Implementing ListAdapter for a RecyclerView Fragment

Question

I've done this successfully with a normal ViewAdapter but I can't seem to get it working with a ListAdapter.

Here is my Fragment that does most of the work:

class CrimeListFragment: Fragment() {

    //Required interface for hosting activities
    interface Callbacks {
        fun onCrimeSelected(crimeId: UUID)
    }

    private var callbacks: Callbacks? = null
    private lateinit var crimeRecyclerView: RecyclerView
    private val crimeListViewModel: CrimeListViewModel by lazy {
        ViewModelProviders.of(this).get(CrimeListViewModel::class.java)
    }

    override fun onAttach(context: Context) {
        super.onAttach(context)

        callbacks = context as Callbacks?
    }

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {

        val view = inflater.inflate(R.layout.fragment_crime_list, container, false)

        crimeRecyclerView =
            view.findViewById(R.id.crime_recycler_view) as RecyclerView
        crimeRecyclerView.layoutManager = LinearLayoutManager(context)
        crimeRecyclerView.adapter = CrimeListAdapter(emptyList())

        return view
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        crimeListViewModel.crimeListLiveData.observe(
            viewLifecycleOwner,
            Observer { crimes ->
                crimes?.let {
                    Log.i(TAG, "Got crimes ${crimes.size}")
                    updateUI(crimes)
                }
            }
        )
    }

    override fun onDetach() {
        super.onDetach()

        callbacks = null
    }

    private fun updateUI(crimes: List<Crime>) {
        crimeRecyclerView.adapter = CrimeListAdapter(crimes)
    }

    companion object {
        fun newInstance(): CrimeListFragment {
            return CrimeListFragment()
        }
    }

    private inner class CrimeHolder(view: View)
        : RecyclerView.ViewHolder(view), View.OnClickListener {

        private lateinit var crime: Crime
        private val titleTextView = itemView.findViewById<TextView>(R.id.crime_title)
        private val dateTextView = itemView.findViewById<TextView>(R.id.crime_date)
        private val solvedImageView = itemView.findViewById<ImageView>(R.id.crime_solved)

        init {
            itemView.setOnClickListener(this)
        }

        fun bind(crime: Crime) {
            this.crime = crime
            titleTextView.text = crime.title
            dateTextView.text = crime.date.toString()
            solvedImageView.visibility = if(crime.isSolved) {
                View.VISIBLE
            } else {
                View.GONE
            }
        }

        override fun onClick(v: View) {
            callbacks?.onCrimeSelected(crime.id)
        }
    }

    private inner class CrimeListAdapter(var crimes: List<Crime>)
        : ListAdapter<Crime, CrimeHolder>(DiffCallback()) {

        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): CrimeHolder {
            val view =
                layoutInflater.inflate(R.layout.list_item_crime, parent, false)

            return CrimeHolder(view)
        }

        override fun onBindViewHolder(holder: CrimeHolder, position: Int) {
            holder.bind(crimes[position])
        }
    }

    private inner class DiffCallback: DiffUtil.ItemCallback<Crime>() {

        override fun areItemsTheSame(oldItem: Crime, newItem: Crime): Boolean {
            return oldItem.id == newItem.id
        }

        override fun areContentsTheSame(oldItem: Crime, newItem: Crime): Boolean {
            return oldItem == newItem
        }
    }
}

And here is the fragment's viewmodel:

class CrimeListViewModel: ViewModel() {

    private val crimeRepository = CrimeRepository.get()
    val crimeListLiveData = crimeRepository.getCrimes() //returns LiveData<List<Crime>>
}

Android documentation has this regarding ListAdapter:

While using a LiveData is an easy way to provide data to the adapter, it isn't required - you can use submitList(List) when new lists are available.

1. I'm supposed to submit a new list instead of creating a new ListAdapter object each time I update the UI. But crimeRecyclerView.adapter has no .submitList() function. So how do I pass on the new list?

2. LiveData is still new to me so I'm not quite clear on this. I already observe a LiveData stored in my viewmodel. So what do I observe this time? Or do I just add code to my existing Observer?

3. Finally when I run the code in this state, phone shows an empty RecyclerView. Only UpdateUI() gets called, none of CrimeListAdapter's functions get called. I'm not sure if this is a real problem or just the consequence of the above.

Answer 1

1.I'm supposed to submit a new list instead of creating a new ListAdapter object each time I update the UI. But crimeRecyclerView.adapter has no .submitList() function. So how do I pass on the new list?

crimeRecyclerView.adapter return RecyclerView.Adapter type

submitList() is a method of ListAdapter, a sub-class of RecyclerView.Adapter

You need to cast from super to sub class before calling that method, like this.

(crimeRecyclerView.adapter as CrimeListAdapter).submitList(crimes)

2.LiveData is still new to me so I'm not quite clear on this. I already observe a LiveData stored in my viewmodel. So what do I observe this time? Or do I just add code to my existing Observer?

Your code for this part is good, no need to do more.

3.Finally when I run the code in this state, phone shows an empty RecyclerView. Only UpdateUI() gets called, none of CrimeListAdapter's functions get called. I'm not sure if this is a real problem or just the consequence of the above.

The best part of using ListAdapter is you do not need to provide a list of data (crimes in your case) to constructor.

Back to your code, you need to change 3 things.

// crimeRecyclerView.adapter = CrimeListAdapter(emptyList())
crimeRecyclerView.adapter = CrimeListAdapter()

and

// crimeRecyclerView.adapter = CrimeListAdapter(crimes)
(crimeRecyclerView.adapter as CrimeListAdapter).submitList(crimes)

and

//private inner class CrimeListAdapter(var crimes: List<Crime>) :
//    ListAdapter<Crime, CrimeHolder>(DiffCallback()) {
//
//    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): CrimeHolder {
//        val view = layoutInflater.inflate(R.layout.list_item_crime, parent, false)
//        return CrimeHolder(view)
//    }
//
//    override fun onBindViewHolder(holder: CrimeHolder, position: Int) {
//        holder.bind(crimes[position])
//    }
//}

private inner class CrimeListAdapter : ListAdapter<Crime, CrimeHolder>(DiffCallback()) {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): CrimeHolder {
        val view = layoutInflater.inflate(R.layout.list_item_crime, parent, false)
        return CrimeHolder(view)
    }

    override fun onBindViewHolder(holder: CrimeHolder, position: Int) {
        holder.bind(getItem(position))
    }
}