show and hide a fullscreen overlay UIView with a spinner; works even in Action/Share extensions that don't have [UIApplication sharedApplication]

showSpinner.m

#define OVERLAY_VIEW_TAG 298739
-(void)showSpinner
{
    UIView *back = [[UIView alloc]initWithFrame:CGRectMake(0, 0, UIScreen.mainScreen.bounds.size.width, UIScreen.mainScreen.bounds.size.height)];
    back.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:0.6];
    back.tag = OVERLAY_VIEW_TAG;
    [self.view addSubview:back];

    UIActivityIndicatorView *spinner = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
    spinner.center = back.center;
    spinner.autoresizingMask = UIViewAutoresizingFlexibleLeftMargin|UIViewAutoresizingFlexibleRightMargin|UIViewAutoresizingFlexibleBottomMargin|UIViewAutoresizingFlexibleTopMargin;
    [back addSubview:spinner];
    [spinner startAnimating];
    self.navigationController.navigationBar.layer.zPosition = -1;
}

-(void)hideSpinner
{
    [[self.view viewWithTag:OVERLAY_VIEW_TAG] removeFromSuperview];
    self.navigationController.navigationBar.layer.zPosition = 1;
}