IOS- Use to Try/Catch for load Json - objective-c

Question

I'm working load data from the server, but sometimes the server is not connected (error).
I wanna use to Try/Catch or something to avoid error app
1: try/cactch at load data
2: try/catch at image
I don't know how to use
I write code is:
#try
{
dispatch_async(htvque, ^{
NSData* data = [NSData dataWithContentsOfURL: [NSURL URLWithString:listChannel]];
NSError* error;
jsonTable = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&error];
if (error) {
NSLog(#"%#", error);
}
else
{
NSMutableArray *arrImage = [jsonTable objectForKey:#"ListImage"];
for (int i =0; i<arrImage.count; i++) {
UIImage * result;
UIImageView *imgView = [[UIImageView alloc] init];
imgView.frame = CGRectMake(0, 30 * i , 20, 20);
imgView.image = result;
#try
{
NSData * data = [NSData dataWithContentsOfURL:[NSURL URLWithString: [arrImage objectAtIndex:i]]];
result = [UIImage imageWithData:data];
[self.view addSubview:imgView];
}
#catch
{}
#finally
{}
}
}
});
}
#catch(NSException * exp)
{
NSLOG(#"abc");
}
#finnaly
{
}

Answer 1

Don't use exceptions for this purpose for your ObjC programs.
In ObjC, just reserve use of exceptions for cases where:
you do not intend to recover
and when you do not intend to recover
-- if an exception is even appropriate there (I just don't use them, but that's a little too 'core for a lot of people).
Anyways - an exception in ObjC indicates a programmer error and is logically something you cannot expect to recover from (unlike other languages). Figure out what your error is instead of "try and swallow" error handling.
The remedy I suggest is to create a new question, which shows the appropriate code, and details the exception, how to reproduce it, etc.
Note: There is actually a handful of oddball Cocoa APIs which will throw in less than exceptional cases, when they should have just used another approach to error handling, such as NSError. The vast majority of Cocoa exceptions you will see in development are problems which you should and can correct (range error, does not respond to selector, reference counting).

Answer 2

I think this will be a reasonably robust way to do what you want. I wanted the example to show lots f error checking without relying on try/catch
dispatch_async(htvque, ^{
NSError* error = nil;
NSData* data = [NSData dataWithContentsOfURL:[NSURL URLWithString:listChannel] options:0 error:&error];
if (error) { NSLog(#"%#", error); return; }
NSDictionary *jsonTable = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&error];
if (error) { NSLog(#"%#", error); return; }
if (![jsonTable isKindOfClass:[NSDictionary class]]) { NSLog(#"jsonTable is not an NSDictionary: %#", jsonTable); return; }
NSArray *images = [jsonTable objectForKey:#"ListImage"];
if (![images isKindOfClass:[NSArray class]]) { NSLog(#"images is not an NSArray: %#", images); return; }
NSMutableArray *dataForImages = [NSMutableArray arrayWithCapacity:images.count];
// Build up an array with all the image data. For simplicity sake, I'll just skip ones the fail to load.
for (NSString *URLString in images) {
if (![URLString isKindOfClass:[NSString class]]) { NSLog(#"URLString is not an NSString: %#", URLString); continue; }
NSData* data = [NSData dataWithContentsOfURL:[NSURL URLWithString:URLString] options:0 error:&error];
if (error) { NSLog(#"%#", error); continue; }
[dataForImages addObject:data];
}
// MUST SWITCH TO MAIN QUEUE BEFORE UPDATING UI!!!!!!!
dispatch_sync(dispatch_get_main_queue(), ^{
// This is just a different way of iterating the array.
[dataForImages enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
UIImage *image = [UIImage imageWithData:obj];
if (!image) { NSLog(#"Could not create image from data at index %d", idx); return; }
UIImageView *imageView = [[UIImageView alloc] initWithImage:image];
imageView.frame = CGRectMake(0, 30 * idx , 20, 20);
[self.view addSubview:imageView];
}];
});
});
This really shouldn't be a working solution, rather a rough outline.