# BaseAdapter with Android ListView
ListView is a ViewGroup that displays a list of vertically scrollable items. The list items are automatically inserted into the list using an `adapter` that is connected to a source, such as an array or a database query, and each item is converted into a row in the ListView.

## BaseAdapter
BaseAdapter, as it's name implies, is the base class for so many concrete adapter implementations on Android. It is abstract and therefore, cannot be directly instantiated.

## Using the BaseAdapter
To use the BaseAdapter with a ListView, a concrete implementation the `BaseAdepter` class that implements the following methods must be created:
* `int getCount()`
* `Object getItem(int position)`
* `long getItemId(int position)`
* `View getView(int position, View convertView, ViewGroup parent)`

Before we create our custom `BaseAdapter` implementation, we need to create the layout for the ListView row and also a model for the items in the ListView.

### Create model class
Each of our ListView rows will conatain an `Item name` and `Item description`, so our Model class is as follows:

**Item.java**

public class Item {
    private String itemName;
    private String itemDescription;
		
    public Items(String name, String description) {
	this.itemName = name;
	this.itemDescription = description;
    }
		
    public String getItemName() {
	return this.itemName;
    }
		
    public String getItemDescription() {
	return itemDescription;
    }
		
}

### ListView row layout
The xml file for the rows of the listview created in the `res/layout` folder is shown below: **layout_list_view_row_items.xml**

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <TextView
        android:id="@+id/text_view_item_name"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <TextView
        android:id="@+id/text_view_item_description"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />
</LinearLayout>

### Create the custom BaseAdapter implementation

public class CustomListAdapter extends BaseAdapter {
    private Context context; //context
    private ArrayList<Item> items; //data source of the list adapter
	
    //public constructor 
    public CustomListAdapter(Context context, ArrayList<Item> items) {
        this.context = context;
        this.items = items;
    }

    @Override
    public int getCount() {
        return items.size(); //returns total of items in the list
    }

    @Override
    public Object getItem(int position) {
        return items.get(position); //returns list item at the specified position
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        //inflate the layout for each list row
        if (convertView == null) {
            convertView = LayoutInflater.from(context).inflate(R.layout.layout_list_view_row_items, parent, false);
            
        }
		
	//get current item to be displayed
        Item currentItem = (Item) getItem(position);
    
	//get the TextView for item name and item description
	TextView textViewItemName = (TextView) convertView.findViewById(R.id.text_view_item_name);
        TextView textViewItemDescription = (TextView) convertView.findViewById(R.id.text_view_item_description);
        
	//sets the text for item name and item description from the current item object
	textViewItemName.setText(currentItem.getItemName());
	textViewItemDescription.setText(currentItem.getItemDescription());
	
	// returns the view for the current row
        return convertView;
    }
}

### Using the custom Adapter
The adapter can simply be used by instantiating it with the required paramters and set as the listview's adapter.

//create the data source
ArrayList<Item> itemsArrayList = generateItemsList(); // calls function to get items list
	
//instantiate the custom list adapter
CustomListAdapter adapter = new CustomListAdapter(this, itemsArrayList);
	
//get the ListView and attach the adapter
ListView itemsListView  = (ListView)findViewById(R.id.list_view_items);
itemsListView.setAdapter(adapter);

### ListView Optimization
To optimize the listview's performance, a new row layout should be inflated only when `convertView == null`. This is because the adapter's `getView` method is called whenever the listview needs to show a new row on the screen. The `convertview` gets recycled in this process. Hence, the row layout should be inflated just once when `convertview == null`, and it contents should be updated on subsequent `getView` calls, instead of inflating a new row on each call which is very expensive.

### Optimization using the ViewHolder pattern
One of the most common operation in Android is finding an inner view inside an inflated layout. This is usually done using the `findViewById` method. This operation is not trivial especially when the lisview has to frequently call `getView` when the list is scrolling.

The aim of the ViewHolder pattern, is to reduce the number of `findViewById` calls in the adapter's `getView`. A ViewHolder is practically a lightweight inner class that holds direct references to all inner views from a row. It is then stored as a **tag** in the row's view after inflating it. This way you’ll only have to use findViewById() when the layout is first created. Here’s a new implementation of our adapter's `getView` with View Holder pattern applied:

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    ViewHolder viewHolder;

    if (convertView == null) {
        convertView = LayoutInflater.from(context).inflate(R.layout.layout_list_view_row_items, parent, false);
        viewHolder = new ViewHolder(convertView);
        convertView.setTag(viewHolder);
    } else {
        viewHolder = (ViewHolder) convertView.getTag();
    }

    Item currentItem = (Item) getItem(position);
    viewHolder.itemName.setText(currentItem.getItemName());
    viewHolder.itemDescription.setText(currentItem.getItemDescription());

    return convertView;
}

private class ViewHolder {
    TextView itemName;
    TextView itemDescription;

    public ViewHolder(View view) {
        itemName = (TextView)view.findViewById(R.id.text_view_item_name);
        itemDescription = (TextView) view.findViewById(R.id.text_view_item_description);
    }
}

The sample code for this tutorial can be found in this [github repo](https://github.com/mayojava/ListViewWithBaseAdapter).

## References
* [BaseAdapter Documentation](https://developer.android.com/reference/android/widget/BaseAdapter.html)
* [http://www.pcsalt.com/android/listview-using-baseadapter-android/](http://www.pcsalt.com/android/listview-using-baseadapter-android/)
* [http://lucasr.org/2012/04/05/performance-tips-for-androids-listview/](http://lucasr.org/2012/04/05/performance-tips-for-androids-listview/)