2.5 Filtering RecyclerViews

Creating a Filter

In order to support filtering items in our RecyclerView, we must change a few things. Take our CustomAdapter from 2.3 Implementation of a Recycler View:

class CustomAdapter(private val dataSet: List<Book>) :
        RecyclerView.Adapter<CustomAdapter.ViewHolder>() {

    /**
     * Provide a reference to the type of views that you are using
     * (custom ViewHolder).
     */
    class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
        val bookName: TextView = view.findViewById(R.id.book_name)
        val author: TextView = view.findViewById(R.id.author)
        val publisher: TextView = view.findViewById(R.id.publisher)
    }

    // Create new views (invoked by the layout manager)
    override fun onCreateViewHolder(viewGroup: ViewGroup, viewType: Int): ViewHolder {
        // Create a new view, which defines the UI of the list item
        val view = LayoutInflater.from(viewGroup.context)
                .inflate(R.layout.book_row_item, viewGroup, false)
        return ViewHolder(view)
    }

    // Replace the contents of a view (invoked by the layout manager)
    override fun onBindViewHolder(viewHolder: ViewHolder, position: Int) {
        // Get element from your dataset at this position and replace the
        // contents of the view with that element
        viewHolder.bookName.text = dataSet[position].bookName
        viewHolder.author.text = dataSet[position].author
        viewHolder.publisher.text = dataSet[position].publisher
    }

    // Return the size of your dataset (invoked by the layout manager)
    override fun getItemCount() = dataSet.size
}

In the RecyclerView adapter, create an ArrayList with the name dataSetFiltered, and pass all the items from our original list. The rest of the adapter is now based off of this filtered list as opposed to the original data set (which should remain unchanged, acting as a source of truth for what an unfiltered list looks like):

class CustomAdapter(private val dataSet: List<Book>) :
        RecyclerView.Adapter<CustomAdapter.ViewHolder>() {
        
     private var dataSetFiltered: List<Book> = dataSet
     
     ...
     
 }  

Return the size of the dataSetFiltered as opposed to the dataSet so the right amount of items to display matches what's filtered:

class CustomAdapter(private val dataSet: List<Book>) :
        RecyclerView.Adapter<CustomAdapter.ViewHolder>() {
        
     private var dataSetFiltered: List<Book> = dataSet
     
     ...
     
     override fun getItemCount(): Int = dataSetFiltered.size
 }  

Now, in the onBindViewHolder get the item for each row from the dataSetFiltered list:

class CustomAdapter(private val dataSet: List<Book>) :
        RecyclerView.Adapter<CustomAdapter.ViewHolder>() {

     private var dataSetFiltered: List<Book> = dataSet
     
     ...
     
     // Replace the contents of a view (invoked by the layout manager)
    override fun onBindViewHolder(viewHolder: ViewHolder, position: Int) {
        // Get element from your dataset at this position and replace the
        // contents of the view with that element
        viewHolder.bookName.text = dataSetFiltered[position].bookName
        viewHolder.author.text = dataSetFiltered[position].author
        viewHolder.publisher.text = dataSetFiltered[position].publisher
    }
     
     override fun getItemCount(): Int = dataSetFiltered.size
}

Lastly, add the capabilities to filter based on some query!

Add the filter variable to your adapter:

class CustomAdapter(private val dataSet: List<Book>) :
        RecyclerView.Adapter<CustomAdapter.ViewHolder>() {

     private var dataSetFiltered: List<Book> = dataSet
     
     var filterTest: CharSequence = ""
        set(value) {
            field = value
            onFilterChange()
        }
     
     ...
     
     // Replace the contents of a view (invoked by the layout manager)
    override fun onBindViewHolder(viewHolder: ViewHolder, position: Int) {
        // Get element from your dataset at this position and replace the
        // contents of the view with that element
        viewHolder.bookName.text = dataSetFiltered[position].bookName
        viewHolder.author.text = dataSetFiltered[position].author
        viewHolder.publisher.text = dataSetFiltered[position].publisher
    }
     
     override fun getItemCount(): Int = dataSetFiltered.size
}

Next, we have to implement onFilterChange:

class CustomAdapter(private val dataSet: List<Book>) :
        RecyclerView.Adapter<CustomAdapter.ViewHolder>() {

     private var dataSetFiltered: List<Book> = dataSet
     
     var filterTest: CharSequence = ""
        set(value) {
            field = value
            onFilterChange()
        }
     
     private fun onFilterChange() {
        dataSetFiltered = if (charString.isEmpty()) {
            // There's no query, should return back the unfiltered list. 
            dataSet 
        } else {
            // The filter function returns a list containing only elements 
            // matching the given predicate. In this case, we can choose 
            // our predicate to be what we want to filter by. Since we are working
            // with books, let's allow filtering by author and title!
            dataSet.filter { book ->
                 book.bookName.contains(charString) || book.author.contains(charString)
            }
        }
        
        notifyDataSetChanged()
     }
     
     // Replace the contents of a view (invoked by the layout manager)
    override fun onBindViewHolder(viewHolder: ViewHolder, position: Int) {
        // Get element from your dataset at this position and replace the
        // contents of the view with that element
        viewHolder.bookName.text = dataSetFiltered[position].bookName
        viewHolder.author.text = dataSetFiltered[position].author
        viewHolder.publisher.text = dataSetFiltered[position].publisher
    }
     
     override fun getItemCount(): Int = dataSetFiltered.size
}

Using notifyDataSetChanged() is costly and often inefficient. We can take advantage of the DiffUtil Android utility class to make updating our RecyclerView more efficient.

Using the Filter

To use filter, we can simply set the value of the filter in our adapter:

adapter.filter = someText

We can also leverage SearchView to filter our RecyclerView by query from the user.