从适配器中的 itemclick 方法如何更改活动中的文本视图

Question

我有一个简单的活动，即 textview 和 listview。我的列表视图使用自定义适配器也在适配器中显示数据，我有 itemClick 方法。单击列表项后，我想更改文本视图。我的问题是我不知道如何从列表适配器中找到文本视图？我会尝试这样的事情：

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

但这给了我一个错误：

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

这是我的适配器：

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;
}

还有我的活动：

 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);
        }

我尝试以不同的方式执行此操作：我将 clickCount 变量发送到活动，并且在 ItemClick 方法中我尝试更改文本视图，但开始时的 clickCount var 发送到活动，但是当 ItemClick 方法完成时 clickCount 更改为null.我该如何解决这个问题？

Answer 1

您不应直接从适配器访问活动。您应该在适配器上设置一个监听器（监听器可以是 Activity）。这是基于观察者模式的。它解耦了组件并促进了可重用性。那么Activity在被调用的时候可以修改自己的Text View。

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，如果您没有在自定义适配器中做任何特别的事情，您始终可以使用内置的列表视图侦听器。

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() );
    }
});