Xamarin android用进度条填充listview异步

Question

如何在 Xamarin.Android 中在 Activity 的 onCreate 上填充 ListView async？ 我知道如何在单击按钮时调用异步方法并添加进度条，但是如何对 onCreate 方法执行相同操作？ 我需要的是在加载 20.000 个项目的列表时打开活动并显示进度条。

在 Eclipse Java 中，我曾经调用 AsyncTask (onpreexecute ... doinbackgroud ...onpostexecute)，但在 Xamarin 中我不知道如何执行此操作。 有这方面的帮助或教程吗？

我使用@Yuri 提供的方法尝试了这种方式，但仍然无法正常工作：

 async protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
        SetContentView(Resource.Layout.PulledItemsList);
        ActionBar.SetDisplayHomeAsUpEnabled(true);
        ActionBar.Title = "Catalog";
        _activity = this;

        //display your activity indicator and start spin
        ProgressDialog progressdialog = new ProgressDialog(this);
        progressdialog.SetCancelable(false);
        progressdialog.Indeterminate = true;
        progressdialog.SetMessage("Loading...");
        progressdialog.SetProgressStyle(ProgressDialogStyle.Spinner);
        progressdialog.Show();
        await Task.Run(() =>
        {
            fillListView();
        })
           .ContinueWith((t) =>
         {
             RunOnUiThread(() =>
                        {
                            progressdialog.Hide();
                            progressdialog.Dismiss();
                        });
         });
 }

还有这个方法：

 async protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
        SetContentView(Resource.Layout.PulledItemsList);
        ActionBar.SetDisplayHomeAsUpEnabled(true);
        ActionBar.Title = "Catalog";
        _activity = this;
        //display your activity indicator and start spin
        ProgressDialog progressdialog = new ProgressDialog(this);
        progressdialog.SetCancelable(false);
        progressdialog.Indeterminate = true;
        progressdialog.SetMessage("Loading...");
        progressdialog.SetProgressStyle(ProgressDialogStyle.Spinner);
        progressdialog.Show();

        await Task.Run(() =>
        {
            RunOnUiThread(() =>
            {
              fillListView();
            });
        });

         progressdialog.Hide();
         progressdialog.Dismiss();

        //disable indicator, list loaded
    }

而fillListView()函数是：

 private void fillListView()
    {
        var listview = FindViewById<ListView>(Resource.Id.listview);
        List<oneimg_twolbl> ItemObjectList = new List<oneimg_twolbl>();
        oneimg_twolbl ItemObject = new oneimg_twolbl(0, "", "");
        List<Items> Itemlist = Items.SelectFromItems();
        if (Itemlist.Count == 0)
        {
            Toast.MakeText(this, GetString(Resource.String.NoItems), ToastLength.Long);
        }
        foreach (Items current_item in Itemlist)
        {
            ItemObject = new oneimg_twolbl(Resource.Drawable.StockQty, current_item.ItemBarcode.ToString().Trim(), current_item.DisplayField.ToString().Trim(), current_item.ItemNum);
            ItemObjectList.Add(ItemObject);
        }
        adapter = new PulledItemList_Adapter(this, ItemObjectList);
        listview.Adapter = adapter;


        listview.ItemClick += delegate (object sender, AdapterView.ItemClickEventArgs args)
        {
            Intent i = new Intent(this, typeof(ManageItemsActivity));
            oneimg_twolbl clicked_Item = adapter.GetItem_bypos(args.Position);
            Items selected_Item = Items.SelectWhereFromItems(clicked_Item.Id);
            i.PutExtra("parameter1", ManageItemsActivity.EDITMODE);
            i.PutExtra("parameter2", selected_Item.ItemCode);
            i.PutExtra("parameter3", selected_Item.ItemNum);
            i.PutExtra("Displayfield", selected_Item.DisplayField);
            StartActivity(i);
        };
    }

适配器类：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using Android.App;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Views;
using Android.Widget;

namespace SocrateMobile.Droid.Adapter
{
class PulledItemList_Adapter : BaseAdapter<oneimg_twolbl>
{
    private Activity context;
    private List<oneimg_twolbl> AllItemList;


    public PulledItemList_Adapter(Activity context, List<oneimg_twolbl> AllItemList)
    {
        this.AllItemList = AllItemList;
        this.context = context;
    }
    public oneimg_twolbl GetItem_bypos(int position)
    {
        return AllItemList[position];
    }
    public override oneimg_twolbl this[int position]
    {
        get { return AllItemList[position]; }
    }

    public override int Count
    {
        get { return AllItemList.Count; }
    }

    public override long GetItemId(int position)
    {
        return position;
    }

    public override View GetView(int position, View convertView, ViewGroup parent)
    {
        Holder_oneimg_twolbl holder = null;
        var view = convertView;

        if (view != null)
            holder = view.Tag as Holder_oneimg_twolbl;


        if (holder == null)
        {
            holder = new Holder_oneimg_twolbl();
            view = context.LayoutInflater.Inflate(Resource.Layout.oneimg_twolbl, null);
            holder.Text = view.FindViewById<TextView>(Resource.Id.text_list4_view);
            holder.Text2 = view.FindViewById<TextView>(Resource.Id.text_list4_view2);
            holder.Image = view.FindViewById<ImageView>(Resource.Id.image_list4_view);
            view.Tag = holder;
        }

        var current_item = AllItemList[position];


        holder.Text.Text = current_item.FirstTxt;
        holder.Text2.Text = current_item.SecondTxt;
        holder.Image.SetImageResource(current_item.FirstImg);
        holder.holderId = current_item.Id;

        if (!holder.Image.HasOnClickListeners)
        {
            holder.Image.Click += delegate
            {
                PulledItemListActivity Itemlistactivity = new                PulledItemListActivity();
                Itemlistactivity.QtyImageClick(holder.holderId);
            };
        }

        return view;
    }



    public class Holder_oneimg_twolbl : Java.Lang.Object
    {
        public TextView Text { get; set; }
        public TextView Text2 { get; set; }
        public ImageView Image { get; set; }

        public int holderId { get; set; }
    }
}
}

Answer 1

根据您的代码，这里是如何做您需要的：

在 OnCreate 调用中

        await Task.Run(() =>
        {
            fillListView();
        });

您填写的listview 函数将如下所示。我不得不注释掉您的一些代码并添加延迟进行测试。您可以恢复您的代码，但请记住，您需要在将对象放入之前设置适配器。然后它将在微调器工作时更新您的屏幕。如果您不需要，则不必调用 NotifyDataSetChanges。

private void fillListView()
        {
            var listview = FindViewById<ListView>(Resource.Id.listView1);
            List<oneimg_twolbl> ItemObjectList = new List<oneimg_twolbl>();



            //oneimg_twolbl ItemObject = new oneimg_twolbl(0, "", "");

            //List<Items> Itemlist = Items.SelectFromItems();
            //if (Itemlist.Count == 0)
            //{
            //    Toast.MakeText(this, "No items", ToastLength.Long);
            //}
            //foreach (Items current_item in Itemlist)
            //{
            //    ItemObject = new oneimg_twolbl(Resource.Drawable.Icon, current_item.ItemBarcode.ToString().Trim(), current_item.DisplayField.ToString().Trim(), current_item.ItemNum);
            //    ItemObjectList.Add(ItemObject);
            //}

            var adapter = new PulledItemList_Adapter(this, ItemObjectList);
            listview.Adapter = adapter;
            for (int i = 0; i < 10; i++)
            {
                RunOnUiThread(() =>
                {
                    ItemObjectList.Add(new oneimg_twolbl(i, i.ToString(), i.ToString()));
                    adapter.NotifyDataSetChanged();
                }); 
                Task.Delay(500).Wait();
            }



            listview.ItemClick += delegate (object sender, AdapterView.ItemClickEventArgs args)
            {
                //Intent i = new Intent(this, typeof(ManageItemsActivity));
                //oneimg_twolbl clicked_Item = adapter.GetItem_bypos(args.Position);
                //Items selected_Item = Items.SelectWhereFromItems(clicked_Item.Id);
                //i.PutExtra("parameter1", ManageItemsActivity.EDITMODE);
                //i.PutExtra("parameter2", selected_Item.ItemCode);
                //i.PutExtra("parameter3", selected_Item.ItemNum);
                //i.PutExtra("Displayfield", selected_Item.DisplayField);
                //StartActivity(i);
            };
        }