You have to create a custom Adapter class for the Spinner and overwrite the two methods getView() for the normal closed view and getDropDownView() for the drop down list view. Both methods must return a View object for a single element.

Have a look at this tutorial it might help you getting started.

I was having problem too. Rather than overriding the class, I hava an easier way to do this.

But first you need to understand the difference between the resource id in the adapter constructor, and the other one in setDropDownViewResource(...). For example,

SimpleAdapter adapter =
    new SimpleAdapter(ab.getThemedContext(), data, R.layout.actionbar_dropdown, new String[] { "EventID", "Icon" },
        new int[] { R.id.event_id, R.id.icon });

adapter.setDropDownViewResource(R.layout.actionbar_list_item);

R.layout.actionbar_dropdown is the style for spinner, and R.layout.actionbar_list_item for every single list item.

I used SimpleAdapter here, since if I use ArrayAdapter, the xml can only be a single TextView.

R.layout.actionbar_list_item contains a TextView whose id is event_id and an ImageView whose id is icon.

R.layout.actionbar_dropdown is almost exactly the same as actionbar_list_item, but the visibility of ImageView of the latter is set to GONE.

In this way every list item has a textview and an imageview, but you will only see a textview on the spinner.

Using the code of the tutorial linked by Flo, I created the following CustomSpinnerAdapter in order to show two different sets of strings, one when the items are displayed, and one when not. I hope it helps someone.

public class CustomSpinnerAdapter extends ArrayAdapter<String> {

Context mContext;
int mTextViewResourceId;
String[] mObjects;
String[] mShortNameObjects;

public CustomSpinnerAdapter(Context context, int textViewResourceId,
                            String[] objects, String[] shortNameObjects) {
    super(context, textViewResourceId, objects);
    mContext = context;
    mTextViewResourceId = textViewResourceId;
    mObjects = objects;
    mShortNameObjects = shortNameObjects;
}

@Override
public View getDropDownView(int position, View convertView,
                            ViewGroup parent) {
    LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

    TextView row = (TextView) inflater.inflate(mTextViewResourceId, parent, false);
    row.setText(mObjects[position]);

    return row;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    return getCustomView(position, convertView, parent);
}

public View getCustomView(int position, View convertView, ViewGroup parent) {
    LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

    TextView row = (TextView) inflater.inflate(mTextViewResourceId, parent, false);
    row.setText(mShortNameObjects[position]);

    return row;
}
}

And the usage inside a Fragment:

CustomSpinnerAdapter mSpinnerAdapter = new CustomSpinnerAdapter(getActivity(), R.layout.spinner_item, getResources().getStringArray(R.array.action_filter), getResources().getStringArray(R.array.action_filter_short_names));

Finally, the layout for the spinner item:

spinner_item.xml

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:textSize="18dip"
    android:gravity="left"
    android:textColor="@color/navdraw_list_item_background_default"
    android:padding="5dip" />

Only sets the dropdown view resource with your alternative layout:

ArrayAdapter<String> genderAdapter = new ArrayAdapter<>(this, R.layout.adapter_spinner_white, Constants.GENDER);
genderAdapter.setDropDownViewResource(R.layout.adapter_spinner_white_dropdown);
view.setAdapter(genderAdapter);

For me, it s only a layout with an extra padding left, because my background of spinner is a rounded drawable and require this extra space.

Just call setUpSpinner() method after getting reference to spinner

// here is setUpSpinner method

private void setupSpinner() {

    // Create adapter for spinner. The list options are from the String array it will use
    // the spinner will use the default layout
    ArrayAdapter spinnerAdapter = ArrayAdapter.createFromResource(this,
            R.array.array_dropdown_options, android.R.layout.simple_spinner_item);

    // Specify dropdown layout style - simple list view with 1 item per line
    spinnerAdapter.setDropDownViewResource(android.R.layout.simple_dropdown_item_1line);

    // Apply the adapter to the spinner
    spinner.setAdapter(spinnerAdapter);
   // spinner is referenced spinner by finViewById.
    
    spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        @Override
        public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
            String selection = (String) parent.getItemAtPosition(position);
            if (!TextUtils.isEmpty(selection)) {
                if (selection.equals(getString(R.string.item_a))) {
                    // your code for selected item whose id equals to id to R.string.item_a
                } else if (selection.equals(getString(R.string.item_b))) {
                    // your code
                } else {
                    // your code
                }
            }
        }

        // Because AdapterView is an abstract class, onNothingSelected must be defined
        @Override
        public void onNothingSelected(AdapterView<?> parent) {
            // code for nothing selected in dropdown
        }
    });
}

I made an extension of ArrayAdapter which preservers as much of its functionality as possible, just sets the long description to the selected item.

First, we need a new model for feeding the adapter. Note that toString() returns the shortDescription so that the ArrayAdapter will show short description in the dropdown:

data class DescriptiveArrayItem(val shortDescription: String, val longDescription: String) {
    override fun toString(): String {
        return shortDescription
    }
}

Second, we create our custom adapter which overrides getView() and shows the long description when the item is selected (unfortunately, a lot of fields and methods in ArrayAdapter are private so I had to copy them):

class DescriptiveArrayAdapter : ArrayAdapter<DescriptiveArrayItem> {
    private var mResource: Int
    private val mFieldId: Int
    private val mContext: Context
    private val mInflater: LayoutInflater

    constructor(context: Context, resource: Int) : super(context, resource) {
        mContext = context
        mFieldId = 0
        mInflater = LayoutInflater.from(context)
        mResource = resource
    }
    constructor(context: Context, resource: Int, textViewResourceId: Int) : super(context, resource, textViewResourceId) {
        mContext = context
        mFieldId = textViewResourceId
        mInflater = LayoutInflater.from(context)
        mResource = resource
    }
    constructor(context: Context, resource: Int, objects: Array<out DescriptiveArrayItem>) : super(context, resource, objects) {
        mContext = context
        mFieldId = 0
        mInflater = LayoutInflater.from(context)
        mResource = resource
    }
    constructor(context: Context, resource: Int, textViewResourceId: Int, objects: Array<out DescriptiveArrayItem>) : super(context, resource, textViewResourceId, objects) {
        mContext = context
        mFieldId = textViewResourceId
        mInflater = LayoutInflater.from(context)
        mResource = resource
    }
    constructor(context: Context, resource: Int, objects: List<DescriptiveArrayItem>) : super(context, resource, objects) {
        mContext = context
        mFieldId = 0
        mInflater = LayoutInflater.from(context)
        mResource = resource
    }
    constructor(context: Context, resource: Int, textViewResourceId: Int, objects: List<DescriptiveArrayItem>) : super(context, resource, textViewResourceId, objects) {
        mContext = context
        mFieldId = textViewResourceId
        mInflater = LayoutInflater.from(context)
        mResource = resource
    }

    override fun getView(position: Int, convertView: View?, parent: ViewGroup): View {
        return createViewFromResource(mInflater, position, convertView, parent, mResource)
    }

    private fun createViewFromResource(inflater: LayoutInflater, position: Int,
                                       convertView: View?, parent: ViewGroup, resource: Int): View {
        val text: TextView?
        val view: View = convertView ?: inflater.inflate(resource, parent, false)
        try {
            if (mFieldId == 0) {
                //  If no custom field is assigned, assume the whole resource is a TextView
                text = view as TextView
            } else {
                //  Otherwise, find the TextView field within the layout
                text = view.findViewById(mFieldId)
                if (text == null) {
                    throw RuntimeException("Failed to find view with ID "
                            + mContext.resources.getResourceName(mFieldId)
                            + " in item layout")
                }
            }
        } catch (e: ClassCastException) {
            Log.e("ArrayAdapter", "You must supply a resource ID for a TextView")
            throw IllegalStateException(
                    "ArrayAdapter requires the resource ID to be a TextView", e)
        }
        val item: DescriptiveArrayItem? = getItem(position)
        text.text = item?.longDescription
        return view
    }
}