Returning a variable value outside a block - Objective-C, iOS

问题

I have the code below that a an connection to an URL and fetches some header responses like http code response and final URL (for redirection case):

- (NSString *)test
{
    __block NSString *cod = @"x";
    NSString *urlString = @"http://www.google.com";
    NSURL *url = [NSURL URLWithString:urlString];
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url
                                                           cachePolicy:NSURLRequestReloadIgnoringCacheData
                                                       timeoutInterval:15.0f];

[request setHTTPMethod:@"HEAD"];

[NSURLConnection sendAsynchronousRequest:request
                                   queue:[NSOperationQueue mainQueue]
                       completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {
                           NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse*)response;
                           NSURL *resolvedURL = [httpResponse URL];
                           NSString *code = [NSString stringWithFormat:@"%ld", (long)[httpResponse statusCode]];
                           NSLog(@"%@", resolvedURL);
                           NSLog(@"%@", code);

                           cod = @"y"; // the idea was use something like 'cod = code', then return 'code' at end.. But it dont works too.
                       }];
return cod; }

As can see, I have declared cod variable as __block type and set with x value. Inside block, I've set cod with y value, but at end of method I got x value for cod. I've tried to use something like cod = code then returns cod, respecting objects type, but anything that I assign inside block, I can't get the value outside it. What am I doing wrong?

回答1:

Look at the method name:

[NSURLConnection sendAsynchronousRequest:...

Whoops! So it's asynchronous. By the time the completion block is called, your test method will have already returned. You can't return the result of an asynchronous call from a synchronous method, because it doesn't make sense. Adapt your class to be coherent with the async nature of networking operations, use callbacks, delegation, etc. All in all, redesign your code.

来源：https://stackoverflow.com/questions/15797721/returning-a-variable-value-outside-a-block-objective-c-ios