Check if NSURL returns 404

问题:

I need to check whether a URL (represented by a NSURL) is available or returns 404. What is the best way to achieve that?

I would prefer a way to check this without a delegate, if possible. I need to block the program execution until I know if the URL is reachable or not.

回答1:

As you may know already that general error can capture by didFailWithError method:

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    NSLog(@"Connection failed! Error - %@ %@",
          [error localizedDescription],
          [[error userInfo] objectForKey:NSErrorFailingURLStringKey]);
}

but for 404 "Not Found" or 500 "Internal Server Error" should able to capture inside didReceiveResponse method:

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    if ([response respondsToSelector:@selector(statusCode)])
    {
        int statusCode = [((NSHTTPURLResponse *)response) statusCode];
        if (statusCode == 404)
        {
            [connection cancel];  // stop connecting; no more delegate messages
            NSLog(@"didReceiveResponse statusCode with %i", statusCode);
        }
    }
}

回答2:

I needed a solution that didn't use a delegate either, so I took pieces of code shown in other answers here and created a simple method that works well in my case (and might be what you are looking for as well):

    -(BOOL) webFileExists {

        NSString *url = @"http://www.apple.com/somefile.html";

        NSURLRequest* request = [NSURLRequest requestWithURL:[NSURL URLWithString:url] cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:5.0];
        NSHTTPURLResponse* response = nil;
        NSError* error = nil;
        [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];
        NSLog(@"statusCode = %d", [response statusCode]);

        if ([response statusCode] == 404)
            return NO;
        else
            return YES;
     }

回答3:

I used the answer from woodmantech above, but changed it based on what I have seen on other similar questions here so that it does not download the whole file to see if it exists.

I changed NSURLRequest to NSMutableURLRequest, and added:

[request setHTTPMethod:@"HEAD"];

This seems to work fine. I am working on my first app, so no real experience yet. Thanks everyone.

 NSMutableURLRequest* request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:url] cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:5.0];
    [request setHTTPMethod:@"HEAD"];
    NSHTTPURLResponse* response = nil;
    NSError* error = nil;
    [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];
    NSLog(@"statusCode = %d", [response statusCode]);

回答4:

You can achieve a synchronous connection by calling:

NSURLRequest* request = [NSURLRequest requestWithURL:[NSURL URLWithString:url]];
NSHTTPURLResponse* response = nil;
NSError* error = nil;
[NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];

Your thread will block until the request has beeen made.