UIApplication有个属性networkActivityIndicatorVisible，用于在UI层面上表征网络正在进行，记下了。

[UIApplication sharedApplication].networkActivityIndicatorVisible = YES; 

example:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    UIApplication* app = [UIApplication sharedApplication];
    app.networkActivityIndicatorVisible = YES;

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        // To test, if Snapshot waits until the loading indicator is hidden
        UIApplication *app = [UIApplication sharedApplication];
        app.networkActivityIndicatorVisible = NO;
    });
    return YES;
}