegate and other ...
countOfParsedSongs++;
// Periodically purge the autorelease pool. The frequency of this action may need to be tuned according to the
// size of the objects being parsed. The goal is to keep the autorelease pool from growing too large, but
// taking this action too frequently would be wasteful and reduce performance.
if (countOfParsedSongs == kAutoreleasePoolPurgeFrequency) {
[downloadAndParsePool release];
self.downloadAndParsePool = [[NSAutoreleasePool alloc] init];
countOfParsedSongs = 0;
}
}
As you see there lines :
[downloadAndParsePool release];
self.downloadAndParsePool = [[NSAutoreleasePool alloc] init];
So exactly that lines causes the exception. If I comment them everything works great.
But I decided not only to comment that lines but also replace NSAutoreleasePool in - (void)downloadAndParse:(NSURL *)url with #autorelease block as it is said that it is more efficient:
- (void)downloadAndParse:(NSURL *)url {
#autoreleasepool {
// initializing internet connection and libxml parser.
if (rssConnection != nil) {
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
} while (!done);
}
// Release resources used only in this thread.
}
}
Now everything works fine. The only problem that I haven't resolved is:
// Periodically purge the autorelease pool. The frequency of this action may need to be tuned according to the
// size of the objects being parsed. The goal is to keep the autorelease pool from growing too large, but
// taking this action too frequently would be wasteful and reduce performance.
So if anybody has any thoughts about this problem can post another one answer and may be try to explain more correctly the bug fix. I will be glad to accept that answer.
Thanks.
This looks like memory problem, please check Apple Technote QA1367 "Finding EXC_BAD_ACCESS bugs in a Cocoa project"
In your code, try this to crash as soon as possible:
[item release], item = nil;
It doesn't solve the problem, just makes the crash happen earlier and hopefully give you a more meaningful callstack to study.
If you're using multi-threading, well... You could try to print "current" thread id into console to verify that everything really is run in thread where you expect them to be running. Especially verify that all UI stuff is in main thread, even when such code is run as side-effect of other code (error popups, maybe).
#include <pthread.h>
- (void)myFunction
{
NSLog(#"Thread (%d)",
pthread_mach_thread_np(pthread_self()));
}
Run your app with Instruments, make sure to change memory verification to happen every 1 or 2 seconds. Slow, but yet again you want to get notified as close to the actual memory problem as possible.
Looking at your code: where did that "done" variable come from and who changes it's value and when? Now it looks pretty magical.
Also you could check the return value of runMode:beforeDate to make sure it was run. If the return value is NO, runLoop was not run at all. Maybe some other part of your code cannot handle such case?
Just my little contribution.
As I've got the same problem, I've discover that in iOS5, you don't need to have your own NSAutoreleasePool in a thread (used by performSelectorOnMainThread).
Then, in your code (a xml parser- same as me), I think you have to separate code from iOS4 and iOS5.
With iOS4, you need NSAutoreleasePool, but not with iOS5.
I'm trying to get data from a website- xml. Everything works fine.
But the UIButton remains pressed until the xml data is returned and thus if theres a problem with the internet service, it can't be corrected and the app is virtually unusable.
here are the calls:
{
AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
if(!appDelegate.XMLdataArray.count > 0){
[UIApplication sharedApplication].networkActivityIndicatorVisible = YES;
[appDelegate GetApps]; //function that retrieves data from Website and puts into the array - XMLdataArray.
}
XMLViewController *controller = [[XMLViewController alloc] initWithNibName:#"MedGearsApps" bundle:nil];
[self.navigationController pushViewController:controller animated:YES];
[controller release];
}
It works fine, but how can I make the view buttons functional with getting stuck. In other words, I just want the UIButton and other UIButtons to be functional whiles the thing works in the background.
I heard about performSelectorInMainThread but I can't put it to practice correctly.
You don’t understand the threading model much and you’re probably going to shoot yourself in the foot if you start adding asynchronous code without really understanding what’s going on.
The code you wrote runs in the main application thread. But when you think about it, you don’t have to write no main function — you just implement the application delegate and the event callbacks (such as touch handlers) and somehow they run automatically when the time comes. This is not a magic, this is simply a Cocoa object called a Run Loop.
Run Loop is an object that receives all events, processes timers (as in NSTimer) and runs your code. Which means that when you, for example, do something when the user taps a button, the call tree looks a bit like this:
main thread running
main run loop
// fire timers
// receive events — aha, here we have an event, let’s call the handler
view::touchesBegan…
// use tapped some button, let’s fire the callback
someButton::touchUpInside
yourCode
Now yourCode does what you want to do and the Run Loop continues running. But when your code takes too long to finish, such as in your case, the Run Loop has to wait and therefore the events will not get processed until your code finishes. This is what you see in your application.
To solve the situation you have to run the long operation in another thread. This is not very hard, but you’ll have to think of a few potential problems nevertheless. Running in another thread can be as easy as calling performSelectorInBackground:
[appDelegate performSelectorInBackground:#selector(GetApps) withObject:nil];
And now you have to think of a way to tell the application the data has been loaded, such as using a notification or calling a selector on the main thread. By the way: storing the data in the application delegate (or even using the application delegate for loading the data) is not very elegant solution, but that’s another story.
If you do choose the performSelectorInBackground solution, take a look at a related question about memory management in secondary threads. You’ll need your own autorelease pool so that you won’t leak autoreleased objects.
Updating the answer after some time – nowadays it’s usually best to run the code in background using Grand Central Dispatch:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// No explicit autorelease pool needed here.
// The code runs in background, not strangling
// the main run loop.
[self doSomeLongOperation];
dispatch_sync(dispatch_get_main_queue(), ^{
// This will be called on the main thread, so that
// you can update the UI, for example.
[self longOperationDone];
});
});
Use NSURLConnection's connectionWithRequest:delegate: method. This will cause the specified request to be sent asynchronously. The delegate should respond to connection:didReceiveResponse: and will be sent that message once the response is completely received.
You can make use of a background operation that gets pushed into the operation queue:
BGOperation *op = [[BGOperation alloc] init];
[[self operationQueue] addOperation:op];
[op release];
I've created specific "commands" that get executed in the background:
#implementation BGOperation
# pragma mark Memory Management
- (BGOperation *)init
{
if ((self = [super init]) != nil)
/* nothing */;
return self;
}
- (void)dealloc
{
self.jobId = nil;
[super dealloc];
}
# pragma mark -
# pragma mark Background Operation
- (void)main
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
[appDelegate GetApps];
[pool release];
return;
}
#end
After completion it might be a good idea to send a notification to the main thread because the internal database has been changed.
It looks as if you might be using NSURLConnection inside your getApps method. If so, you should convert it to an asynchronous call.