改掉Navigation Controller的back button事件

//在viewDidLoad中加入這兩行，override返回鍵的方法

self.navigationItem.backBarButtonItem.target = self;
self.navigationItem.backBarButtonItem.action = @selector(backButtonDidPressed:);

//宣告這個方法來執行你要在back前做的事

- (void)backButtonDidPressed:(id)aResponder {
    //do your stuff
    //but don't forget to dismiss the viewcontroller
    [self.navigationController popViewControllerAnimated:YES];
}