I just spent a day tracking down a really weird bug. It was a UILabel being overreleased, although in the code there was no over-releasing. If I commented out the release of the label, code was fine.
I tracked it down to a UIView that was being released in a background thread. Which didn't seem right as you're supposed to access all UI elements from the main thread. I then found a nib file that was being loaded in a background thread using:
[[NSBundle mainBundle] loadNibNamed:#"nib name" owner:self options:nil];
I don't need to go into the details of what the code was doing, but my question is:
If loadNibNamed is called in a background thread and the view loaded is cached by the nib loading process, then the main thread wants to load that nib too - but loads it from a cache. When my object is released - is the view loaded from the nib in the background also released in the background..?? Doesn't really make sense as the background thread would no longer exist anyway....
I don't know, but I resolved it by wrapping the loadNibNamed part of the code in a GCD dispatch onto main thread synchronously.
From Mike Ash's article
Dangerous Cocoa Calls
NSBundle
This one has half of the problems of NSHost. NSBundle returns shared objects, but is not thread safe, so it's main-thread only. It's still safe to use from the main thread. The reason I mark it as dangerous is because the fact that it's unsafe to use from secondary threads is not really documented, but rather has to be inferred from the fact that it's not thread safe and the fact that the instances are shared, and it can be tempting to use it from other threads.
All UI-related operations should be done from the main thread.
Related
I experimented and was able to instantiate UIKit class (UIImageView in this case) in a background thread without issue. I have seen app crashes by doing things with UIKit on a background thread. Behind the scene, what causes exception when dealing with UIKit on a non-main thread? For example, if I stay completely away from dealing with the view hierarchy, is it safe?
It is not safe. There are some documented tasks that are safe to call on a background task, e.g. drawing a string into an image, accesing UIFont, pushing/popping/using current graphics context. However, mostly the behavior is undefined or documented as unsafe. You never know what shared resources the classes are using in the background (or they will in future releases). They could use shared memory pools, for example.
From my experience, in most cases you won't see a problem if the UIView is not added to any UIWindow but still it is not safe.
For example, I think that loading things from nibs (using UINib) must be done only from the main thread... and any view can load subviews from nibs.
In my code, I subclassed NSView and in its drawRect method, I am spawning three threads to perform the drawing.
-(void)drawRect:(NSRect)dirtyRect
{
[[self window] setAllowsConcurrentViewDrawing:YES];
[self setCanDrawConcurrently:YES];
[NSThread detachNewThreadSelector:#selector(DrawText) toTarget:self withObject:nil];
[NSThread detachNewThreadSelector:#selector(DrawRectangle) toTarget:self withObject:nil];
[NSThread detachNewThreadSelector:#selector(DrawGradient) toTarget:self withObject:nil];
//Wherease these functions DrawText, DrawRectangle and DrawGradient performs their task as suggested by name.
//In DrawText, DrawRectangle, and DrawGradient lockFocus and unlockFocus is being
// used for multithreaded drawing.
}
When I run the same program from Xcode, it is running fine. Output is shown below.
But when I run it from the outside, there is problem and output is shown below.
First, I would like to know is it right way to draw from a secondary thread? Or what is another way to draw from a secondary thread?
What is the reason behind this problem?
Ken Aspeslagh is somewhat incorrect about drawing from secondary threads (he is correct it is generally a bad idea). From what I can see of your code you don't have a good use case for drawing on a secondary thread. Can you explain why you want to do this?
you yourself have already discovered setCanDrawConcurrently: which explicitly talks of invoking drawRect: from a background thread. Note that the views window must have allowsConcurrentViewDrawing set to YES for this to work (it is the default).
Apples own Cocoa Drawing Guide has a section on drawing from secondary threads. I have highlighted some portions I think are relevant to you.
The Application Kit maintains a unique graphics context for each window and thread combination. Because each thread has its own graphics context object for a given window, it is possible to use secondary threads to draw to that window. There are some caveats, however.
During the normal update cycle for windows, all drawing requests are sent to your application’s main thread for processing. The normal update cycle happens when a user event triggers a change in your user interface. In this situation, you would call the setNeedsDisplay: or setNeedsDisplayInRect: method (or the display family of methods) from your application’s main thread to invalidate the portions of your view that require redrawing. You should not call these methods from any secondary threads.
If you want to update a window or view from a secondary thread, you must manually lock focus on the window or view and initiate drawing yourself. Locking focus configures the drawing environment for that window's graphics context. Once locked, you can configure the drawing environment, issue your drawing commands as usual, and then flush the contents of the graphics context to the window buffer.
In order to draw regularly on a secondary thread, you must notify the thread yourself. The simplest way to send regular notifications is using an NSTimer or NSAnimation object. For more information on how to animate content, see “Advanced Drawing Techniques.”
The Cocoa Threading Programming Guide also says this:
If you want to use a thread to draw to a view, bracket all drawing code between the lockFocusIfCanDraw and unlockFocus methods of NSView
Just an aside, GCD block invocation is probably a much nicer method for performing small sets of operations in the background than NSThread.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
// you can put each of these calls in their own queue if you want
[self DrawText];
[self DrawRectangle];
[self DrawGradient];
});
However, that likely has nothing to do with your problem; I mention it only because I think it will serve you better to use GCD queues.
You should only be drawing to the screen from the main thread.
Edit: It is apparently really complicated so you're better off drawing to the screen from the main thread. ;)
If you need to render something that takes too much time to avoid blocking the main thread, consider using a thread to do the drawing to an offscreen context, and then copy that context to the screen on the main thread.
I read about NSGraphicsContext Restriction at Thread guide.
Here, I found the following line:
If you do any drawing from a secondary thread, you must flush your drawing calls manually. Cocoa does not automatically update views with content drawn from secondary threads, so you need to call the flushGraphics method of NSGraphicsContext when you finish your drawing. If your application draws content from the main thread only, you do not need to flush your drawing calls.
After calling flushGraphics, it works fine.
I'm familiar with the delegate pattern and nilling my delegates, especially when doing asynchronous calls which are still in progress when my view controllers disappear. I nil the delegate, and the callback successfully returns on a nil object.
I'm now experimenting with using completion blocks to make my code a little easier to read.
I call a network service from my view controller, and pass a block which updates my UITableView. Under normal circumstances it works fine. However, if I leave the view before it completes, the completion handler block is executed - but the UITableView is now a zombie.
Whats the usual pattern for handling this?
UPDATE WITH CODE SAMPLE
This is an iPad app, I have two view controllers on screen at once, like a split view. One is the detail, and the other is a grid of images. I click an image and it tell the detail to load the info. However, if i click the images too fast before they have chance to do the network call - I have the problems. On changing images the code below is called which counts the favourites of a image....
So here is my dilemma, if I use the code below - it works fine but it leaks in instruments if you switch images before the network responds.
If I remove the __block and pass in self, then it crashes with zombies.
I can't win... I'm sure i'm missing something fundamental about using blocks.
__block UITableView *theTable = [self.table retain];
__block IndexedDictionary *tableData = [self.descriptionKeyValues retain];
FavouritesController *favourites = [Container controllerWithClass:FavouritesController.class];
[favourites countFavouritesForPhoto:self.photo
completion:^(int favesCount) {
[tableData insertObject:[NSString stringWithFormat:#"%i", favesCount]
forKey:#"Favourites:" atIndex:1];
[theTable reloadData];
[tableData release];
[theTable release];
}];
Any tips? Thanks
SECOND UPDATE
I changed the way I loaded the favourites. Instead of the favourites being a singleton, I create an instance on each photo change. By replacing this and killing the old one - the block has nowhere to callback (i guess it doesn't even exist) and my code now just looks like the below, and it appear to be working:
[self.favourites countFavouritesForPhoto:self.photo
completion:^(int favesCount) {
[self.descriptionKeyValues insertObject:[NSString stringWithFormat:#"%i", favesCount]
forKey:#"Favourites:" atIndex:1];
[self.table reloadData];
}];
It doesn't leak, and doesn't appear to be crashing either.
I recommend you test that the tableview is not nil at the start of the block. It sounds like the tableview is properly discarded when its parent view goes off-screen, so after that point, no tableview operations are valid.
Retaining the UITableView within the block is a bad idea, because datasource/tableview updates can result in implicit method calls and notifications that will not be relevant if the tableview is not on-screen.
Block will retain any object that it references, except for those annotated with __block. If you want not to execute completion blocks at all, just make some property like isCancelled and check whether it is YES before calling completion block.
So you have a background operation which has to call back another object after it finishes and the object can be destroyed in the meantime. The crashes you describe happen when you have non retained references. The problem as you see is that the referred object goes away and the pointer is invalid. Usually, what you do is unregister the delegate inside the dealloc method so that the background task continues, and whenever it is ready to communicate the results back it says "Shoot, my callback object is nil", and at least it doesn't crash.
Still, handling manually weak references is tedious and error prone. You can forget to nil a delegate inside a dealloc method and it may go without notice for months before you encounter a situation where the code crashes.
If you are targeting iOS 5.0 I would read up upon ARC and the weak references it provides. If you don't want to use ARC, or need to target pre 5.x devices, I would recommend using zeroing weak reference libraries like MAZeroingWeakRef which work also for 3.x devices.
With either ARC's weak references or MAZeroingWeakRef, you would implement the background task with one of these fancy weak reference objects pointing back to your table. Now if the pointed object goes away, the weak pointer will nil itself and your background task won't crash.
I have a window that displays some data in an NSTableView. This data is loaded in the background. The data-loading-thread is started in the windowDidLoad: method. If the window is closed before loading has finished, the background thread should be cancelled. I do this by signalling the thread in the windowWillClose: delegate method and waiting for the background thread to finish.
Now this all works perfectly. But I have one problem: How can I update the data in the table view? I have tried calling reloadData via performSelectorOnMainThread: but this leads to a race condition: The reloadData call is sometimes queued on the main thread after the window close command, and will execute after the window has closed, and everything goes up in flames.
What's the best way to control and communicate with a background thread?
Well, you know, this is exactly what makes the use of threading complex: you always face synchronization issues.
What I suggest is, instead of calling [tableView reloadData] from your thread, simply signal your controller (by calling a method controllerShouldReloadTable) and let your controller do the check if windowWillClose has been called or not. There might be a chance that your controller has been also released by the time controllerShouldReloadTable, and to fix this you will definitely need to retain the controller from the secondary thread.
On a side note, I would cancel the thread in viewDidUnload (for symmetry).
Most important: I would use asynchronous calls and a delegate class so that the whole multithreading issue is solved at its root.
EDIT: Sending asynchronously a request will not block the sending thread waiting for the response. Instead, asynchronous send (for NSURLConnection is called start) immediately returns (so, no blocking) and when the response is received, a delegate method will be called (i.e., connectionDidFinishLoading:) so that you can updated the model and the UI. Take a look at NSURLConnection docs, but as usual, I strongly suggest using [ASIHTTPRequest][2], which has many advantages.
I am doing my download with an object which was inherited from NSOperation. I have read the documentation and when my operation finished I must call the
[self.delegate performSelectorOnMainThread:#selector(operationDidFinish:) withObject:self waitUntilDone:YES];
method. It needs to be called on the main thread, because the UIKit is not thread safe and the documentation says this in these non thread safe frameworks cases.
In the delegate method I am drawing a pdf or an image, but because it is drawn on the main thread the User Interface is very laggy until the drawing is finished.
Maybe can you suggest me a good way to avoid this problem?
I am assuming you are downloading image data, decoding it, and rendering the image. The last bit, render, has to happen on the main thread. Can you move the decode part to your download thread? For example, use CGImage calls to decode a png or jpeg, so you have a CGImageRef ready to use before calling operationDidFinish. If you can work with the PDF as images, it would be better to convert it than decode it in the main thread.