WP7导航 - NullReferenceException

Question

I need to navigate to a certain page the first time my app is run, to gather login details etc. I'm using IsloatedStorageSettings to save a value to determine if this is the first run of the app or not, which works fine.

My problem is actually navigating to my 'first run' page when the app is run for the first time, using NavigationService, it seems NavigationService is not created at this point so is still null. When is NavigationService created or how can I work around this?

My code (in the constructor of my main page:

if ((bool)settings["firstRun"])
 { 
    if (NavigationService != null)
    {
        NavigationService.Navigate(new Uri("/FirstRun.xaml", UriKind.Relative));
    }
    else
    {
        MessageBox.Show("Navigation service must be null?");   //always prompts
    }                
 }
else
{
   InitializeComponent();
} 

Answer 1

Peter Torr有一篇很棒的博客文章，介绍了重定向初始导航的细节，但对于用户登录，我建议您使用全屏弹出窗口或在“普通”起始页面上设置登录控件并切换可见性根据您的第一次运行条件。

Answer 2

Add in class

    private bool m_onNavigatedToCalled = false;

In ctor

   this.LayoutUpdated += new EventHandler(MainPage_LayoutUpdated);

Then in code

    void MainPage_LayoutUpdated(object sender, EventArgs e)
    {
        if (m_onNavigatedToCalled)
        {
            m_onNavigatedToCalled = false;
            Dispatcher.BeginInvoke(() =>
            {
                if (NavigationService != null)
                {
                    MessageBox.Show("Navigation not null?"); //always prompts
                }
                else
                {
                    MessageBox.Show("Navigation service must be null?");   
                } 
                //StartApp(); do all stuff here to keep the ctor lightweight
            }
            );
        }
    }