NSThread causes UIWebView Crash - iphone

Question

i would like to explain my issue from the beginning. I am creating a app with UIWebView. Also I am capturing the screen at the moment user using the app and create a video using that image array.
I am using NSTimer to get screen shots of the screen. I worked well but i got one issue. That was when user start scrolling the web page or when user hold his touch on the screen NSTimer got pause and after he releases the touch NSTimer continue.
Here is my NSTimer used to get Screen shots of the screen.
assetWriterTimer = [NSTimer scheduledTimerWithTimeInterval:1.0f/24.0f
target:self
selector:#selector (writeSample)
userInfo:nil
repeats:YES];
I googled this issue and most of the people got same issue as i got. So they found a answer for that and that is using NSRunLoop. They said it stops pausing the NSTimer.
So I used NSRunLoop and add NSTimer into the NSRunLoop and here is the code after I did that.
NSRunLoop* runLoop = [NSRunLoop currentRunLoop];
assetWriterTimer = [NSTimer scheduledTimerWithTimeInterval:1.0f/24.0f
target:self
selector:#selector (writeSample)
userInfo:nil
repeats:YES];
[runLoop addTimer:assetWriterTimer forMode:NSRunLoopCommonModes];
[runLoop run];
This code worked well and it didnt stops pausing NSTimer. But after i used NSRunLoop My app got really really slow. Also it took lots of time to load the webpage into the WebView and scrolling also very slow.
So I started to search in google again and i found the issue. That was all those things are doing in main thread and iphone cant process every thing in same time. So people suggest to use NSThread to over come the problem. So what i did was create a NSThread and attach the process to the custom thread.
Here is my code after i apply NSThread to it.
[NSThread detachNewThreadSelector:#selector(launchTimer) toTarget:self withObject:nil];
-(void)launchTimer{
NSRunLoop* runLoop = [NSRunLoop currentRunLoop];
assetWriterTimer = [NSTimer scheduledTimerWithTimeInterval:1.0f/24.0f
target:self
selector:#selector (writeSample)
userInfo:nil
repeats:YES];
[runLoop addTimer:assetWriterTimer forMode:NSRunLoopCommonModes];
[runLoop run];
}
-(void) writeSample{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
//did some coding here to take screen shot
[pool release];
}
This solves the slowing the app issue forever and my app is running again very smoothly but it brings up serious problem now. That is happening because of using threads i guess. My custom thread and main thread getting conflict now and causes the app to crash. I searched the google for the reason. I found some questions related to this kind of problem and some people answered those and said Any UI changes need to be done in Main thread. Also UIWebView WebThread must run in main thread.
Those are some error reports i am getting,
Those errors are like some thread conflicting. But I have no idea how to solve this problem. I used my code to some native app that is not having any Web Views or Maps. So it works well with out crashing. But when i use this to app that is having WebView or Map its crashing some times and give me this errors.
I am in soo much trouble in here and If any one can give any idea or sample code to handle threads , handle thread priorities , or using NSTimer and NSRunLoop with out using NSThreads , or Overcome the app slowness without using NSThread , Any answer would be very very very helpful and I am looking forward to hear from you soon.
Thanks.

Answer 1

My guess is the code that you wrote to capture screen shots is causing the problem. A simple test would be to comment out all the code in the writeSample method and see if the app still crashes. I say this because you are right that all UI related tasks should work in the Main Thread at all times.
I would suggest to use a separate thread for timer so that your timer isn't paused and then from the writeSample method force the screen capture code to run on the main thread.
Hope this helps

Answer 2

First off:
As soon as you are using timers, you are working with run loops — as you can easily see from the reference:
Timers work in conjunction with run loops.
(That’s the first sentence of the second paragraph)
With that out of the way, to your syptoms:
Blocking a thread by capturing images, causes it to stutter.
This shouldn’t surprise you!
Moving the work off of that thread, releases the strain on the first one.
Shocking, I know!
Well, given that you didn’t show us any of the code that causes your synchronization issues, (read: crashes) this sarcastic answer is all the help I can offer you.
That and the tip to read and internalize the document I linked to above…
Edit
I just forgot to mention, this:
NSRunLoop* runLoop = [NSRunLoop currentRunLoop];
assetWriterTimer = [NSTimer scheduledTimerWithTimeInterval:blablabla];
[runLoop addTimer:assetWriterTimer forMode:NSRunLoopCommonModes];
[runLoop run];
doesn’t make much sense because…
-scheduledTimerWithTimeInterval:target:selector:userInfo:repeats:
Creates and returns a new NSTimer object and schedules it on the current run loop in the default mode.
(Which is explained in the magnificent documentation, as well.)
In essence: when you’re on your own private thread, the default mode is sufficient. You have to make sure that the run loop is actually running, though.
This is actually accomplished by the last line of that code snippet, but it effectively causes your method to not return until the timer is cancelled…