I've implemented a NSOperationQueue for each file upload to dropbox. I am setting the delegate for each operation that operation's thread. I do not get a callback to any of the delegate methods.
This same procedure works fine if use the main thread.
The problem is that dropbox delegate is registered in another thread. I am using a NSOperation w/ NSQueue I just continue to poll that the operation is not complete.
while (_state != DropboxOperationStateFinished)
{
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
}
Related
I don't understand it at all but NSTimer in my app definitely is running in background. I have a NSLog in method run by the timer and it is logging while it's in background. It's on iPhone 4 with iOS 4.2.1. I have declared location background support in Info.plist.
I read the docs and many discussions here and elsewhere and it shouldn't be possible. Is it an iOS bug? Or undocumented feature? I don't want to use it and find out in near future, for example with coming of iOS 4.3 that Apple silently "fixed" it and the app won't be working.
Does anybody know more about it?
NSTimer is going to fire whenever the main runloop is running. Apple makes no promises that I know of to unschedule timers or to prevent the main runloop from running. It's your responsibility to unschedule your timers and release resources when you move to the background. Apple isn't going to do it for you. They may, however, kill you for running when you are not supposed to or using too many seconds.
There are many holes in the system that will allow an app to run when it isn't authorized to. It would be very expensive for the OS to prevent this. But you cannot rely on it.
You can have a timer fire while in background execution mode. There are a couple of tricks:
You need to opt into background execution with beginBackgroundTaskWithExpirationHandler.
If you create the NSTimer on a background thread, you need to add it to the mainRunLoop manually.
- (void)viewDidLoad
{
// Avoid a retain cycle
__weak ViewController * weakSelf = self;
// Declare the start of a background task
// If you do not do this then the mainRunLoop will stop
// firing when the application enters the background
self.backgroundTaskIdentifier =
[[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
[[UIApplication sharedApplication] endBackgroundTask:self.backgroundIdentifier];
}];
// Make sure you end the background task when you no longer need background execution:
// [[UIApplication sharedApplication] endBackgroundTask:self.backgroundTaskIdentifier];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// Since we are not on the main run loop this will NOT work:
[NSTimer scheduledTimerWithTimeInterval:0.5
target:self
selector:#selector(timerDidFire:)
userInfo:nil
repeats:YES];
// This is because the |scheduledTimerWithTimeInterval| uses
// [NSRunLoop currentRunLoop] which will return a new background run loop
// which will not be currently running.
// Instead do this:
NSTimer * timer =
[NSTimer timerWithTimeInterval:0.5
target:weakSelf
selector:#selector(timerDidFire:)
userInfo:nil
repeats:YES];
[[NSRunLoop mainRunLoop] addTimer:timer
forMode:NSDefaultRunLoopMode];
// or use |NSRunLoopCommonModes| if you want the timer to fire while scrolling
});
}
- (void) timerDidFire:(NSTimer *)timer
{
// This method might be called when the application is in the background.
// Ensure you do not do anything that will trigger the GPU (e.g. animations)
// See: http://developer.apple.com/library/ios/DOCUMENTATION/iPhone/Conceptual/iPhoneOSProgrammingGuide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html#//apple_ref/doc/uid/TP40007072-CH4-SW47
NSLog(#"Timer did fire");
}
Notes
Apps only get ~ 10 mins of background execution - after this the timer will stop firing.
As of iOS 7 when the device is locked it will suspend the foreground app almost instantly. The timer will not fire after an iOS 7 app is locked.
I've created an NSOperation in the queue like so:
ImageLoadingOperation *operation = [[ImageLoadingOperation alloc] initWithImageURL:url target:self action:#selector(didFinishLoadingImageWithResult:)];
[operationQueue addOperation:operation];
[operation release];
And this works fine but if the view gets popped before the operation finishes the App crashes with "EXC_BAD_ACCESS"
I've tried to cancel the the operation Queue by calling cancelAllOperations but as its already in process it doesn't prevent the App from crashing. The docos say that if the operation is running it is up to the operation to detect that it has been canceled and respond appropriately but not too sure how I would implement this?
Any ideas?
It is a general problem for View calling some network and then callback.
My solution is you can retain the view before you call the operation. And then, when the operation finishes, you release the view.
- (void)longTask {
[self retain];
}
- (void)longTaskDidFinish {
// do something if you want
[self release];
}
You will have to either override the "cancel" operation in your ImageLoadingOperation class, or have your ImageLoadingOperation add itself as KVO observer to the "cancelled" property. There - you can intelligently cancel your operation in such way that it won't crash.
Also, if your ImageLoadingOperation runs in the background, it would be wiser to defer your access to the views somehow to the main thread (where all drawing takes place). You could use a dispatch_sync(dispatch_get_main_queue(), ^{}); or even performSelectorOnMainThread for actual access to the related view.
You see - the whole point of using an operation queue is to remove dependencies, and let things run in parallel, but your operation must synchronize with the view-system changes, and that must be designed for completeness and robustness.
You could retain the view before the callback of operation is called, as vodkhang mentioned above. But that will prolong the life of the view unnecessarily because since the view is popped you don't want the operation to continue any more.
Here is a sketch about what you should do to respond to the cancel command:
- (void)start{
if(self.isCancelled){
[self markAsFinished];
return;
}
//start your task asynchronously
}
//If you want to cancel the downloading progress immediately, implement your own 'cancel' method
- (void)cancel{
[super cancel];
if(self.isExecuting){
{
......
cancel load process
......
}
[self markAsFinished];
}
}
- (void)markAsFinished{
......
change 'finished' to YES' generate KVO notifications on this key path
change 'executing' to 'YES'; generate KVO notification on this key path
......
}
This sketch is based on ASIHTTPRequest networking library, and
there is an official guide on how you should respond to cancel command.
In the CocoaXMLParser class of Apple's CocoaXMLParser example, the following code appears:
rssConnection = [[NSURLConnection alloc] initWithRequest:theRequest delegate:self];
[self performSelectorOnMainThread:#selector(downloadStarted) withObject:nil waitUntilDone:NO];
if (rssConnection != nil) {
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
} while (!done);
}
According to the NSRunLoop documentation "In general, your application does not need to either create or explicitly manage NSRunLoop objects. Each NSThread object, including the application’s main thread, has an NSRunLoop object automatically created for it as needed." In the context of this, why is the run-loop explicitly managed in this example? Would it not be created and destroyed automatically by the thread generated by the NSURLConnection request?
In that code, the run loop is basically just being told to run forever, so that that thread can continue to process incoming background data from the NSURLConnection. Even though a run-loop is created for you, by default the thread would terminate when that method ended.
In general when doing something like that it's easier to put everything in an NSOperation which then goes in an NSOperationQueue (although if you are implementing NSUrlConnection callbacks you have to provide a few extra methods in the NSOperation class).
I have got a memory bug that seems to boil down to something happening in a thread. I am having difficulties troubleshooting this.
I have a UIViewController, that when active, i.e. the user is using its view, retrieves updates from a web service in an NSThread.
This is done every 3 minutes and this delay is controlled by a:
[self performSelector:#selector(timerDone) withObject:nil afterDelay:180.0];
The timerDone method now starts the NSThread that retrieves the web service data and also it sends the performSelector message again. This is a little "check for updates, populate views, shut everything down, repeat" routine that works just fine.
Now, the user can of course suddenly tap a button an load up a second UIViewController. When this happens I call:
[NSObject cancelPreviousPerformRequestsWithTarget:self selector:#selector(timerDone) object:nil];
And do my cleaning up in the dealloc method.
My question is now: What happens if the NSThread was running while the user changed the view and set in motion the deconstruction of this object that is the starting point of the NSThread?
Should I keep a BOOL around that tells me if the NSThread is still active, and if so, what to do with the NSThread if this is the case.
The threading is done like this:
- (void) runTimer {
[self performSelector:#selector(timerDone) withObject:nil afterDelay:180];
}
- (void) timerDone {
[self performSelector:#selector(runTimer) withObject:nil afterDelay:2];
[NSThread detachNewThreadSelector:#selector(updateAllVisibleElements) toTarget:self withObject:nil];
}
- (void) updateAllVisibleElements {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
//call approiate web service
[pool release];
}
You have two problems here: first, you're using performSelector:withObject:afterDelay: to do what an NSTimer does best (periodic callback). cancelPreviousPerformRequestsWithTarget:selector:object: can be quite expensive, and because of your threading is likely creating race conditions.
Second problem: each thread has its own run loop, and both mechanisms (performSelector:... and NSTimer) and are tied to the current thread's run loop.
Here's what I recommend: Create a single, long-lived NSThread with its own explicit run loop for all your update needs. Look at the Threading Programming Guide for some good example code of this. On that thread, set up a 3-minute repeating NSTimer. Every 3 minutes, update.
If you need to schedule an update outside the three-minute cycle, then you use performSelector:onThread:withObject:waitUntilDone: to call your updateAllVisibileElements. The way I generally do this is to encapsulate all of the thread logic into a single object (WebServiceController or whatever). It creates it own NSThread and saves it in an ivar. Then I use code like this:
- (void)requestUpdate
{
if ([NSThread currentThread] != self.thread)
{
[self performSelector:#selector(update) onThread:self.thread withObject:nil waitUntilDone:NO];
return;
}
else
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
//call approiate web service
[pool drain];
}
}
One more note: you mention that the background thread "populates views." A background thread should never call into UIKit. UIKit is not thread safe and should only be called on the main thread. I typically achieve this by posting notifications onto the main thread which the view controllers observe. The "updating" object should not know anything about the UI. That breaks the Model-View-Controller paradigm of Cocoa.
I have an iphone app, it run some thread to compute search. The search is made calling a time consuming function from a library.
I need to exit from the thread when the app is terminating, otherwise the thread continue to run and the search create problem when i reopen the app.
I tried to subscribe in the thread
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(mainApplicationWillTerminate) name:#"UIApplicationWillTerminateNotification" object:nil];
And in mainApplicationWillTerminate
-(void)mainApplicationWillTerminate;
{
[NSThread exit];
}
The problem is still present, any idea?
As stated in the docs you should avoid using [NSThread exit]. In general, to avoid memory leaks and other disasters, a thread should never be stopped "from the outside". A thread should always exits by itself.
In your thread main loop you should check if the thread was cancelled:
if ([[NSthread currentThread] isCancelled]) {
return;
}
To cancel it you call its "cancel" method from another thread.
In your case you should setup an application delegate (see UIApplicationDelegate)
- (void)applicationWillTerminate:(UIApplication *)application
{
[myThread cancel];
}
Better have a look to the nice NSOperation class also.