recyclerview.md

RecyclerView

• android:clipToPadding="false" removes the top margin of the screen disappears as you scroll down the screen.
  • Search https://developer.android.com/codelabs/basic-android-kotlin-training-internet-images#4 for android:clipToPadding="false" to find a good visual explanation.
• findViewHolderForItemId(id: Long) returns a ViewHolder of the given ID.
  • This method is available only if RecyclerView.Adapter.hasStableIds() is true.
• findViewHolderForAdapterPosition(position: Int) returns a ViewHolder at the given adapter position.
• findViewHolderForLayoutPosition(position: Int) returns a ViewHolder at the given layout position.
• setHasFixedSize(true)
  • should be set if a vertical RecyclerView has android:layout_height="match_parent".
  • should NOT be set if a vertical RecyclerView has android:layout_height="wrap_content".
  • setHasFixedSize(true) refers to the entire size of RecyclerView, not each child item.
• setItemAnimator(null) removes animation.
• smoothScrollToPosition(position: Int) smooth-scrolls to the given adapter position.
• addOnItemTouchListener(...) intercepts touch events.
• addOnScrollListener(...) intercepts scroll events.
  • You may want to pass SimpleOnItemTouchListener instead of OnItemTouchListener if you don't want to override all the methods of the latter.

Position

Layout position

• is the position of a ViewHolder in the latest layout calculation, which is updated every 16 milliseconds.
• is the position of a ViewHolder the user sees.

Adapter position

• is the position of a ViewHolder, which is updated when the adapter content is updated.
• is the position of a ViewHolder, which may not be reflected to UI yet.
• should be used in the click listener of a ViewHolder.

Layout position and adapter position are inconsistent only during the following short period.

1. The adapter content is updated.
2. The Adapter.notify*() is called.
3. The next frame is rendered, which will occur in 16 milliseconds.

How to scroll to top after adapter.submitList(...)

adapter.registerAdapterDataObserver(
    object : RecyclerView.AdapterDataObserver() {
        override fun onItemRangeInserted(positionStart: Int, itemCount: Int) {
            binding.recyclerView.scrollToPosition(0)
        }
    }
)

RecyclerView.Adapter

• adapts the data so that it can be displayed in a ViewHolder.
• onCreateViewHolder(parent: ViewGroup, viewType: Int)
  • takes a layout file and creates a ViewHolder.
  • ViewHolder(s) are created only as many as necessary for screen.
• onBindViewHolder(holder: ViewHolder, position: Int)
  • binds an item to a ViewHolder.
  • whose analogy is to replace a person in a moving vehicle in Disneyland attractions.
  • position should NOT be used in a click listener because position is only the snapshot of a position when binding occurs. Use getBindingAdapterPosition() instead.
  • It is recommended to call ViewDataBinding.executePendingBindings() at the end of onBindViewHolder() if data binding is used. Otherwise the size of the ViewHolder may not be correctly measured before it is rendered, so re-rendering will occur.
    • https://developer.android.com/topic/libraries/data-binding/generated-binding#dynamic_variables
• notifyDataSetChanged() should be called after ListAdapter.submitList(...) or UI does not change.
• getItemId(position: Int) returns the ID of an item at a given position.
  • This method is available only if RecyclerView.Adapter.hasStableIds() is true.
• setHasStableIds(hasStableIds: Boolean) takes true when each item has an ID.

RecyclerView.ViewHolder

• is created by RecyclerView.Adapter.onCreateViewHolder(...) only as many as necessary for screen.
• contains an item by RecyclerView.Adapter.onBindViewHolder(...).
• As you scroll, a ViewHolder that goes off screen will be moved to the opposite end of the screen to display a new item.
• getBindingAdapterPosition()
  • returns an adapter position.
  • returns RecyclerView.NO_POSITION between notifyDataSetChanged() and the new frame has not been rendered. FYI, frames are rendered every 16 milliseconds.
• getItemId() returns the ID of an item bound to the ViewHolder.
• getAbsoluteAdapterPosition() returns a MergeAdapter position.
• getLayoutPosition() returns a layout position.

How to save and store RecyclerView's state during a configuration change or a system-initiated process death

class MyFragment : Fragment() {
    private lateinit var binding: MyFragmentBinding

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        savedInstanceState?.getParcelable<Parcelable>(LAYOUT_MANAGER_STATE)?.let {
            binding.recyclerView.layoutManager?.onRestoreInstanceState(it)
        }
    }

    override fun onSaveInstanceState(outState: Bundle) {
        outState.putParcelable(LAYOUT_MANAGER_STATE, binding.recyclerView.layoutManager?.onSaveInstanceState())
    }

    companion object {
        private const val LAYOUT_MANAGER_STATE = "layout_manager_state"
    }
}