Recently I've been researching and working with NSURLConnection and concurrency.
There seem to be several different approaches, and the ones I've tried (dispatch queues and operation queues) all seemed to work properly, eventually.
One problem I encountered with concurrency and NSURLConnection, is the delegate methods not being called. After some research I found out the NSURLConnection needs to either be scheduled in the main runloop, or the NSOperation should be running on the main thread. In the first case I'm invoking NSURLConnection like this:
connection = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:NO];
[connection scheduleInRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];
[connection start];
And in the latter case like this:
- (void)start
{
if (![NSThread isMainThread])
{
[self performSelectorOnMainThread:#selector(start) withObject:nil waitUntilDone:NO];
return;
}
connection = [[NSURLConnection alloc] initWithRequest:request delegate:self];
}
The delegate methods handle everything else, and both seem to work properly.
In the case when I was using a dispatch queue, I did the same as with the first case (schedule the NSURLConnection in the main runloop).
My question is, what's the difference between these two approaches? Or are they actually the same, but just a different way of implementing them?
The second question is, why is this necessary? I'm also using an NSXMLParser inside an NSOperation, and this doesn't seem to require a main runloop or main thread, it just works.
I think I figured it out myself. Since both NSURLConnection and NSXMLParser are asynchronous, they require a run loop for the delegate messages when they're running in the background.
As far as I know now, the main thread automatically keeps the run loop running; the main run loop. So both solutions for NSURLConnection I posted will make sure the main run loop is used for the asynchronous part, either by telling the connection to use the main run loop for the delegate messages, or by moving the entire operation onto the main thread, which will automatically schedule the connection on the main thread as well.
What I've come up with now, is to keep a run loop running on my custom NSOperation classes, so I no longer have to perform any scheduling or thread checking. I've implemented the following at the end of the (void)start method:
// Keep running the run loop until all asynchronous operations are completed
while (![self isFinished]) {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
}
Related
I have an app that successfully uses the synchronous methods to download files (NSData's initWithContentsOfURL and NSURLConnection's sendSynchronousRequest), but now I need to support large files. This means I need to stream to disk bit by bit. Even though streaming to disk and becoming asynchronous should be completely orthoganal concepts, Apple's API forces me to go asynchronous in order to stream.
To be clear, I am tasked with allowing larger file downloads, not with re-architecting the whole app to be more asynchronous-friendly. I don't have the resources. But I acknowledge that the approaches that depend on re-architecting are valid and good.
So, if I do this:
NSURLConnection* connection = [ [ NSURLConnection alloc ] initWithRequest: request delegate: self startImmediately: YES ];
.. I eventually have didReceiveResponse and didReceiveData called on myself. Excellent. But, if I try to do this:
NSURLConnection* connection = [ [ NSURLConnection alloc ] initWithRequest: request delegate: self startImmediately: YES ];
while( !self.downloadComplete )
[ NSThread sleepForTimeInterval: .25 ];
... didReceiveResponse and didReceiveData are never called. And I've figured out why. Weirdly, the asynchronous download happens in the same main thread that I'm using. So when I sleep the main thread, I'm also sleeping the thing doing the work. Anyway, I have tried several different ways to achieve what I want here, including telling the NSURLConnection to use a different NSOperationQueue, and even doing dispatch_async to create the connection and start it manually (I don't see how this couldn't work - I must not have done it right), but nothing seems to work. Edit: What I wasn't doing right was understanding how Run Loops work, and that you need to run them manually in secondary threads.
What is the best way to wait until the file is done downloading?
Edit 3, working code:
The following code actually works, but let me know if there's a better way.
Code executing in the original thread that sets up the connection and waits for the download to complete:
dispatch_queue_t downloadQueue = dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0 );
dispatch_async(downloadQueue, ^{
self.connection = [ [ NSURLConnection alloc ] initWithRequest: request delegate: self startImmediately: YES ];
[ [ NSRunLoop currentRunLoop ] run ];
});
while( !self.downloadComplete )
[ NSThread sleepForTimeInterval: .25 ];
Code executing in the new thread that responds to connection events:
-(void)connection:(NSURLConnection*) connection didReceiveData:(NSData *)data {
NSUInteger remainingBytes = [ data length ];
while( remainingBytes > 0 ) {
NSUInteger bytesWritten = [ self.fileWritingStream write: [ data bytes ] maxLength: remainingBytes ];
if( bytesWritten == -1 /*error*/ ) {
self.downloadComplete = YES;
self.successful = NO;
NSLog( #"Stream error: %#", self.fileWritingStream.streamError );
[ connection cancel ];
return;
}
remainingBytes -= bytesWritten;
}
}
-(void)connection:(NSURLConnection*) connection didFailWithError:(NSError *)error {
self.downloadComplete = YES;
[ self.fileWritingStream close ];
self.successful = NO;
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
self.downloadComplete = YES;
[ self.fileWritingStream close ];
self.successful = YES;
}
... didReceiveResponse and didReceiveData are never called. And I've
figured out why. Weirdly, the asynchronous download happens in the
same main thread that I'm using. It doesn't create a new thread. So
when I sleep the main thread, I'm also sleeping the thing doing the
work.
Exactly. The connection is driven by the run loop; if you sleep the thread, the run loop stops, and that prevents your connection from doing its thing.
So don't do anything special. Let the app sit there, with the run loop running. Maybe put a little spinner on the screen to entertain the user. Go about your business if you can. If at all possible, let the user continue to use the application. Your delegate method will be called when the connection is complete, and then you can do what you need to do with the data.
When you move your code to a background thread, you'll again need a run loop to drive the connection. So you'll start create a run loop, schedule your connection, and then just return. The run loop will keep running, and your delegate method will again be called when the connection completes. If the thread is done, you can then stop the run loop and let the thread exit. That's all there is to it.
Example: Let's put this in concrete terms. Let's say that you want to make a number of connections, one at a time. Stick the URL's in a mutable array. Create a method called (for example) startNextConnection that does the following things:
grabs an URL from the array (removing it in the process)
creates an URL request
starts a NSURLConnection
return
Also, implement the necessary NSURLConnectionDelegate methods, notably connectionDidFinishLoading:. Have that method do the following:
stash the data somewhere (write it to a file, hand it to another thread for parsing, whatever)
call startNextConnection
return
If errors never happened, that'd be enough to retrieve the data for all the URLs in your list. (Of course, you'll want startNextConnection to be smart enough to just return when the list is empty.) But errors do happen, so you'll have to think about how to deal with them. If a connection fails, do you want to stop the entire process? If so, just have your connection:didFailWithError: method do something appropriate, but don't have it call startNextConnection. Do you want to skip to the next URL on the list if there's an error? Then have ...didFailWithError: call startNextRequest.
Alternative: If you really want to keep the sequential structure of your synchronous code, so that you've got something like:
[self downloadURLs];
[self waitForDownloadsToFinish];
[self processData];
...
then you'll have to do the downloading in a different thread so that you're free to block the current thread. If that's what you want, then set up the download thread with a run loop. Next, create the connection using -initWithRequest:delegate:startImmediately: as you've been doing, but pass NO in the last parameter. Use -scheduleInRunLoop:forMode: to add the connection to the download thread's run loop, and then start the connection with the -start method. This leaves you free to sleep the current thread. Have the connection delegate's completion routine set a flag such as the self.downloadComplete flag in your example.
I hesitate to provide this answer because the others are correct that you really should structure your app around the asynchronous model. Nevertheless:
NSURLConnection* connection = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:NO];
NSString* myPrivateMode = #"com.yourcompany.yourapp.DownloadMode";
[connection scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:myPrivateMode];
[connection start];
while (!self.downloadComplete)
[[NSRunLoop currentRunLoop] runMode:myPrivateMode beforeDate:[NSDate distantFuture]];
Do not do this on the main thread. Your app is just as likely to be terminated for blocking the main thread as for downloading too big a file to memory.
By the way, given that you're downloading to a file instead of memory, you should consider switching from NSURLConnection to NSURLDownload.
I think your sleepForInterval is blocking the NSURLConnection's activity -
No run loop processing occurs while the thread is blocked.
From the NSThread documentation.
I think you might have to rethink how you're setting your downloadComplete variable. Consider using your connectionDidFinishLoading:connection delegate method to determine when the download is complete instead of your loop + sleep?
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
self.downloadComplete = YES;
// release the connection, and the data object
[connection release];
[receivedData release];
}
From the NSURLConnection guide.
You can use the connection:connection didFailWithError:error delegate method to ensure you're dealing with situations where the download does not complete.
I am am trying to get multiple NSURLConnections to run in parallel (synchronously), however if it is not running on the main thread (block of code commented out below) the URL connection doesn't seem to work at all (none of the NSURLConnection delegate methods are triggered). Here is the code I have (implementation file of an NSOperation subclass):
- (void)start
{
NSLog(#"DataRetriever.m start");
if ([self.DRDelegate respondsToSelector:#selector(dataRetrieverBeganExecuting:)])
[self.DRDelegate dataRetrieverBeganExecuting:identifier];
if ([self isCancelled]) {
[self finish];
} else {
/*
//If this block is not commented out NSURLConnection works, but not otherwise
if (![NSThread isMainThread])
{
[self performSelectorOnMainThread:#selector(start) withObject:nil waitUntilDone:NO];
return;
}*/
SJLog(#"operation for <%#> started.", _url);
[self willChangeValueForKey:#"isExecuting"];
_isExecuting = YES;
[self didChangeValueForKey:#"isExecuting"];
NSURLRequest * request = [NSURLRequest requestWithURL:_url];
_connection = [[NSURLConnection alloc] initWithRequest:request
delegate:self];
if (_connection == nil)
[self finish];
} //not cancelled
}//start
Ran through it with a debugger, and after the end of this start method none of the NSURLConnection delegates trigger (I set breakpoints there). But on the main thread it works just fine. Any ideas of what's up? Thanks!
Background threads don't automatically have an active run loop on them. You need to start up the run loop after you create the NSURLConnection in order to get any input from it. Fortunately, this is quite simple:
[[NSRunLoop currentRunLoop] run];
When you say that you are running the connections synchronously, you are incorrect. The default mode of NSURLConnection is asynchronous -- it creates and manages a new background thread for you, and calls back to the delegate on the original thread. You therefore don't need to worry about blocking the main thread.
If you do actually want to perform a synchronous connection, you would use sendSynchronousRequest:returningResponse:error:, which will directly return the data. See "Downloading Data Synchronously" for details.
NSURLConnection needs an active run loop to actually work; the easiest way to ensure this is to just run it from the main thread.
Note that NSURLConnection normally runs asynchronously (and if you run one synchronously, what it really does is run one asynchronously on another thread and then block until that completes), so except for whatever processing you do in your delegate methods it shouldn't have much of an effect on UI responsiveness.
So I'm starting a new NSThread that I want to be able to use later by calling performSelector:onThread:.... From how I understand it calling that methods add that call to the runloop on that thread, so on its next iteration it will pop all these calls and subsequently call them until there is nothing left to call. So I need this kind of functionality, an idle thread ready for work that I just can call upon it. My current code looks like this:
- (void)doInitialize
{
mThread = [[NSThread alloc] initWithTarget:self selector:#selector(runThread) object:nil];
[mthread start];
}
- (void)runThread
{
NSAutoReleasePool *pool = [[NSAutoReleasePool alloc] init];
// From what I understand from the Google machine is that this call should start the
// runloop on this thread, but it DOESN'T. The thread dies and is un-callable
[[NSRunLoop currentRunLoop] run];
[pool drain];
}
- (void)scheduleSomethingOnThread
{
[self performSelector:#selector(hardWork) onThread:mThread withObject:nil waitUntilDone:NO];
}
But the thread is not kept alive, and the performSelector:onThread does not do anything. How do I go about this the right way?
A run loop requires at least one "input source" to run. The main run loop does, but you have to add a source manually to get a secondary run loop's -run method to do anything. There's some documentation on this here.
One naïve way to get this to work would be just to put [[NSRunLoop currentRunLoop] run] in an infinite loop; when there's something to do, it'll do it, and return immediately otherwise. The problem is that the thread will take a decent amount of processor time simply waiting for something to occur.
Another solution is to install an NSTimer on this run loop to keep it alive.
But, if possible, you should use a mechanism designed for this sort of thing. If possible, you may want to use NSOperationQueue for background operations.
this piece of code should force the thread to wait forever
BOOL shouldKeepRunning = YES; // global
NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
[runLoop addPort:[NSMachPort port] forMode:NSDefaultRunLoopMode]; // adding some input source, that is required for runLoop to runing
while (shouldKeepRunning && [runLoop runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]]); // starting infinite loop which can be stopped by changing the shouldKeepRunning's value
I have url connection, which normally works fine
NSURLConnection *connection = [[NSURLConnection alloc] initWithRequest:request
delegate:delegate];
But when I create a modal window, no request ever receives response:
[NSApp runModalForWindow:window];
If I comment this line out, thus creating a 'standard' window, everything works.
I tried implementing all methods from NSURLConnectionDelegate, not a single of them called.
I suspect this is something about 'run loops', but have little experience in this area. Does anybody have experience in this?
Thank you
If you're targeting 10.5+, you can tell the NSURLConnection to also run in NSModalPanelRunLoopMode (the mode your current thread's runloop would be in while presenting a modal view) via
-(void)scheduleInRunLoop:(NSRunLoop *)aRunLoop forMode:(NSString *)mode
where aRunLoop would probably be [NSRunLoop currentRunLoop] and the mode would be NSModalPanelRunLoopMode. More info in the NSURLConnection doc.
If you're supporting earlier OSs, you may have to get creative (i.e. with multithreading). Good discussion of this issue pre-10.5 here.
I haven't bumped into the situation you're having, but I suggest spawning and starting a connection in background thread.
I also met the same problem that didn't get the delegate method called when using NSURLConnection in a Modal Window.
after some investigation, following code resolve it.
NSURLConnection* conn = [[NSURLConnection alloc] initWithRequest:requst delegate:self startImmediately:NO];
[conn scheduleRunLoop:[NSRunLoop currentLoop] forMode:NSModalPanelRunLoopMode];
[conn start];
However, when connectionDidFinishLoading called, [NSApp stopModal] doesn't work, need call [NSApp abortModal] instead.
See also:
Objective-C Asynchronous Web Request with Cookies
I spent a day writing this code and can anyone tell me what is wrong here?
WSHelper is inherited from NSObject, I even tried NSDocument and NSObjectController and everything..
-(void) loadUrl: (NSString*) urlStr{
url = [[NSURL alloc] initWithString:urlStr];
request = [NSURLRequest requestWithURL:url cachePolicy: NSURLRequestReloadIgnoringCacheData timeoutInterval: 60.0];
connection = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:YES];
if(connection)
{
receivedData = [[NSMutableData data] retain];
//[connection start];
}
else
{ display error etc... }
NSApplication * app = [NSApplication sharedApplication];
[app runModalForWindow: waitWindow];// <-- this is the problem...
}
-(void)connection: (NSURLConnection*)connection didReceiveData:(NSData*)data{
progressText = #"Receiving Data...";
[receivedData appendData:data];
}
-(void)connection: (NSURLConnection *)connection didFailWithError:(NSError *)error{
progressText = #"Error...";
NSAlert * alert = [[NSAlert alloc] init];
[alert setMessageText:[error localizedDescription]];
[alert runModal];
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection{
progressText = #"Done...";
pData = [[NSData alloc] initWithData:receivedData];
[self hideWindow];
}
The code just wont do anything, it doesnt progress at all. I even tried it with/without startImmediately:YES but no luck !!!, this is executed in main window so even the thread and its run loop is running successfully.
I tried calling synchronous request, and it is working correctly !! But I need async solution.
I have added CoreServices.Framework in project, is there anything more I should be adding to the project? any compiler settings? Or do i have to initialize anything before I can use NSURLConnection?
Any solution to run NSURLConnection on different thread on its own NSRunLoop, Objective-C and MAC Development has no sample code anywhere in documentation that makes everything so difficult to code.
I also met the same problem that didn't get the delegate method called when using NSURLConnection in a Modal Window.
after some investigation, following code resolve it.
NSURLConnection* conn = [[NSURLConnection alloc] initWithRequest:requst delegate:self startImmediately:NO];
[conn scheduleRunLoop:[NSRunLoop currentRunLoop] forMode:NSModalPanelRunLoopMode];
[conn start];
However, when connectionDidFinishLoading called, [NSApp stopModal] doesn't work, need call [NSApp abortModal] instead.
Firstly you're making starting the connection too complicated. Change to:
connection = [[NSURLConnection alloc] initWithRequest:request delegate:self]
Remove [connection start]. Now:
Is your app definitely running the run loop normally? NSURLConnection requires this to work.
Are you able to perform a synchronous load of the URL request?
In the debugger, can you see that url is what you expect it to be? What is it?
Is it possible that you're deallocating WSHelper before any delegate messages are received? NSURLConnection is asynchoronous after all.
One does not need to do anything special to use NSURLConnection, it's a straightforward part of the Foundation framework. No special compiler settings required. No initialization before use. Nothing. Please don't start blindly trying stuff like bringing in CoreServices.Framework.
As sending the request synchronously works, there must be something wrong with your handling of the asynchronous aspect. It could be:
The runloop is not running in NSDefaultRunLoopMode so the connection is unable to schedule itself.
Some other part of your code is calling -cancel on the connection before it has a chance to load.
You are managing to deallocate the connection before it has a chance to load.
Real problem
Ah, in fact I've just realised what's going on. You are calling:
-[NSApp runModalForWindow:]
Read the description of what this method does. It's not running the run loop like NSURLConnection expects. I'd say that really, you don't want to be presenting a window quite like this while running a URL connection for it.
I'd also suggest that you implement the -connection:didReceiveResponse: delegate method too. You want to check here that the server is returning the expected status code.
You say that you're using this in a modal dialog? A modal dialog puts the run loop into a different mode. You should be able to get this to work by scheduling it to run in the modal dialog run loop mode, in addition to the normal run loop mode. Try adding this line of code after you allocate connection in loadURL:
[connection scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSModalPanelRunLoopMode];
Hope that helps.
How do you know it isn't doing anything? Are there any error or warning messages during the compile? Are any error messages showing up on console when the program is running?
Have you tries setting breakpoints in your code and following through what you expect to be happening?