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Populating a Xamarin.Android ListView with data

  • Artikkeli

To add rows to a ListView you need to add it to your layout and implement an IListAdapter with methods that the ListView calls to populate itself. Android includes built-in ListActivity and ArrayAdapter classes that you can use without defining any custom layout XML or code. The ListActivity class automatically creates a ListView and exposes a ListAdapter property to supply the row views to display via an adapter.

The built-in adapters take a view resource ID as a parameter that gets used for each row. You can use built-in resources such as those in Android.Resource.Layout so you don't need to write your own.

Using ListActivity and ArrayAdapter<String>

The example BasicTable/HomeScreen.cs demonstrates how to use these classes to display a ListView in only a few lines of code:

[Activity(Label = "BasicTable", MainLauncher = true, Icon = "@drawable/icon")]
public class HomeScreen : ListActivity {
   string[] items;
   protected override void OnCreate(Bundle bundle)
   {
       base.OnCreate(bundle);
       items = new string[] { "Vegetables","Fruits","Flower Buds","Legumes","Bulbs","Tubers" };
       ListAdapter = new ArrayAdapter<String>(this, Android.Resource.Layout.SimpleListItem1, items);
   }
}

Handling row clicks

Usually a ListView will also allow the user to touch a row to perform some action (such as playing a song, or calling a contact, or showing another screen). To respond to user touches there needs to be one more method implemented in the ListActivity – OnListItemClick – like this:

Screenshot of a SimpleListItem

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();
}

Now the user can touch a row and a Toast alert will appear:

Screenshot of Toast that appears when a row is touched

Implementing a ListAdapter

ArrayAdapter<string> is great because of its simplicity, but it's extremely limited. However, often times you have a collection of business entities, rather than just strings that you want to bind. For example, if your data consists of a collection of Employee classes, then you might want the list to just display the names of each employee. To customize the behavior of a ListView to control what data is displayed you must implement a subclass of BaseAdapter overriding the following four items:

  • Count – To tell the control how many rows are in the data.

  • GetView – To return a View for each row, populated with data. This method has a parameter for the ListView to pass in an existing, unused row for re-use.

  • GetItemId – Return a row identifier (typically the row number, although it can be any long value that you like).

  • this[int] indexer – To return the data associated with a particular row number.

The example code in BasicTableAdapter/HomeScreenAdapter.cs demonstrates how to subclass BaseAdapter:

public class HomeScreenAdapter : BaseAdapter<string> {
   string[] items;
   Activity context;
   public HomeScreenAdapter(Activity context, string[] items) : base() {
       this.context = context;
       this.items = items;
   }
   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.SimpleListItem1, null);
       view.FindViewById<TextView>(Android.Resource.Id.Text1).Text = items[position];
       return view;
   }
}

Using a custom adapter

Using the custom adapter is similar to the built-in ArrayAdapter, passing in a context and the string[] of values to display:

ListAdapter = new HomeScreenAdapter(this, items);

Because this example uses the same row layout (SimpleListItem1) the resulting application will look identical to the previous example.

Row view re-Use

In this example there are only six items. Since the screen can fit eight, no row re-use required. When displaying hundreds or thousands of rows, however, it would be a waste of memory to create hundreds or thousands of View objects when only eight fit on the screen at a time. To avoid this situation, when a row disappears from the screen its view is placed in a queue for re-use. As the user scrolls, the ListView calls GetView to request new views to display – if available it passes an unused view in the convertView parameter. If this value is null then your code should create a new view instance, otherwise you can re-set the properties of that object and re-use it.

The GetView method should follow this pattern to re-use row views:

public override View GetView(int position, View convertView, ViewGroup parent)
{
   View view = convertView; // re-use an existing view, if one is supplied
   if (view == null) // otherwise create a new one
       view = context.LayoutInflater.Inflate(Android.Resource.Layout.SimpleListItem1, null);
   // set view properties to reflect data for the given row
   view.FindViewById<TextView>(Android.Resource.Id.Text1).Text = items[position];
   // return the view, populated with data, for display
   return view;
}

Custom adapter implementations should always re-use the convertView object before creating new views to ensure they do not run out of memory when displaying long lists.

Some adapter implementations (such as the CursorAdapter) don't have a GetView method, rather they require two different methods NewView and BindView which enforce row re-use by separating the responsibilities of GetView into two methods. There is a CursorAdapter example later in the document.

Enabling fast scrolling

Fast Scrolling helps the user to scroll through long lists by providing an additional 'handle' that acts as a scroll bar to directly access a part of the list. This screenshot shows the fast scroll handle:

Screenshot of fast-scrolling with a scroll handle

Causing the fast scrolling handle to appear is as simple as setting the FastScrollEnabled property to true:

ListView.FastScrollEnabled = true;

Adding a section index

A section index provides additional feedback for users when they are fast-scrolling through a long list – it shows which 'section' they have scrolled to. To cause the section index to appear the Adapter subclass must implement the ISectionIndexer interface to supply the index text depending on the rows being displayed:

Screenshot of H appearing above section that starts with H

To implement ISectionIndexer you need to add three methods to an adapter:

  • GetSections – Provides the complete list of section index titles that could be displayed. This method requires an array of Java Objects so the code needs to create a Java.Lang.Object[] from a .NET collection. In our example it returns a list of the initial characters in the list as Java.Lang.String .

  • GetPositionForSection – Returns the first row position for a given section index.

  • GetSectionForPosition – Returns the section index to be displayed for a given row.

The example SectionIndex/HomeScreenAdapter.cs file implements those methods, and some additional code in the constructor. The constructor builds the section index by looping through every row and extracting the first character of the title (the items must already be sorted for this to work).

alphaIndex = new Dictionary<string, int>();
for (int i = 0; i < items.Length; i++) { // loop through items
   var key = items[i][0].ToString();
   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]);
}

With the data structures created, the ISectionIndexer methods are very simple:

public Java.Lang.Object[] GetSections()
{
   return sectionsObjects;
}
public int GetPositionForSection(int section)
{
   return alphaIndexer[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;
}

Your section index titles don't need to map 1:1 to your actual sections. This is why the GetPositionForSection method exists. GetPositionForSection gives you an opportunity to map whatever indices are in your index list to whatever sections are in your list view. For example, you may have a "z" in your index, but you may not have a table section for every letter, so instead of "z" mapping to 26, it may map to 25 or 24, or whatever section index "z" should map to.