使用TableView的Xamarin IOS导航

Question

I am sure this is probably a very simple solution, but I am quite new to xamarin and c# in general. I have created a tableview, and when a cell in that view is pressed I want it to navigate to a new view, named SecondViewController(). I have the following class in my ViewController.cs

     public void changeview()
    {

           SecondViewController controller = this.Storyboard.InstantiateViewController("SecondViewController") as SecondViewController;
        this.NavigationController.PushViewController(controller, true);
    }

Then in my TableSource.cs class, I have the following code to call changeview()

   public override void RowSelected(UITableView tableView, NSIndexPath indexPath)
    {

        ViewController view = new ViewController();
        view.changeview();
        tableView.DeselectRow(indexPath, true);
    }

I get no errors when it compiles, but when it runs I get an error on the following code

   SecondViewController controller = this.Storyboard.InstantiateViewController("SecondViewController") as SecondViewController;

which reads

  System.NullReferenceException: Object reference not set to an instance of an object

why does this not work? if I use this same code within a button class it works great, to test to make sure I was calling the class correctly I changed the code within changeview() to a UIAlertView and when a cell was pressed the alertview worked. I am not sure where to go from here, possibly there is a better method for changing views for this purpose? any help is greatly appreciated!

Answer 1

You will need to have a global reference or access to the current active navigation controller of your application in order to make a push.

What you can do is to create an instance of an AppDelegate and set the Navigation Controller to a variable.

In your AppDelegate.cs

[Register ("AppDelegate")]
public partial class AppDelegate : UIApplicationDelegate
{
    //application layer declarations
    public UINavigationController navCtrl { get; set; }
    /*
    .
    .  the rest of the codes
    .
    .*/
}

In your ViewController.cs (First view) or the root view controller where you instantiated the navigation controller:

public override void ViewDidLoad() {
    AppDelegate appD = UIApplication.SharedApplication.Delegate as AppDelegate;

    // assign the UINavigationController to the variable being used to push
    // assuming that you have already initialised or instantiated a UINavigationController
    appD.navCtrl = this.NavigationController; // Your UINavigation controller should be here
    /*
    .
    . the rest of the codes
    .
    */
}

In your TableSource.cs Class.

private AppDelegate appD = UIApplication.SharedApplication.Delegate as AppDelegate;

public override void RowSelected (UITableView tableView, NSIndexPath indexPath){
    SecondViewController secondView = UIStoryboard.FromName ("Main", null).InstantiateViewController ("SecondViewController") as SecondViewController;
    appD.navCtrl.PushViewController (secondView, true);
}

This should be able to do what you want.

Answer 2

You should keep a reference of your ViewController in your TableSource.cs class as

public ViewController ParentController{ get; set;}

When initializing the data source,

tableClassObject.ParentController = this;

In Row Selected, it should be

public override void RowSelected(UITableView tableView, NSIndexPath indexPath)
    {
        tableView.DeselectRow(indexPath, true);
        ParentController.changeview();
    }

This is simplest solution.

Answer 3

       SecondViewController controller = this.Storyboard.InstantiateViewController("SecondViewController") as SecondViewController;

Do you used Storyboard Id for "SecondViewController". If not goto stroyboard add storyboard Id to your SecondViewController