Question

I am currently using a function in my app’s didFinishLaunchingWithOptions that retrieves a file, saves it to the application directory.

I have found that when there is a weak connection the app will crash when this is happening. I read that there is a 20 second time limit Apple allows before crashing the app. Is this correct? If so, I believe this is causing my issue as the app works flawlessly with the exception of being on a very weak connection.

How could I modify my logic below to try and compensate for this?

- (void)writeJsonToFile
{   
//applications Documents dirctory path
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];

//live json data url
NSString *stringURL = @"http://link-to-my-data.json";
NSURL *url = [NSURL URLWithString:stringURL];
NSData *urlData = [NSData dataWithContentsOfURL:url];

//attempt to download live data
if (urlData)
{   
    NSString *filePath = [NSString stringWithFormat:@"%@/%@", documentsDirectory,@"data.json"];
    [urlData writeToFile:filePath atomically:YES];
}
//copy data from initial package into the applications Documents folder
else
{
    //file to write to
    NSString *filePath = [NSString stringWithFormat:@"%@/%@", documentsDirectory,@"data.json"];

    //file to copy from
    NSString *json = [ [NSBundle mainBundle] pathForResource:@"data" ofType:@"json" inDirectory:@"html/data" ];
    NSData *jsonData = [NSData dataWithContentsOfFile:json options:kNilOptions error:nil];

    //write file to device
    [jsonData writeToFile:filePath atomically:YES];
}

}

Answer 1

It’s a very bad idea to run this sort of thing on the main thread: I assume you are – basically, you’ll block the entire UI while you wait for the network operation to complete.

dataWithContentsOfURL is not a good idea for this sort of thing. It will be much better to use NSURLConnection or one of the wrapper libraries like AFNetworking, because you can handle cases like when the connection times out gracefully.

These libraries also have built-in methods to asynchronously download the data, which prevents the main UI thread from being locked.