In this piece we want to look at android adapters as well as adapterviews.

ListAdapter

ListAdapter is an interface that’s used to bind a ListView to its data.

This inteface resides inside the android.widget package:

package android.widget;

It derives from android.widget.Adapter, an interface that contains several signatures to be implemented by it’s implementers.

public interface ListAdapter extends Adapter {}

Majority of the times that data needs to come from a Cursor object, however it’s not mandatory. ListView can display any data as long as it’s contained in a ListAdapter.

Methods in ListAdapter

This interface has only two method signatures. The rest it inherits from the Adapter interface.

1. public boolean areAllItems Enabled() : This method clearly returns a boolean. This boolean indicates whether all items in this adapter are enabled. If true then it means that all the items in the List can be selected and clicked.
2. isEnabled(int position): This method takes the position of the item. It will return true if that item(at that position) is not a separator. Remember separators are list items that can neither be clicked nor selected.

Classes that Implement ListAdapter

ListAdapter is an interface that’s implemented by 7 classes and one interface.

All these are defined inside the android.widget package:

SpinnerAdapter

SpinnerAdapter is an interface that binds android.widget.Spinner to its data.

This interface lives inside the android.widget package:

package android.widget;

Like the ListAdapter, this interface derives majority of its methods from the android.widget.Adapter interface.

public interface SpinnerAdapter extends Adapter {}

Spinners in android show data in dropdowns. They are the equivalence of ComboBox in say Swing or WinForms.

However,because of the SpinnerAdapter, spinners are very customizable.

This is because spinner adapter allows for definition of two different views:

1. One showing the data in the spinner itself.
2. One showing the data in a dropdown list when the spinner is pressed.

Methods in SpinnerAdapter

We said SpinnerAdapter inherits all its methods from the Adapter apart from the getDropDownView().

public View getDropDownView(int position,View convertView,ViewGroup parent): This method will return a android.view.View object that will display data at the specified position in the dropdown popup.

• The position parameter is the index of the item whose view we want to return
• The convertView is the old view we are recycling. Normally you check if it’s null first before using it. You can also create a new View as well.
• The parent parameter is the parent view onto which we will attach this View.

Classes that Implement SpinnerAdapter

SpinnerAdapter is an interface that’s implemented by 6 classes and 1 interface. Some of classes are abstract but most concrete.

All these are defined inside the android.widget package:

1. BaseAdapter – A super class of common implementations for an android.widget.Adapter interface.It implements both ListAdapter and SpinnerAdapter interfaces.
2. ArrayAdapter – A BaseAdapter child which uses an array of arbitrary objects as data source.
3. CursorAdapter – An abstract BaseAdapter child used to expose data from android.database.Cursor to a ListView.
4. ResourceCursorAdapter – An abtract CursorAdapter child that provides an easy way to create views defined in an xml file.
5. SimpleAdapter – An easy BaseAdapter child used to map static data to views defined in an XML file.
6. SimpleCursorAdapter – A ResourceCursorAdapter child that provides an easy way to map columns from a cursor to TextViews or ImageViews defined in an XML file.
7. ThemedSpinnerAdapter – This is an interface that provides extensions over SpinnerAdapter and is capable of inflating drop-down views againts a different theme than normal views.

BaseAdapter

BaseAdapter as the name suggests BaseAdapter is a base adapter, or super adapter. An adapter is a class that acts as a bridge between an adapterview and the underlying data source.

By being a base adapter BaseAdapter provides a common implementation for adapters that can be used in:

1. ListView,GridView – By use of listAdapter.
2. Spinner – By use of spinnerAdapter.

BaseAdapter Details

1. BaseAdapter was introduced back in Android API level 1.

2. It’s actually an abstract class in that it has some abstract methods that need overriding. Therefore you have to either choose to implement those methods or make your child class as well.

   public abstract class BaseAdapter ..{}

3. BaseAdapter derives from java.lang.Object.

   public abstract class BaseAdapter extends Object ...{}

4. BaseAdapter implements ListAdapter and SpinnerAdapter. SpinnerAdapter is used for spinners while ListAdapter for ListViews and GridViews.

   public abstract class BaseAdapter extends Object implements ListAdapter, SpinnerAdapter{..}

BaseAdapter Children and GrandChildren

BaseAdapter has several direct and indirect sub-classes.

Android AdapterViews

AdapterView

Some types of views rely on an Adapter for the purpose of data binding.

These types of views are called AdapterViews.

AdapterView belongs to android.widget package.

package android.widget

It’s a generic class with the Adapter as the generic type.

android.widget.AdapterView<T extends android.widget.Adapter>

Examples of AdapterViews

There are three abstract classes inheriting from the AdapterView directly:

There are also many classes that do inherit from this class indirectly. Here the most popular among them:

Android AbsListView

This is an abstract class that meant to be used to implement virtualized lists(and grids,carousels,stacks etc) of items.

This class derives from AdapterView.

public abstract class AbsListView extends AdapterView<ListAdapter>

Several classes derive from this class:

AbsListView has been available since Android API Level 1 and resides in the android.widget package.

Normally to render items, you won’t use AbsListView, instead you’ll use its children like ListView and GridView.

AbsListView is just the base class that provides the necessary abstraction to hold any type of list, be it grid, carousel or stack for these views.

AbsListView’s subclasses like ListView and GridView rely on adapter for data binding. This is because AbsListView itself is an AdapterView.

Quick AbsListView Usage Examples

1. How to determine if an AbsListView can scroll vertically Up or Down.

We want to create a method that can help us determine if our abslistview widget can scroll vertically in both directions, up or down.

We’ll start by creating a method returns us a boolean value. This method takes a View object as well as an integer. This view is what we will cast to our AbsListView.

The integer on the other hand will represent the direction of scroll as follows:

• -1 – Scroll Up.
• 1 – Scroll Down.

    public static boolean canScrollVertically(View targetView, int direction) {..}

Here’s the full method:

    @SuppressWarnings("deprecation")
    public static boolean canScrollVertically(View targetView, int direction) {
        if (Build.VERSION.SDK_INT < 14) {
            if (direction < 0) {
                if (targetView instanceof AbsListView) {
                    final AbsListView absListView = (AbsListView) targetView;
                    return absListView.getChildCount() > 0
                            && (absListView.getFirstVisiblePosition() > 0
                            || absListView.getChildAt(0).getTop() < absListView.getPaddingTop());
                } else {
                    return ViewCompat.canScrollVertically(targetView, direction) || targetView.getScrollY() > 0;
                }
            }else{
                if (targetView instanceof AbsListView) {
                    final AbsListView absListView = (AbsListView) targetView;
                    if (absListView.getCount() <= 0) {
                        return false;
                    } else if (absListView.getLastVisiblePosition() < (absListView.getCount() - 1)) {
                        return true;
                    } else {
                        View lastView = absListView.getChildAt(absListView.getLastVisiblePosition() - absListView.getFirstVisiblePosition());
                        return !(lastView != null && lastView.getBottom() <= absListView.getMeasuredHeight());
                    }
                } else if (targetView instanceof ScrollView) {
                    ScrollView scrollView = (ScrollView) targetView;
                    View child = scrollView.getChildAt(0);
                    return child != null && scrollView.getScrollY() < (child.getHeight() - scrollView.getMeasuredHeight());
                } else if (targetView instanceof WebView) {
                    WebView webView = (WebView) targetView;
                    return webView.getScrollY() < (webView.getContentHeight() * webView.getScale() - webView.getMeasuredHeight());
                } else {
                    return ViewCompat.canScrollVertically(targetView, direction);
                }
            }
        }else{
            returnViewCompat.canScrollVertically(targetView,direction);
        }
    }

Then here is how we can expose that method:

    /**
     * Determine if the view can scroll up
     */
    public static boolean canScrollUp(View targetView) {
        return canScrollVertically(targetView, -1);
    }

    /**
     * Determine if the view can scroll down
     */
    public static boolean canScrollDown(View targetView) {
        return canScrollVertically(targetView, 1);
    }

AbsSpinner

AbsSpinner is the parent class for Spinner.

Spinner is a view that allows us display items in a dropdown manner, allowing user pick a single item at a time.

Spinner relies on abstractions defined in this AbsSpinner class, inheriting several concrete and abstract methods.

The fact that this class has abstract methods means it’s also abstract.

public abstract class AbsSpinner...{}

This class is an adapterview. So it’s children will rely on an Adapter for binding of data. Specifically a SpinnerAdapter.

public abstract class AbsSpinner extends AdapterView<SpinnerAdapter>

This class like other adapterviews is defined in the android.widget package. It’s existed since the first release of Android, API Level 1.

You won’t use this class in your day to day projects since it’s a base class meant to provide abstraction to the Spinner class. Instead you’ll use Spinner.

This class has 3 children:

Android AdapterViewAnimator

This is a base class for some adapterviews to help in implementation of animations when switching from one view to another.

This class is abstract and derives from AdapterView.

public abstract class AdapterViewAnimator
extends AdapterView<Adapter>...{}

This class has existed since API 11.

Furthermore this class implements android.widget.Adanceable interface. Advanceable interface allows our AdapterViewAnimator to progress through its set of children .

AdapterViewAnimator has two direct children: