Question

我正在 xamarin android

中创建自定义列表

这是我的代码：

using System;

using Android.App;
using Android.Content;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;

namespace FinalList
{
	[Activity (Label = "FinalList", MainLauncher = true, Icon = "@drawable/icon")]
	public class MainActivity :ListActivity
	{
		string[] items;
		protected override void OnCreate (Bundle bundle)
		{  
		
			base.OnCreate (bundle);
			items = new string[] { "aBulbs","bFliuits","cFruits","dpriraji","eqsbciob","frTubers","guhVegetables","hvwert","iwxamarin","jyaaaa","kVegetables","lFruits","mVegetables","nwert","pxamarin","ryaaaa"};
			//ListAdapter = new ArrayAdapter<String> (this, Android.Resource.Layout.SimpleListItem1, items);
			ListView lv =FindViewById<ListView> (Android.Resource.Id.List);
			ListAdapter = new HomeScreenAdapter(this, items);
			ListView.FastScrollEnabled = true;
			//listview.ChoiceMode = Android.Widget.ChoiceMode.Single;
		   // listview.SetItemChecked (1, true);
			lv.SetItemChecked (3, true);
		}
		protected override void OnListItemClick(ListView l, View v, int position, long id)
		{
			var t = items[position];
			Android.Widget.Toast.MakeText(this, t, Android.Widget.ToastLength.Short).Show();
		}
	}
}

这是我的axml文件：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
   <ListView
     android:id="@+id/List"
     android:layout_height="match_parent"
     android:layout_width="match_parent"
     android:fastScrollEnabled="true"
     />
</LinearLayout>

这是我的HomeScreenAdapter文件：

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 FinalList
{
	[Activity (Label = "HomeScreenAdapter")]			
	public class HomeScreenAdapter : BaseAdapter<string>,ISectionIndexer {
		String[] items;
		Activity context;
		Dictionary<string, int> alphaIndex;
		String[] sections;
		Java.Lang.Object[] sectionsObjects;
		public HomeScreenAdapter(Activity context, string[] items) : base() {
			this.context = context;
			this.items = items;
			 alphaIndex = new Dictionary<string, int>();
			for (int i = 0; i < items.Length; i++) { // loop through items
				String s = items [i];
				String key = s.Substring (0, 1);
				key= key.ToUpper ();
				if (!alphaIndex.ContainsKey(key))
					alphaIndex.Add(key, i); // add each 'new' letter to the index
			}
		 sections = new string[alphaIndex.Keys.Count];
			alphaIndex.Keys.CopyTo(sections, 0); // convert letters list to string[]
			// Interface requires a Java.Lang.Object[], so we create one here
			 sectionsObjects = new Java.Lang.Object[sections.Length];
			for (int i = 0; i < sections.Length; i++) {
				sectionsObjects[i] = new Java.Lang.String(sections[i]);
			}
		}
		public override long GetItemId(int position)
		{
			return position;
		}
		public override string this[int position] {  
			get { return items[position]; }
		}
		public override int Count {
			get { return items.Length; }
		}
		public override View GetView(int position, View convertView, ViewGroup parent)
		{
			View view = convertView; // re-use an existing view, if one is available
			if (view == null) // otherwise create a new one
				view = context.LayoutInflater.Inflate(Android.Resource.Layout.SimpleListItemChecked, null);
			view.FindViewById<TextView>(Android.Resource.Id.Text1).Text = items[position];
			return view;
		}
		public Java.Lang.Object[] GetSections()
		{
			return sectionsObjects;
		}
		public int GetPositionForSection(int section)
		{
			return alphaIndex[sections[section]];
		}
		public int GetSectionForPosition(int position)
		{      // this method isn't called in this example, but code is provided for completeness
			int prevSection = 0;
			for (int i = 0; i < sections.Length; i++)
			{
				if (GetPositionForSection(i) > position)
				{
					break;
				}
				prevSection = i;
			}
			return prevSection;
		}


	}
}

当我构建解决方案时，我得到 null对象引用异常 它说：

object reference is not set to an instance of an object at line
lv.setitemchecked(1,true);

如何删除此例外？

Answer 1

您必须在base.OnCreate (bundle);：

之后在您的活动中夸大您的布局

SetContentView(Resource.Layout."YourLayoutName");

Answer 2

你应该使用

ListView lv =FindViewById<ListView> (Android.Resource.Id.List);
        ListAdapter = new HomeScreenAdapter(this, items);
        lv.FastScrollEnabled = true;
        //lv.ChoiceMode = Android.Widget.ChoiceMode.Single;
       // lv.SetItemChecked (1, true);
        lv.SetItemChecked (3, true);

而不是

ListView lv =FindViewById<ListView> (Android.Resource.Id.List);
        ListAdapter = new HomeScreenAdapter(this, items);
        ListView.FastScrollEnabled = true;
        //listview.ChoiceMode = Android.Widget.ChoiceMode.Single;
       // listview.SetItemChecked (1, true);
        lv.SetItemChecked (3, true);

记得setContentView（resource.layout.your_layout）;太

在xamarin android中定制listview

2 个答案: