从内部块返回值（Objective-C）

我已经尝试了几个小时，从一个块内部获取一个值，我不明白如何在完成时使用处理程序，实际上不知道该如何使用。
这是我的代码：

+ (void)downloadUserID:(void(^)(NSString *result))handler {
    //Now redirect to assignments page

    __block NSMutableString *returnString = [[NSMutableString alloc] init]; //'__block' so that it has a direct connection to both scopes, in the method AND in the block


    NSURL *homeURL = [NSURL URLWithString:@"https://mistar.oakland.k12.mi.us/novi/StudentPortal/Home/PortalMainPage"];
    NSMutableURLRequest *requestHome = [[NSMutableURLRequest alloc] initWithURL:homeURL];
    [requestHome setHTTPMethod:@"GET"]; // this looks like GET request, not POST

    [NSURLConnection sendAsynchronousRequest:requestHome queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *homeResponse, NSData *homeData, NSError *homeError) {
         // do whatever with the data...and errors
         if ([homeData length] > 0 && homeError == nil) {
             NSError *parseError;
             NSDictionary *responseJSON = [NSJSONSerialization JSONObjectWithData:homeData options:0 error:&parseError];
             if (responseJSON) {
                 // the response was JSON and we successfully decoded it

                 //NSLog(@"Response was = %@", responseJSON);
             } else {
                 // the response was not JSON, so let's see what it was so we can diagnose the issue

                 returnString = (@"Response was not JSON (from home), it was = %@", [[NSMutableString alloc] initWithData:homeData encoding:NSUTF8StringEncoding]);
                 //NSLog(returnString);
             }
         }
         else {
             //NSLog(@"error: %@", homeError);
         }
    }];
    //NSLog(@"myResult: %@", [[NSString alloc] initWithData:myResult encoding:NSUTF8StringEncoding]);
    handler(returnString);
}

- (void)getUserID {
    [TClient downloadUserID:^(NSString *getIt){
        NSLog([NSString stringWithFormat:@"From get userID %@", getIt]);
    }];

}

所以我想returnString从downloadUserID方法NSLog 。我首先尝试返回，然后我意识到您无法从一个块内进行返回。因此，现在我一直在尝试使用:(void(^)(NSString *result))handler来尝试从另一个类方法访问它。

所以我downloadUserID从getUserID方法中调用，并尝试记录该returnString字符串。它只是一直保持零。它只是打印From get userID而已。

我如何访问方法returnString块内的那个downloadUserID？