从另一个类Xamarin c＃填充ListView

Question

I'm trying to populate a ListView from Another class which run on another thread but I failed to do that. Here is the main Activity :

public ListView myList;
List < string>contacts = new List< string>();

    protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
        SetContentView(Resource.Layout.Control);
        myList = FindViewById<ListView>(Resource.Id.contactsList);
        myList.ItemLongClick += MyList_ItemLongClick;
    }

    public void insertContact(string ContactInfo)
    {
        myList=FindViewById<ListView>(Resource.Id.contactsList);
        contacts.Add(ContactInfo);
        ArrayAdapter<string> myArray = new ArrayAdapter<string>(Application.Context, Android.Resource.Layout.SimpleListItem1, contacts);
        list.Adapter = null;
        list.Adapter = myArray;
        myList.DeferNotifyDataSetChanged();
    }

    public void startAsync(){
        Contacts con = new Contacts();
        Thread thread = new Thread(()=> con.Start());
        thread.Start();
    }

I'm firing a method from another class to grap contacts. There it just get some info and should send it to main Activity The other class doing like

MainAct main = new MainAct();
main.insertContact("new Contact");

but It did nothing. I tried to make a method to get the listView from the main activity (like mentioned in some thread) and pass it as second paramter to add contact like :

public ListView getListView()
        {
            ListView myList = FindViewById<ListView>(Resource.Id.contactsList);
            return myList;
        }

and after than calling this line which has the ListView :

main.insertContact("new Contact", main.getListView());

but I got an exception :

Java.Lang.NullPointerException: < Timeout exceeded getting exception details>

I tried from the other class to run the line like :

main.RunOnUIThread(()=>     main.insertContact("new Contact", main.getListView()));

but got the same exception ! What did I missed ?

NOTE: If I put both classes in one class ( I mean all the code in one class ) It works without a problem with RunOnUIThread(()=> ); . but When I move the method to another class, I got the exception.

EDIT 1: When I catch the exception I got nullpointer

Exception of type 'Java.Lang.NullPointerException' was thrown.

Answer 1

1) If Contacts is your another class, I suggest you rename it as MyContacts , because there is a Contacts class,it will confused us.

2) You don't need to new MainAct , just pass the context to your another class.

---

I have tried to reproduce your question, and I add a comment for these codes:

Here is your MyContacts class:

   public class MyContacts
    {
        MainActivity mContext;
        public MyContacts(MainActivity context) {
            this.mContext = context;
        }

        public void Start() {
            mContext.insertContact("new Contact");

            mContext.insertContact("new Contact1");

            mContext.insertContact("new Contact2");
        }
    }

You MainActivity :

public class MainActivity : Activity
{
    public ListView myList;
    List<string> contacts = new List<string>();
    protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);

        // Set our view from the "main" layout resource
        SetContentView(Resource.Layout.Main);
        myList = FindViewById<ListView>(Resource.Id.contactsList);

        //we usually init the data in OnCreate method.
        startAsync();


    }
    public void insertContact(string ContactInfo)
    {
        //this is redundant, because you have Instantiated the myList in the OnCreate method.
        //myList = FindViewById<ListView>(Resource.Id.contactsList);
        contacts.Add(ContactInfo);

        //You can use this instead of Application.Context.
        ArrayAdapter<string> myArray = new ArrayAdapter<string>(this, Android.Resource.Layout.SimpleListItem1, contacts);

        myList.Adapter = myArray;

        //from your question, I can't find any information about the list property.
        //list.Adapter = null;
        //list.Adapter = myArray;

        //myList.DeferNotifyDataSetChanged();
    }

    //From your question, I can't find where have you call this method, I will call it in the OnCreate method
    public void startAsync()
    {
        MyContacts con = new MyContacts(this);
        Thread thread = new Thread(() => con.Start());
        thread.Start();
    }
}