從適配器中的itemclick方法如何更改活動中的textview

Question

我有一個簡單的活動，即textview和listview。 我的列表視圖使用自定義適配器在我有itemClick方法的適配器中也顯示數據。 我想在單擊列表項后更改textview。 我的問題是我不知道如何從列表適配器中找到文本視圖？ 我會嘗試這樣的事情：

  TextView TeamSelect = (TextView)activity.FindViewById(Resource.Id.tvSelectPlayers);
           TeamSelect.SetText("Players", TextView.BufferType.Normal);

但這給我一個錯誤：

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

這是我的適配器：

public HomePlayersAdapter(Activity context)
    { 
        mInflater = LayoutInflater.From(context);
        mSelectedItemsIds = new SparseBooleanArray();
        this.context = context;
public override int Count            //return number of items in the list
    {
        get { return homePlayers.Count; }
    }
    public override Java.Lang.Object GetItem(int position)
    {
        return position;
    }

   public Player GetObject(int position)
    {
        return this.homePlayers.ElementAt(position);
    }
    public override long GetItemId(int position)
    {
        return position;
    }

    public override View GetView(int position, View convertView, ViewGroup parent)
     var y = (itemPos.IndexOf(position) + 1);
     var item = homePlayers[position];
        if (convertView == null || holder == null)
        {
            convertView = mInflater.Inflate(Resource.Layout.RowPlayers, null);
            holder = new ViewHolder();
            holder.playerName = convertView.FindViewById<TextView>(Resource.Id.tvRow); //set holder label with label list id in the view
            convertView.Tag = (holder);
     }
        else
        {
            holder = (ViewHolder)convertView.Tag;
        }
        holder.playerName.SetText(item.firstName + " " + item.lastName, TextView.BufferType.Normal);//set data label
        holder.playerName.TextSize = 30;

        if (clickCount >= 15)
        {
            subs = 1;
            Android.Widget.Toast.MakeText(context, "ok", Android.Widget.ToastLength.Short).Show();
        }
        else
        {
            subs = 0;
        }

        y = (itemPos.IndexOf(position) + 1);
        if (itemPos.Contains(position))
        {
            holder.playerName.SetTextColor(mInflater.Context.Resources.GetColor(Resource.Drawable.green));
            holder.playerName.SetText(y + ". " + item.firstName + " " + item.lastName, TextView.BufferType.Normal);//set data label
        }
        else
        {                holder.playerName.SetTextColor(mInflater.Context.Resources.GetColor(Resource.Drawable.white));
        }
        return convertView;
    }
   public void itemClick(int position)
    {

        if (!itemPos.Contains(position))
        {
            clickCount++;
            var selectFixtureActivity = new Intent(context, typeof(SelectPlayers));
            selectFixtureActivity.PutExtra("clickCount", clickCount);

            holder.playerName.SetTextColor(mInflater.Context.Resources.GetColor(Resource.Drawable.green));
            itemPos.Add(position);
            NotifyDataSetChanged();
            insertPlayer(position);
    }
        else
        {
            clickCount--;
            var selectFixtureActivity = new Intent(context, typeof(SelectPlayers));
            selectFixtureActivity.PutExtra("clickCount", clickCount);
            holder.playerName.SetTextColor(mInflater.Context.Resources.GetColor(Resource.Drawable.white));
            deletePlayer(position);
            int po = itemPos.IndexOf(position);
            itemPos.RemoveAt(po);
            NotifyDataSetChanged();
 class ViewHolder : Java.Lang.Object
{
    public TextView playerName;
}

我的活動：

 protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        SetContentView(Resource.Layout.SelectPlayer);

        homeListAdapter = new HomePlayersAdapter(this);
        listView = FindViewById<ListView>(Resource.Id.lvSelectPlayers);
        TextView TeamSelect = FindViewById<TextView>(Resource.Id.tvSelectPlayer);
        listView.Adapter = homeListAdapter;


        listView.ChoiceMode = ChoiceMode.Multiple;
        this.listView.ChoiceMode = ChoiceMode.Multiple;
        var nameHome = Intent.GetStringExtra("nameHome");
        var clickCount = Intent.GetStringExtra("clickCount");

        TextView homeTeam = (TextView)FindViewById(Resource.Id.tvHomeTeam);
        homeTeam.SetText(nameHome, Button.BufferType.Normal);

        this.listView.ItemClick += (sender, e) =>
        {
           homeListAdapter.itemClick(e.Position);
        }

我嘗試以不同的方式執行此操作：我將clickCount變量發送到活動中，並且在ItemClick方法中嘗試更改文本視圖，但是開始時的clickCount var被發送到了實際情況，但是當ItemClick方法完成后clickCount的值更改為null。我可以解決這個問題嗎？

Answer 1

您不應直接從適配器訪問活動。 您應該在適配器上設置一個偵聽器（該偵聽器可以是Activity）。 這基於觀察者模式。 它使組件分離，並提高了可重用性。 然后，該活動在被調用時可以修改其自己的文本視圖。

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.your_layout);
    ListView myListView = (ListView)findViewById(R.id.your_list);
    final TextView myTextView = (TextView)findViewById(R.id.your_text);
    YourAdapter adapter = new YourAdapter();
    adapter.setListener(new CustomListener() { // You write this method in your adapter
        @Override
        public void onItemClicked(Item clickedItem) {
            myListView.setText( clickedItem.toString() );
        }
    });
}

或者，如果您在自定義適配器中沒有做任何特別的事情，則可以始終使用內置的列表視圖偵聽器。

myListView.setOnItemClickListener( new OnItemClick() { // This is not onClick, but onItemClick
    @Override
    public void onItemClicked(AdapterView<?> parent, View view, int position, long id) {
        Object clickedItem = parent.getItemAtPosition(position);
        myTextView.setText( clickedItem.toString() );
    }
});