ios - 可以显示消息导致崩溃吗？

Question

I have this code:

[NSURLConnection sendAsynchronousRequest:urlRequest queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *error)
 {         
     NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;

     if ( error != nil )
     {
         // Display a message to the screen.
         UIAlertView *message = [[UIAlertView alloc] initWithTitle:@"There was a server error getting your business plan. We use a remote server to backup your work." 
                    message:@"Please make sure your phone is connected to the Internet. If the problem persists, please let us know about this." 
                delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];

         [message show];

but it executes when a NSUrlConnection returns from the server. It creates a crash on rare occasions. Is it possible? It seems like such a harmless piece of code.

Thanks!

Answer 1

Is the NSURLConnection returning a result on some odd thread? I don't know, but I suspect that UIAlertView is only meant to work on the UI thread, since it starts with UI.

 (dispatch_async(dispatch_get_main_queue(), ^{ 
    UIAlertView *message = [[UIAlertView alloc] initWithTitle:@"There was a server error getting your business plan. We use a remote server to backup your work." 
                message:@"Please make sure your phone is connected to the Internet. If the problem persists, please let us know about this." 
            delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];

     [message show];
 });)

Sorry, haven't compiled this, there's probably a typo in there somewhere.

Answer 2

If it's entering the conditional block to display the error it's not because of the UIAlert, it's because the NSURLConnection encountered an error. I would output to the console the error information so that you can see what the error is when it gets into these rare occasions and solve the issue with the NSURLConnection

Answer 3

The issue is that, you are not displaying the alertView in the main thread. All UI related code is need to be work on main thread.

I got a similar crash when I displayed the alertView on another thread.

You need to change the method like:

[NSURLConnection sendAsynchronousRequest:urlRequest queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *error)
 {         
     NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;

     if ( error != nil )
     {
         // Display a message to the screen.
        dispatch_async(dispatch_get_main_queue(), ^{

          UIAlertView *message = [[UIAlertView alloc] initWithTitle:@"There was a server error getting your business plan. We use a remote server to backup your work." 
                    message:@"Please make sure your phone is connected to the Internet. If the problem persists, please let us know about this." 
                delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];

          [message show];
       });
     }
}

or change it like:

 [NSURLConnection sendAsynchronousRequest:urlRequest queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *error)
 {         
     NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;

     if ( error != nil )
     {
         // Display a message to the screen.
          UIAlertView *message = [[UIAlertView alloc] initWithTitle:@"There was a server error getting your business plan. We use a remote server to backup your work." 
                    message:@"Please make sure your phone is connected to the Internet. If the problem persists, please let us know about this." 
                delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];
         [message performSelectorOnMainThread:@selector(show) withObject:nil waitUntillDone:NO];

     }
}