Question

此方法返回将由getView方法创建的视图类型的数量。

＆＃13;

public class CustomAdapter : BaseAdapter{

		private  const int TYPE_ITEM = 0;
		private const int TYPE_SEPARATOR = 1;

		private List<string> mData;
		private TreeSet sectionHeader;
	
		private LayoutInflater mInflater;

		public CustomAdapter(Context context, List<string> Data) {
			mInflater = (LayoutInflater) context
				.GetSystemService(Context.LayoutInflaterService);
			this.mData=Data;
		}

		public void addItem(string item) {
			mData.Add(item);
			NotifyDataSetChanged();
		}

		public void addSectionHeaderItem(string item) {
			mData.Add(item);
			//sectionHeader.Add(mData.Count - 1);
			NotifyDataSetChanged();
		}

		public int getItemViewType(int position) {
			return sectionHeader.Contains(position) ? TYPE_SEPARATOR : TYPE_ITEM;
		}


		public int getViewTypeCount {
			get{ return 2; }
		}


		public override int Count {
			get {return mData.Count;}
		}


		public override Java.Lang.Object GetItem(int position) {
			return mData[position];
		}
			
		public override long GetItemId(int position) {
			return position;
		} 

		public override View GetView(int position, View convertView, ViewGroup parent) {
			ViewHolder holder = null;
			int rowType = getItemViewType(position);

			if (convertView == null) {
				holder = new ViewHolder();
				switch (rowType) {
				case TYPE_ITEM:
					convertView = mInflater.Inflate(Resource.Layout.textViewItemsSeparator, parent);
					holder.textView = (TextView) convertView.FindViewById(Resource.Id.textviewHeaderItems);
					break;
				case TYPE_SEPARATOR:
					convertView = mInflater.Inflate(Resource.Layout.textViewHeaderItems, parent);
					holder.textView = (TextView) convertView.FindViewById(Resource.Id.textviewItemsSeparator);
					break;
				}
				convertView.Tag=holder;
			} else {
				holder = (ViewHolder)convertView.Tag as ViewHolder;
			}
			holder.textView.Text=mData[position];

			return convertView;
		}

		public  class ViewHolder:Java.Lang.Object {
			public TextView textView;
		}

	}

＆＃13;

ListView lst;
		string[] items = new string[] { "Alternative Rock","Classical",...........};
		
		List<string> listItems;
		
		private CustomAdapter mAdapter;
		public override void OnCreate (Bundle savedInstanceState)
		{
			base.OnCreate (savedInstanceState);


			// Create your fragment here
		}

		public override View OnCreateView (LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
		{
			// Use this to return your custom view for this Fragment
			// return inflater.Inflate(Resource.Layout.YourFragment, container, false);


			listItems = new List<string> (items);

			return inflater.Inflate (Resource.Layout.GenerFragment, container, false);
		}

		public override void OnActivityCreated(Bundle savedInstanceState)
		{
			base.OnActivityCreated(savedInstanceState);


			lst = View.FindViewById<ListView> (Resource.Id.lstGenres);
	
			//lst.Adapter = new ArrayAdapter<string>(Activity, Resource.Layout.textViewHeaderItems,Resource.Id.textviewHeaderItems, items);
			//lst = View.FindViewById<ListView> (Resource.Id.lst_genre);

			//lst.SetAdapter(new ArrayAdapter<String>(this.Activity, Resource.Layout.GenerFragment, items));
			//mAdapter=new CustomAdapter();
			for (int i = 0; i < listItems.Count(); i++) {
				mAdapter.addItem (listItems[i]);
				if (i == 0) {
					mAdapter.addSectionHeaderItem ("Music");
				} else if(i==13) {
					mAdapter.addSectionHeaderItem ("Audio");
				}
			}
			lst.Adapter = new CustomAdapter (Activity, listItems);

＆＃13;

我花了很多时间寻找错误，但我不知道为什么它是null。虽然它从列表中获得了数据

mAdapter.addItem（listItems [i]）; - ＆GT;我在设备上调试时出现null异常。哪里不对？

Answer 1

OnActivityCreated中的

您正在引用listItems

for (int i = 0; i < listItems.Count(); i++) {

但是，listItems为空。您在OnCreateView初始化它，尚未执行。在尝试引用listItems之前，您需要确保private CustomAdapter mAdapter;已初始化。

此外，您正在尝试向mAdapter添加项目，但它从未被初始化（据我所见）

你在这里声明它，但在你将其初始化之前它将是NULL

//mAdapter=new CustomAdapter();

这是初始化，已注释掉

mAdapter.addItem (listItems[i]);

当你试图在这里引用它时，它仍然是null，并将抛出一个空引用异常

   define(MAX_SLEEP, 3); // Decide a good value for number of tries
   $sleep = 0; // Initialize value, always a good habit from C :)
   $done = false; // Sentinel value 
   $flock = new Flock; // You need to implement this class
    do {
        if (! $flock->locked()) { // We have a green light
            $flock->lock(); // Lock right away

            //DO STUFF;

            $flock->unlock(); // Release the lock so others can access
            $done = true; // Allows the loop to exit

        } else if ($sleep++ > MAX_SLEEP) { // Giving up, cannot write
            // Handle exception, there are many possibilities:
            //     Log exception and do nothing (definitely log)
            //     Force a write 
            //     See if another process has been running for too long
            //     Check for timestamp of the lock file, maybe left behind after a reboot
        } else {
            sleep(SLEEP_TIME);
        }
    } while(! $done);

＆＃34;中的错误空例外＆＃34;构造xamarin android。哪里不对？

1 个答案: