How to add an action to share button in navigation bar with Xcode

I am trying to add an action to my share button in navigation bar but I don't know how and where to define my "shareAction" method. To add share button, I have the following code in viewWillAppear :

UIBarButtonItem *shareButton = [[UIBarButtonItem alloc]
                                initWithBarButtonSystemItem:UIBarButtonSystemItemAction
                                target:self
                                action:@selector(shareAction:)];
self.navigationItem.rightBarButtonItem = shareButton;

in your implementation.m file

- (void) viewWillAppear 
{
    [super viewWillAppear:animated];
    UIBarButtonItem *shareButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemAction
                                    target:self
                                    action:@selector(shareAction:)];
    self.navigationItem.rightBarButtonItem = shareButton;
}

-(void)shareAction:(id)sender
{
    NSLog(@"share action");
}