Customizing a ListView's Appearance with Xamarin.Android

Artikkeli
04/26/2018

The appearance of a ListView is dictated by the layout of the rows being displayed. To change the appearance of a ListView, use a different row layout.

Built-in Row Views

There are twelve built-in Views that can be referenced using Android.Resource.Layout:

TestListItem – Single line of text with minimal formatting.
SimpleListItem1 – Single line of text.
SimpleListItem2 – Two lines of text.
SimpleSelectableListItem – Single line of text that supports single or multiple item selection (added in API level 11).
SimpleListItemActivated1 – Similar to SimpleListItem1, but the background color indicates when a row is selected (added in API level 11).
SimpleListItemActivated2 – Similar to SimpleListItem2, but the background color indicates when a row is selected (added in API level 11).
SimpleListItemChecked – Displays check marks to indicate selection.
SimpleListItemMultipleChoice – Displays check boxes to indicate multiple-choice selection.
SimpleListItemSingleChoice – Displays radio buttons to indicate mutually-exclusive selection.
TwoLineListItem – Two lines of text.
ActivityListItem – Single line of text with an image.
SimpleExpandableListItem – Groups rows by categories, and each group can be expanded or collapsed.

Each built-in row view has a built in style associated with it. These screenshots show how each view appears:

The BuiltInViews/HomeScreenAdapter.cs sample file (in the BuiltInViews solution) contains the code to produce the non-expandable list item screens. The view is set in the GetView method like this:

view = context.LayoutInflater.Inflate(Android.Resource.Layout.SimpleListItem1, null);

The view's properties can then be set by referencing the standard control identifiers Text1, Text2 and Icon under Android.Resource.Id (do not set properties that the view does not contain or an exception will be thrown):

view.FindViewById<TextView>(Android.Resource.Id.Text1).Text = item.Heading;
view.FindViewById<TextView>(Android.Resource.Id.Text2).Text = item.SubHeading;
view.FindViewById<ImageView>(Android.Resource.Id.Icon).SetImageResource(item.ImageResourceId); // only use with ActivityListItem

The BuiltInExpandableViews/ExpandableScreenAdapter.cs sample file (in the BuiltInViews solution) contains the code to produce the SimpleExpandableListItem screen. The group view is set in the GetGroupView method like this:

view = context.LayoutInflater.Inflate(Android.Resource.Layout.SimpleExpandableListItem1, null);

The child view is set in the GetChildView method like this:

view = context.LayoutInflater.Inflate(Android.Resource.Layout.SimpleExpandableListItem2, null);

The properties for the group view and the child view can then be set by referencing the standard Text1 and Text2 control identifiers as shown above. The SimpleExpandableListItem screenshot (shown above) provides an example of a one-line group view (SimpleExpandableListItem1) and a two-line child view (SimpleExpandableListItem2). Alternately, the group view can be configured for two lines (SimpleExpandableListItem2) and the child view can be configured for one line (SimpleExpandableListItem1), or both group view and child view can have the same number of lines.

Accessories

Rows can have accessories added to the right of the view to indicate selection state:

SimpleListItemChecked – Creates a single-selection list with a check as the indicator.
SimpleListItemSingleChoice – Creates radio-button-type lists where only one choice is possible.
SimpleListItemMultipleChoice – Creates checkbox-type lists where multiple choices are possible.

The aforementioned accessories are illustrated in the following screens, in their respective order:

To display one of these accessories pass the required layout resource ID to the adapter then manually set the selection state for the required rows. This line of code shows how to create and assign an Adapter using one of these layouts:

ListAdapter = new ArrayAdapter<String>(this, Android.Resource.Layout.SimpleListItemChecked, items);

The ListView itself supports different selection modes, regardless of the accessory being displayed. To avoid confusion, use Single selection mode with SingleChoice accessories and the Checked or Multiple mode with the MultipleChoice style. The selection mode is controlled by the ChoiceMode property of the ListView.

Handling API Level

Earlier versions of Xamarin.Android implemented enumerations as integer properties. The latest version has introduced proper .NET enumeration types which makes it much easier to discover the potential options.

Depending on which API level you are targeting, ChoiceMode is either an integer or an enumeration. The sample file AccessoryViews/HomeScreen.cs has a block commented out if you wish to target the Gingerbread API:

// For targeting Gingerbread the ChoiceMode is an int, otherwise it is an
// enumeration.

lv.ChoiceMode = Android.Widget.ChoiceMode.Single; // 1
//lv.ChoiceMode = Android.Widget.ChoiceMode.Multiple; // 2
//lv.ChoiceMode = Android.Widget.ChoiceMode.None; // 0

// Use this block if targeting Gingerbread or lower
/*
lv.ChoiceMode = 1; // Single
//lv.ChoiceMode = 0; // none
//lv.ChoiceMode = 2; // Multiple
//lv.ChoiceMode = 3; // MultipleModal
*/

Selecting Items Programmatically

Manually setting which items are 'selected' is done with the SetItemChecked method (it can be called multiple times for multiple selection):

// Set the initially checked row ("Fruits")
lv.SetItemChecked(1, true);

The code also needs to detect single selections differently from multiple selections. To determine which row has been selected in Single mode use the CheckedItemPosition integer property:

FindViewById<ListView>(Android.Resource.Id.List).CheckedItemPosition

To determine which rows have been selected in Multiple mode you need to loop through the CheckedItemPositions SparseBooleanArray. A sparse array is like a dictionary that only contains entries where the value has been changed, so you must traverse the entire array looking for true values to know what has been selected in the list as illustrated in the following code snippet:

var sparseArray = FindViewById<ListView>(Android.Resource.Id.List).CheckedItemPositions;
for (var i = 0; i < sparseArray.Size(); i++ )
{
   Console.Write(sparseArray.KeyAt(i) + "=" + sparseArray.ValueAt(i) + ",");
}
Console.WriteLine();

Creating Custom Row Layouts

The four built-in row views are very simple. To display more complex layouts (such as a list of emails, or tweets, or contact info) a custom view is required. Custom views are generally declared as AXML files in the Resources/Layout directory and then loaded using their resource Id by a custom adapter. The view can contain any number of display classes (such as TextViews, ImageViews and other controls) with custom colors, fonts and layout.

This example differs from the previous examples in a number of ways:

Inherits from Activity , not ListActivity . You can customize rows for any ListView , however other controls can also be included in an Activity layout (such as a heading, buttons or other user interface elements). This example adds a heading above the ListView to illustrate.
Requires an AXML layout file for the screen; in the previous examples the ListActivity does not require a layout file. This AXML contains a ListView control declaration.
Requires an AXML layout file to render each row. This AXML file contains the text and image controls with custom font and color settings.
Uses an optional custom selector XML file to set the appearance of the row when it is selected.
The Adapter implementation returns a custom layout from the GetView override.
ItemClick must be declared differently (an event handler is attached to ListView.ItemClick rather than an overriding OnListItemClick in ListActivity).

These changes are detailed below, starting with creating the activity's view and the custom row view and then covering the modifications to the Adapter and Activity to render them.

Adding a ListView to an Activity Layout

Because HomeScreen no longer inherits from ListActivity it doesn't have a default view, so a layout AXML file must be created for the HomeScreen's view. For this example, the view will have a heading (using a TextView) and a ListView to display data. The layout is defined in the Resources/Layout/HomeScreen.axml file which is shown here:

<?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">
    <TextView android:id="@+id/Heading"
        android:text="Vegetable Groups"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:background="#00000000"
        android:textSize="30dp"
        android:textColor="#FF267F00"
        android:textStyle="bold"
        android:padding="5dp"
    />
    <ListView android:id="@+id/List"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:cacheColorHint="#FFDAFF7F"
    />
</LinearLayout>

The benefit of using an Activity with a custom layout (instead of a ListActivity) lies in being able to add additional controls to the screen, such as the heading TextView in this example.

Creating a Custom Row Layout

Another AXML layout file is required to contain the custom layout for each row that will appear in the list view. In this example the row will have a green background, brown text and right-aligned image. The Android XML markup to declare this layout is described in Resources/Layout/CustomView.axml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout  xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="fill_parent"
   android:layout_height="wrap_content"
   android:background="#FFDAFF7F"
   android:padding="8dp">
    <LinearLayout android:id="@+id/Text"
       android:orientation="vertical"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:paddingLeft="10dip">
        <TextView
         android:id="@+id/Text1"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:textColor="#FF7F3300"
         android:textSize="20dip"
         android:textStyle="italic"
         />
        <TextView
         android:id="@+id/Text2"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:textSize="14dip"
         android:textColor="#FF267F00"
         android:paddingLeft="100dip"
         />
    </LinearLayout>
    <ImageView
        android:id="@+id/Image"
        android:layout_width="48dp"
        android:layout_height="48dp"
        android:padding="5dp"
        android:src="@drawable/icon"
        android:layout_alignParentRight="true" />
</RelativeLayout >

While a custom row layout can contain many different controls, scrolling performance can be affected by complex designs and using images (especially if they have to be loaded over the network). See Google's article for more information on addressing scrolling performance issues.

Referencing a Custom Row View

The implementation of the custom adapter example is in HomeScreenAdapter.cs. The key method is GetView where it loads the custom AXML using the resource ID Resource.Layout.CustomView, and then sets properties on each of the controls in the view before returning it. The complete adapter class is shown:

public class HomeScreenAdapter : BaseAdapter<TableItem> {
   List<TableItem> items;
   Activity context;
   public HomeScreenAdapter(Activity context, List<TableItem> items)
       : base()
   {
       this.context = context;
       this.items = items;
   }
   public override long GetItemId(int position)
   {
       return position;
   }
   public override TableItem this[int position]
   {
       get { return items[position]; }
   }
   public override int Count
   {
       get { return items.Count; }
   }
   public override View GetView(int position, View convertView, ViewGroup parent)
   {
       var item = items[position];
       View view = convertView;
       if (view == null) // no view to re-use, create new
           view = context.LayoutInflater.Inflate(Resource.Layout.CustomView, null);
       view.FindViewById<TextView>(Resource.Id.Text1).Text = item.Heading;
       view.FindViewById<TextView>(Resource.Id.Text2).Text = item.SubHeading;
       view.FindViewById<ImageView>(Resource.Id.Image).SetImageResource(item.ImageResourceId);
       return view;
   }
}

Referencing the Custom ListView in the Activity

Because the HomeScreen class now inherits from Activity, a ListView field is declared in the class to hold a reference to the control declared in the AXML:

ListView listView;

The class must then load the Activity's custom layout AXML using the SetContentView method. It can then find the ListView control in the layout then creates and assigns the adapter and assigns the click handler. The code for the OnCreate method is shown here:

SetContentView(Resource.Layout.HomeScreen); // loads the HomeScreen.axml as this activity's view
listView = FindViewById<ListView>(Resource.Id.List); // get reference to the ListView in the layout

// populate the listview with data
listView.Adapter = new HomeScreenAdapter(this, tableItems);
listView.ItemClick += OnListItemClick;  // to be defined

Finally the ItemClick handler must be defined; in this case it just displays a Toast message:

void OnListItemClick(object sender, AdapterView.ItemClickEventArgs e)
{
   var listView = sender as ListView;
   var t = tableItems[e.Position];
   Android.Widget.Toast.MakeText(this, t.Heading, Android.Widget.ToastLength.Short).Show();
}

The resulting screen looks like this:

Customizing the Row Selector Color

When a row is touched it should be highlighted for user feedback. When a custom view specifies as background color as CustomView.axml does, it also overrides the selection highlight. This line of code in CustomView.axml sets the background to light green, but it also means there is no visual indicator when the row is touched:

android:background="#FFDAFF7F"

To re-enable the highlight behavior, and also to customize the color that is used, set the background attribute to a custom selector instead. The selector will declare both the default background color as well as the highlight color. The file Resources/Drawable/CustomSelector.xml contains the following declaration:

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_pressed="false"
  android:state_selected="false"
  android:drawable="@color/cellback" />
<item android:state_pressed="true" >
  <shape>
     <gradient
      android:startColor="#E77A26"
        android:endColor="#E77A26"
        android:angle="270" />
  </shape>
</item>
<item android:state_selected="true"
  android:state_pressed="false"
  android:drawable="@color/cellback" />
</selector>

To reference the custom selector, change the background attribute in CustomView.axml to:

android:background="@drawable/CustomSelector"

A selected row and the corresponding Toast message now looks like this:

Preventing Flickering on Custom Layouts

Android attempts to improve the performance of ListView scrolling by caching layout information. If you have long scrolling lists of data you should also set the android:cacheColorHint property on the ListView declaration in the Activity's AXML definition (to the same color value as your custom row layout's background). Failure to include this hint could result in a 'flicker' as the user scrolls through a list with custom row background colors.

Jaa