RecyclerView Click, Grid & DiffUtil

class SubjectAdapter(
    private val items: List<Subject>,
    private val onClick: (Subject) -> Unit            // the screen decides what a click does
) : RecyclerView.Adapter<SubjectAdapter.VH>() {

    class VH(val binding: ItemSubjectRowBinding) : RecyclerView.ViewHolder(binding.root)

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int) =
        VH(ItemSubjectRowBinding.inflate(LayoutInflater.from(parent.context), parent, false))

    override fun onBindViewHolder(holder: VH, position: Int) {
        val s = items[position]
        holder.binding.tvName.text = s.name
        holder.binding.root.setOnClickListener { onClick(s) }
    }
    override fun getItemCount() = items.size
}

// Activity
binding.recyclerView.adapter = SubjectAdapter(subjects) { subject ->
    startActivity(SubjectActivity.newIntent(this, subject.name))
}

class SubjectListAdapter(private val onClick: (Subject) -> Unit) :
    ListAdapter<Subject, SubjectListAdapter.VH>(DIFF) {     // no list field – ListAdapter keeps it

    class VH(val binding: ItemSubjectRowBinding) : RecyclerView.ViewHolder(binding.root)

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int) =
        VH(ItemSubjectRowBinding.inflate(LayoutInflater.from(parent.context), parent, false))

    override fun onBindViewHolder(holder: VH, position: Int) {
        val s = getItem(position)
        holder.binding.tvName.text = s.name
        holder.binding.root.setOnClickListener { onClick(s) }
    }

    companion object {
        val DIFF = object : DiffUtil.ItemCallback<Subject>() {
            override fun areItemsTheSame(old: Subject, new: Subject) = old.name == new.name   // same row?
            override fun areContentsTheSame(old: Subject, new: Subject) = old == new          // same data?
        }
    }
}

// usage
val adapter = SubjectListAdapter { openSubject(it) }
binding.recyclerView.adapter = adapter
adapter.submitList(subjects)                      // later: adapter.submitList(newList)

package com.codingeasily.studentdiary.ui.notes

import android.view.LayoutInflater
import android.view.ViewGroup
import androidx.core.content.ContextCompat
import androidx.core.view.isVisible
import androidx.recyclerview.widget.DiffUtil
import androidx.recyclerview.widget.ListAdapter
import androidx.recyclerview.widget.RecyclerView
import com.bumptech.glide.Glide
import com.codingeasily.studentdiary.R
import com.codingeasily.studentdiary.data.Note
import com.codingeasily.studentdiary.databinding.ItemNoteBinding
import com.codingeasily.studentdiary.util.DateUtils
import java.io.File

/**
 * ListAdapter + DiffUtil: when the list changes, only the changed rows are redrawn.
 */
class NoteAdapter(
    private val onClick: (Note) -> Unit,
    private val onToggleDone: (Note) -> Unit,
    private val onToggleFavorite: (Note) -> Unit
) : ListAdapter<Note, NoteAdapter.NoteViewHolder>(DIFF) {

    inner class NoteViewHolder(val binding: ItemNoteBinding) : RecyclerView.ViewHolder(binding.root)

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): NoteViewHolder {
        val binding = ItemNoteBinding.inflate(LayoutInflater.from(parent.context), parent, false)
        return NoteViewHolder(binding)
    }

    override fun onBindViewHolder(holder: NoteViewHolder, position: Int) {
        val note = getItem(position)
        val b = holder.binding
        val ctx = b.root.context

        b.tvTitle.text = note.title
        b.tvTitle.alpha = if (note.isDone) 0.5f else 1f
        b.tvSubject.text = note.subject
        b.tvContent.text = note.content
        b.tvContent.isVisible = note.content.isNotBlank()
        b.tvDue.isVisible = note.dueAt != null
        note.dueAt?.let { b.tvDue.text = ctx.getString(R.string.due_at, DateUtils.formatShort(it)) }

        val color = when (note.priority) {
            Note.PRIORITY_HIGH -> R.color.priority_high
            Note.PRIORITY_LOW -> R.color.priority_low
            else -> R.color.priority_medium
        }
        b.priorityStrip.setBackgroundColor(ContextCompat.getColor(ctx, color))

        // Remove the old listener first, otherwise setChecked() would fire it while recycling
        b.cbDone.setOnCheckedChangeListener(null)
        b.cbDone.isChecked = note.isDone
        b.cbDone.setOnCheckedChangeListener { _, _ -> onToggleDone(note) }

        b.btnFavorite.setImageResource(if (note.isFavorite) R.drawable.ic_star else R.drawable.ic_star_outline)
        b.btnFavorite.setOnClickListener { onToggleFavorite(note) }

        b.ivThumb.isVisible = note.imagePath != null
        note.imagePath?.let { Glide.with(b.ivThumb).load(File(it)).centerCrop().into(b.ivThumb) }

        b.root.setOnClickListener { onClick(note) }
    }

    companion object {
        private val DIFF = object : DiffUtil.ItemCallback<Note>() {
            override fun areItemsTheSame(old: Note, new: Note) = old.id == new.id
            override fun areContentsTheSame(old: Note, new: Note) = old == new
        }
    }
}

// 2 columns (Student Diary – SubjectsFragment)
binding.recyclerView.layoutManager = GridLayoutManager(requireContext(), 2)

// horizontal carousel
binding.rvBanners.layoutManager = LinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false)
PagerSnapHelper().attachToRecyclerView(binding.rvBanners)   // snap one card at a time

// header that spans both columns in a grid
(binding.recyclerView.layoutManager as GridLayoutManager).spanSizeLookup =
    object : GridLayoutManager.SpanSizeLookup() {
        override fun getSpanSize(position: Int) = if (position == 0) 2 else 1
    }

// From Student Diary – NoteListFragment
val swipe = object : ItemTouchHelper.SimpleCallback(0, ItemTouchHelper.LEFT or ItemTouchHelper.RIGHT) {
    override fun onMove(rv: RecyclerView, vh: RecyclerView.ViewHolder, target: RecyclerView.ViewHolder) = false
    override fun onSwiped(vh: RecyclerView.ViewHolder, direction: Int) {
        val position = vh.bindingAdapterPosition
        if (position == RecyclerView.NO_POSITION) return
        val note = adapter.currentList[position]
        viewModel.delete(note)
        Snackbar.make(binding.root, getString(R.string.note_deleted, note.title), Snackbar.LENGTH_LONG)
            .setAction(R.string.undo) { viewModel.restore(note) }
            .show()
    }
}
ItemTouchHelper(swipe).attachToRecyclerView(binding.recyclerView)

viewModel.notes(filter).observe(viewLifecycleOwner) { list ->
    adapter.submitList(list)
    binding.emptyView.isVisible = list.isEmpty()     // "No notes here yet. Tap + to add one."
}

sealed class Row {
    data class Header(val title: String) : Row()
    data class Item(val note: Note) : Row()
}

override fun getItemViewType(position: Int) = when (getItem(position)) {
    is Row.Header -> TYPE_HEADER
    is Row.Item -> TYPE_ITEM
}

override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
    val inflater = LayoutInflater.from(parent.context)
    return if (viewType == TYPE_HEADER) HeaderVH(ItemHeaderBinding.inflate(inflater, parent, false))
           else NoteVH(ItemNoteBinding.inflate(inflater, parent, false))
}