Best way to performselectoronmainthread in objective c?

Question

I'm writing a client-server app to iPhone. And I have a question about threading. When I access my online database from the device, I need to do this on a separate thread to not freeze the UI/main thread. But when responding to the data I've fetched from the database I call this method on the main thread: performSelectorOnMainThread. The thing is that this only lets me send one argument/object to the method (WithObject), sometimes I have more arguments I want to pass. and another thing about it is that I HAVE TO pass this one object. I can't pass nil, if I do the app crashes.

This is my code today.. and I'm worried that I'm using the methods and threading the wrong way.

- (IBAction)testServerAction:(id)sender { [self.imageView setHidden:YES]; [self.activityView setHidden:NO]; [self.activityView startAnimating]; dispatch_queue_t testServer = dispatch_queue_create("Test-Server-Thread", NULL); dispatch_async(testServer, ^{ if ([self.arrayWithServerConnections count] > 0) { NSString *messageToShow; if ([self testServerMethod]) { messageToShow = @"Server is working!"; [self performSelectorOnMainThread:@selector(showMessageBoxWithString:) withObject:messageToShow waitUntilDone:YES]; [self performSelectorOnMainThread:@selector(threadedUIActivityRemover:) withObject:nil waitUntilDone:YES]; }else { messageToShow = @"Server is NOT working!"; [self performSelectorOnMainThread:@selector(showMessageBoxWithString:) withObject:messageToShow waitUntilDone:YES]; [self performSelectorOnMainThread:@selector(threadedUIActivityRemover:) withObject:nil waitUntilDone:YES]; } } }); dispatch_release(testServer); } -(void)threadedUIActivityRemover:(NSString *)string { [self.imageView setHidden:NO]; [self.activityView setHidden:YES]; [self.activityView stopAnimating]; }

Is this a good way of doing this, is there anything besides performSelectorOnMainThread this you can point me to, that works better?

As you can see I pass nil to an NSString argument in this example, because I HAVE to pass something, if I don't have NSString as an arg in the method, the app crashes evan when passing nil.. Why is this?.. please make this a bit clearer for me!

//Thanks!

Yuji · Accepted Answer · 2011-08-29 16:40:32Z

10

Well, you're already using dispatch_async. Then you should just use

 dispatch_async(dispatch_get_main_queue(),^ { ... } );

from inside your background thread to perform things on the main thread. For example,

 if ([self testServerMethod]) { dispatch_async(dispatch_get_main_queue(),^ { [self showMessageBoxWithString: @"Server is working!"]; [self threadedUIActivityRemover:nil]; } ); }else ...

It doesn't have any constraint on the number of arguments for the methods you call.

answered Aug 29, 2011 at 16:40

Yuji

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4 Comments

Objective Coder Over a year ago

This worked liked a charm and was EXACLY what I was requesting.. performSelectorOnMainThread seemed a little bit to constraint to me! This is great.. Is there anything I should be careful about when switching between threads this way? is there any time I shouldn't use async etc.?

Yuji Over a year ago

You know, in the olden days we only had detachNewThreadSelector and performSelectorOnMainThread. Now we have dispatch_* and NSOperationQueue. Everything is easier now. Usually async just works, if you don't have to do anything in the background thread after you update the UI.

Objective Coder Over a year ago

ok, glad I'm not from the olden days then =). this seems to work very smoothly. Thanks a bunch for the help! I'll read some documentation get a better understanding of how dispatch_* works.. Thanks!

gheese Over a year ago

i had problems using dispatch_async(dispatch_get_main_queue() ( never worked out exactly why ) but the UI would hang, but it worked with performSelectorOnMainThread

Peter DeWeese · Accepted Answer · 2012-08-02 13:05:05Z

2

Pass a collection, such as a dictionary of unarchived objects.

edited Aug 2, 2012 at 13:05

Peter DeWeese

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answered Aug 29, 2011 at 16:41

justin

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2 Comments

Objective Coder Over a year ago

Yes, I can do that.. But, it seems.. weird. Why can't there be a performSelectorOnMainThread without having to pass anything.. One more thing. I can't perform a selector on my delegate from this method either.. I have to call a method in self. This also means that I have to write a lot of methods for just this. Methods that only are there to be called from this method.

justin Over a year ago

you can pass nil as an argument to performSelectorOnMainThread - if the app crashes, it's not a result of the implementation in Foundation.framework.

Peter DeWeese · Accepted Answer · 2012-08-02 13:08:49Z

1

You can also use an NSInvocation.

edited Aug 2, 2012 at 13:08

Peter DeWeese

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answered Aug 29, 2011 at 16:43

justin

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1 Comment

justin Over a year ago

GIYF - this was one of the top results: stackoverflow.com/questions/2963933/…

justin · Accepted Answer · 2011-08-29 16:46:52Z

since you are passing instance variables, another option would be to pass self and make self thread safe.

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