Question

I am developing an app in which I have a table. In the table cell I have an imageview ( the images are displayed via url ) and a textview/webview. I start threads for each row to get the images in the

- (UITableViewCell *)tableView:(UITableView *)theTableView cellForRowAtIndexPath:(NSIndexPath *)indexPath 

method ( if image is not already got ) and set the text of textview/webview from an array.

The problem arises when the images are being received and I pop the view, the application crashes giving the following message:

bool _WebTryThreadLock(bool), 0x1a0670: Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now…

Now the situation becomes more weird if I don’t release the textview/webview that I added to the cell, then every thing works fine.

Edit: the crash does not happens when I replace textview/webview with a label

Hoping I am clear in my question. If any thing is confusing please comment. I need to resolve this.

Thanks,

Nikhil

Answer 1

That’s a huge mistake to use threads. Try to avoid using threads if you have other solutions !

In your case, just use an asynchronous NSURLConnection which will take care of downloading your image while not slowing your app 😉

Here is part of the code :

- (void) startDownload {
    self.activeDownload = [NSMutableData data];

    NSURLConnection *conn = [[NSURLConnection alloc] initWithRequest:
                             [NSURLRequest requestWithURL:
                              [NSURL URLWithString:@"blablabla"]] delegate:self];
    self.imageConnection = conn;
    [conn release];
}

#pragma mark -
#pragma mark Download support (NSURLConnectionDelegate)

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    [self.activeDownload appendData:data];
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    NSLog(@"ERROR DOWNLOADING");
    // Clear the activeDownload property to allow later attempts
    self.activeDownload = nil;

    // Release the connection now that it's finished
    self.imageConnection = nil;
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    NSLog(@"FINISH DOWNLOAD");

    UIImage *image = [[UIImage alloc] initWithData:self.activeDownload];
    self.activeDownload = nil;
    self.imageConnection = nil;

    //do whatever you want with your image

    [image release];
}