如何在Objective-C中捕获JSON错误？

Question

Follow this tutorial: http://www.raywenderlich.com/5492/working-with-json-in-ios-5 , I make simple App like that:

#define kLatestKivaLoansURL [NSURL URLWithString: @"http://api.kivaws.org/v1/loans/search.json?status=fundraising"] 

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        NSData* data = [NSData dataWithContentsOfURL: kLatestKivaLoansURL];
        NSError* error;
        NSDictionary* json = [NSJSONSerialization JSONObjectWithData:data 
                                                             options:kNilOptions
                                                               error:&error];
        NSArray* latestLoans = [json objectForKey:@"loans"];
        NSLog(@"Error: %@",error);
        NSLog(@"loans: %@",latestLoans); 

        dispatch_async(dispatch_get_main_queue(), ^(){
            self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
            self.viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
            self.window.rootViewController = self.viewController;
            [self.window makeKeyAndVisible];
        });

    });

    return YES;
}

@end

When Network not OK or JSON link Error, I get same break: " * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'data parameter is nil"

How to catch this Error? I just want to display an Alert message, not break.

How many kind of Errors when parse JSON data?

Answer 1

You should implement a check on NSData* data whether it is nil or not? If its nil then you should not run the code line

NSDictionary* json = [NSJSONSerialization JSONObjectWithData:data 
                                                             options:kNilOptions
                                                               error:&error];

as you are trying to convert nil data into Dictionary .

You can also check network availability by implementing Reachability classes in you code. A sample application demonstrates how to use the SystemConfiguration framework to monitor the network state of an iPhone or iPod touch

http://developer.apple.com/library/ios/#samplecode/Reachability/Introduction/Intro.html