NSURLSessionTask Never Calls Back When WiFi Off

Question

When I turn off my WiFi connection and run the following code on the iPhone 6s 10.2 simulator, the callback is never executed. I expected the callback to fire fairly quickly with an error like "No Internet connection".

NSLog(@"request-start");
NSURLRequest* request = [[NSURLRequest alloc] initWithURL:[NSURL URLWithString:@"https://www.google.com"] cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:0];
task = [[NSURLSession sharedSession] dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
    NSLog(@"request-end");
}];
[task resume];

My Problem

I'm fetching data when the app first loads. If wifi is turned off, I need to show an error. If I set a timeout, it is obeyed - but it would need to be 10+ seconds and I'd rather not make them wait. I've also tried to detect the network status with reachability, but the network status is often unknown when the app is first loaded.

Answer 1

timeoutIntervalForResource

This property determines the resource timeout interval for all tasks within sessions based on this configuration. The resource timeout interval controls how long (in seconds) to wait for an entire resource to transfer before giving up. The resource timer starts when the request is initiated and counts until either the request completes or this timeout interval is reached , whichever comes first.

The default value is 7 days.

and

timeoutIntervalForRequest

Important

Any upload or download tasks created by a background session are automatically retried if the original request fails due to a timeout. To configure how long an upload or download task should be allowed to be retried or transferred, use the timeoutIntervalForResource property.

The default value is 60.

So, without your timeout set, your connection will run for 7 days.

In general an NSURLSession background session does not fail a task if something goes wrong on the wire. Rather, it continues looking for a good time to run the request and retries at that time. This continues until the resource timeout expires (that is, the value in the timeoutIntervalForResource property in the NSURLSessionConfiguration object you use to create the session). The current default for that value is one week!

Source