I've this bit of code:
- (IBAction)registerAction:(id)sender {
[NSThread detachNewThreadSelector:#selector(registerThread) toTarget:self withObject:nil];
}
- (void)registerThread {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
MyDelegate *delegate = (MyDelegate *)[[UIApplication sharedApplication] delegate];
NSInteger locationID = [delegate.api checkCoordinate:[NSString stringWithFormat:#"%f,%f",
location.coordinate.latitude, location.coordinate.longitude]];
NSNumber *status = [api registerWithUsername:usernameField.text
password:passwordField.text email:emailField.text andLocation:locationID];
[self performSelectorOnMainThread:#selector(registrationDoneWithStatus:) withObject:[NSNumber numberWithInt:1] waitUntilDone:NO];
[pool release];
}
it works nicely, but sometimes I get this error:
void _WebThreadLockFromAnyThread(bool), 0x6157e30: Obtaining the web lock from a thread other than the main thread or the web thread. UIKit should not be called from a secondary thread.
And it seems that only using the delegate I get this error, and I don't know how to resolve.
Thanks in advance :)
I've run into the same problem recently.
There may be some active views (eg. UITextField,UITextView). Try resignFirstResponder those views before accessing delegate
You fix the problem by very carefully thinking through your application's concurrency architecture and ensuring that you aren't exercising anything from a thread that should only be done on the main thread.
In this case, you are causing the UIKit to execute code from a secondary thread. If you were to set a breakpoint on _WebThreadLockFromAnyThread, you would know exactly where.
It is exceedingly atypical to use the app's delegate from a secondary thread in anything but the most extremely controlled circumstances.
tl;dr You can't make an app threaded by detaching a new thread against a random selector.
Related
I'm doing an app that loads the contents of viewControllers using NSThread while is reading an XML file.
I have it done as follows:
-(void)viewDidAppear:(BOOL)animated
{
// Some code...
[NSThread detachNewThreadSelector:#selector(loadXML) toTarget:self withObject:nil];
[super viewDidAppear:YES];
}
-(void)loadXML{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
// Read XML, create objects...
[pool release];
}
My problem is that I don't know how to stop the NSThread if the user changes to another viewController while the NSThread is loading, doing that the app crashes.
I've tried to cancel or exit the NSThread as follows but without success:
-(void)viewsDidDisappear:(BOOL)animated{
[NSThread cancel];
// or [NSThread exit];
[super viewDidDisappear:YES];
}
Can anyone help? Thanks.
When you detach new thread, you can no more cancel or exit it from viewDidDisappear etc. These UI specific methods execute only on main thread so the exit/cancel applies to the main thread which is obviously wrong.
Instead of using the detach new thread method, declare NSThread variable in .h and initialize it using initWithTarget: selector: object: method and cancel it whenever/wherever you want to..
you can also use [NSThread exit]; method of NSThread.
It's better to let a thread end gracefully, i.e. reach its natural conclusion, if you can. It sounds like in your case you can afford to. Also be sure that you're updating the user interface from the main thread, not a secondary thread, as UIKit is not thread safe.
You wrote:
... the app stops responding while the thread finishes...
Once you flag a thread for cancelling or exit, you have to manually stop whatever the thread was called to do. An example:
....
- (void) doCalculation{
/* Do your calculation here */
}
- (void) calculationThreadEntry{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; NSUInteger counter = 0;
while ([[NSThread currentThread] isCancelled] == NO){
[self doCalculation];
counter++;
if (counter >= 1000){ break;
} }
[pool release]; }
application:(UIApplication *)application
- (BOOL)
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{
/* Start the thread */
[NSThread detachNewThreadSelector:#selector(calculationThreadEntry)
toTarget:self withObject:nil];
// Override point for customization after application launch. [self.window makeKeyAndVisible];
return YES;
}
In this example, the loop is conditioned on the thread being in a non-cancelled state.
I am trying to make a searchbar to search for things I receive using NSURLConnection.
right now, if I search for something, that string is send away as an URL with an asynchronous request, which gives me data.
NSURLRequest *theRequest=[NSURLRequest requestWithURL:[NSURL URLWithString:urlString] cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:20.0];
[theConnection cancel];
[theConnection release];
theConnection=[[NSURLConnection alloc] initWithRequest:theRequest delegate:self];
That data is parsed and when it is successful I post a notification
-(void)connectionDidFinishLoading:(NSURLConnection *)connection
{
xmlParser = [[NSXMLParser alloc] data];
[xmlParser setDelegate:xmlGeocoder];
BOOL success = [xmlParser parse];
if(success == YES){
NSLog(#"No Errors");
[[NSNotificationCenter defaultCenter] postNotificationName:#"getArray" object:self];
}else{
NSLog(#"Error Error Error!!!");
[[NSNotificationCenter defaultCenter] postNotificationName:#"failToGetArray" object:self];
}
}
and my searchresultsTableView is reloaded.
self.array1 = [array2 copy];
[self.searchDisplayController.searchResultsTableView reloadData];
All these methods are depending on eachother, so B can't be executed, when A is still busy.
I am using NSNotificationCenter to tell them to execute those code.
But I want to try NSOperation and I have no idea HOW to implement that.
Do I have to put my search requests in an operation or every method I'm using?
Can someone give me a sample code to give me the idea how this should be done?
Thanks in advance...
NSOperation is very useful. To use it you extend NSOperation and override the "main" method.
In the main method you do your calculations/web request etc. So NSOperation is best for tasks you can wrap into a few simple steps, after each step you test if everything is good and either continue to the next step or cancel the operation. Once this is done you can simply instantiate your custom NSOperation and hand it off to a NSOperationQueue object and it will take care of the threading, starting, stopping cleaning up etc.
In the example below I have written a protocol to handle the completion of the task, I would advise you take this approach instead of using notification - unless you have multiple objects that needs to be notified instantly.
Make a new class that extends the NSOperation class:
//This object takes a "searchTerm" and waits to be "started".
#import <Foundation/Foundation.h>
#protocol ISSearchOperationDelegate
- (void) searchDataReady:(NSArray*) searchResult;
#end
#interface ISSearchOperation : NSOperation {
id <ISSearchOperationDelegate> delegate;
NSString *searchTerm;
}
#property(nonatomic, retain) NSString *searchTerm;
#property(nonatomic, assign) id delegate;
- (id) initWithSearchTerm:(NSString*) searchString;
#end
When an object extending NSOperation is added to an NSOperationQueue, the queue object
tries to call a "main" method on the NSOperation, you must therefore wrap your task in this method.
(notice that after each completed sub-task I test if it went well and "return" if not. The NSOperation class
has a property called isCancelled This property can be set by the NSOperationQueue, so you must also
test if that has been set during your completion of main. So to recap, you test from the inside of main if each step went as you wanted and you test if something on the outside has cancelled your task.):
- (id) initWithSearchTerm:(NSString*) searchString {
if (self = [super init]) {
[self setSearchTerm:searchString];
}
return self;
}
- (void) main {
[self performSelector:#selector(timeOut) withObject:nil afterDelay:4.0];
if ([self isCancelled]) return;
NSData *resultData = [self searchWebServiceForString:self.searchTerm];
if (resultData == nil) return;
if ([self isCancelled]) return;
NSArray *result = [self parseJSONResult:resultData];
if ([self isCancelled]) return;
if (result == nil) return;
[NSObject cancelPreviousPerformRequestsWithTarget:self];
[delegate performSelectorOnMainThread:#selector(searchDataReady:) withObject:result waitUntilDone:YES];
}
//I have not copied the implementation of all the methods I call during main, but I hope you understand that they are just "tasks" that each must be successfully completed before the next sub-task can be computed.
So first of I put a timeout test in there, then I get my data from the web service and then I parse it.
Ok to get all this going you need a queue.
So in the class you want to be the delegate for this operation you do this:
somewhere set up a queue:
NSOperationQueue *q = [[NSOperationQueue alloc] init];
[self setQueue:q];
[q release];
- (void) doSearch:(NSString*) searchString {
[queue cancelAllOperations];
ISSearchOperation *searchOperation = [[ISSearchOperation alloc] initWithSearchTerm:searchString];
[searchOperation setDelegate:self];
[queue addOperation:searchOperation]; //this makes the NSOperationQueue call the main method in the NSOperation
[searchOperation release];
}
//the delegate method called from inside the NSOperation
- (void) searchDataReady:(NSArray*) results {
//Data is here!
}
Some of the advantages with NSOperations is that from the caller point of view, we simply make an object, set a delegate, wait for the reply. But behind the scenes a series of threaded tasks that can be cancelled at any time is run, and in a manner that can handle if threaded stuff fails.
As you can see in the doSearch method it starts out by canceling any previous operations, I did this in an app where I would search a web service each time a user typed a letter in a word. That means that if the user searched for "hello world" - I would do a search for "h", then "he", then "hel", then hell", then "hello" etc.
I wanted to stop and clean up the "h" task as soon as the user typed the "e", because it was then obsolete.
I found out NSOperation was the only way that gave the responsiveness of threading and none of the mess that usually comes with spawning many threads on top of each other.
Hope you can use it to get started:)
I am checking network reachability in applicationDidFinishLaunching:
[self performSelectorInBackground:#selector(performReachabilityCheck) withObject:nil];
Background thread
-(void)performReachabilityCheck{
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
internetReach = [[Reachability reachabilityForInternetConnection] retain];
[internetReach startNotifer];
[self updateInterfaceWithReachability: internetReach];
[pool release]; pool = nil;
}
I'm not sure why my app fails to launch in time?
Is [self updateInterfaceWithReachability: internetReach]; correctly updating the UI in the main thread? If not, that could be a problem.
Otherwise, I would suggest you make sure that your applicationDidFinishLaunching: is correctly returning quickly as you expect.
Another thing to try is to break into the debugger as the app is firing up but before it has failed to launch. Check the backtrace and make sure the main event loop is in a sensible state (as it sounds like it isn't).
In my iPhone app I need to connect to a web server as this can take some time I'm using threads like this:
[NSThread detachNewThreadSelector:#selector(sendStuff) toTarget:self withObject:nil];
- (void)sendStuff {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
//Need to get the string from the textField to send to server
NSString *myString = self.textField.text;
//Do some stuff here, connect to web server etc..
[pool release];
}
On the row where I use self.textField I get a warning in console saying:
void _WebThreadLockFromAnyThread(bool), 0x5d306b0: Obtaining the web lock from a thread other than the main thread or the web thread. UIKit should not be called from a secondary thread.
How can I use the textField without getting this error?
It depends a little bit on what you want to do with the textField. If reading the value is the only thing, you can do:
[NSThread detachNewThreadSelector:#selector(sendStuff) toTarget:self withObject:self.textField.text];
- (void)sendStuff:(NSString*)myString {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
//Do someting with myString
[pool release];
}
If you want to change a value on the textField you could:
[self.textField performSelectorOnMainThread:#selector(setText:) withObject:#"new Text"];
Perform any selectors that handle UI updates on the main thread. You can do this with the NSObject method -performSelectorOnMainThread:withObject:waitUntilDone:
Why not:
[NSThread detachNewThreadSelector: #selector(sendStuff:) toTarget: self withObject: self.textField.text];
?
This is indeed unsafe behaviour. The MainThread is the only one that should interface the UI. Have your thread return for instance a string to the mainthread and have a method there update the UI. You can do for instance do this by passing a selector to the other thread method, and then have the other thread call the selector on the mainthread.
[self performSelectorOnMainThread:#selector(myMethod) withObject:nil waitUntilDone:NO];
will work
I'm having a problem when trying to add a UITabBar as a subview of my AppDelegate's window.
The link above shows a screenshot of the messy state of the screen.
TabBarInAMessyState.png
The results are unpredictable. In this picture only the UITabBarItem's titles were affected, but sometimes the TabBar background is not shown (consequently we can see the window's background). Sometimes the NavigationBar is also affected (not show in this picture).
When I start the Application I first have to check if there's network connection, so It is called a method (verifyNetworkAvailability:) that will run in a thread different from the main thread. This is done in order not to freeze the application.
- (void)applicationDidFinishLaunching:(UIApplication *)application {
[window makeKeyAndVisible];
// check if there's network connection in another thread
[NSThread detachNewThreadSelector: #selector(verifyNetworkAvailability:) toTarget:self withObject:self];
}
- (void) verifyNetworkAvailability:(MyAppDelegate*) appDelegate {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
// Check if there's network connection..
// If so, call the verifyNetworkAvailabilityDidEnd method
[appDelegate verifyNetworkAvailabilityDidEnd];
[pool release];
}
- (void) verifyNetworkAvailabilityDidEnd {
[window addSubview:tabBarController.view];
}
I'd like to know if it is possible to add the tabBarController.view in this way (by a method call done in thread other than the main thread).
Thanks in advance
Try this
- (void) verifyNetworkAvailability:(MyAppDelegate*) appDelegate {
// other code here ...
[appDelegate performSelectorOnMainThread:#selector(verifyNetworkAvailabilityDidEnd) withObject:nil waitUntilDone:NO];
}
UIKit has some trouble when you try to access it from any thread but the main thread. Think about dispatching a notification to have your primary app thread to add the view rather than adding the view directly in your secondary thread.