如果无法访问数据，iOS应用程序将崩溃

Question

I'm using the following to request data using NSJSONSerialization . The problem I'm having is that if the data is inaccessible (eg no network connection) the app crashes. How could I go about stopping the app from crashing if the network or server is down?

I'm calling [self requestData]; in the viewDidLoad: method

-(void)requestData {

    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL
                                                          URLWithString:@"http://example.com/api/nodes"]];

    NSData *response = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];

    NSError *jsonParsingError = nil;

    NSDictionary *publicData =  [NSJSONSerialization JSONObjectWithData:response
                                                                options:0
                                                                  error:&jsonParsingError];
    publicDataArray = [publicData objectForKey:@"data"];
    for(publicDataDict in publicDataArray) {
        NSLog(@"data output is %@",[publicDataDict objectForKey:@"title"]);

    }
}

thanks for any help

Answer 1

Some thoughts:

1. Use Reachability for checking network connection
2. Always use asynchronous request, else it'll block your UI till the app get the response from server.
3. Always use exception handling

Here the issue is:

You are calling a synchronous request in the viewDidLoad using sendSynchronousRequest . But the server is down, so you won't get the result, and it still expect any data to come. But your app won't load untill that request finishes. Due to this springboards application-watchdog will terminate your app.

What is Watch dog ?

watchdog — In order to keep the user interface responsive, iOS includes a watchdog mechanism. If your application fails to respond to certain user interface events (launch, suspend, resume, terminate) in time, the watchdog will kill your application and generate a watchdog timeout crash report. The amount of time the watchdog gives you is not formally documented, but it's always less than a network timeout.

Please check this Technical question on Apple site.

Answer 2

Why don't you check if [NSURLConnection sendSynchronousRequest:] got any error?

NSError *requestError = nil;
NSData *response = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:&requestError];

if (requestError)
{
    NSLog(@"sync. request failed with error: %@", requestError);
}
else
{
    // handle data
}

And you really should check if NSJSONSerialization had an error too:

NSError *jsonParsingError = nil;

NSDictionary *publicData =  [NSJSONSerialization JSONObjectWithData:response
                                                            options:0
                                                              error:&jsonParsingError];
if (jsonParsingError)
{
    NSLog(@"JSON parsing failed with error: %@", jsonParsingError);
}
else
{
    // do something
}