Android Development

Categorizing in RecyclerViews

April 3, 2018

We are all con­stant­ly cat­e­go­riz­ing bits of infor­ma­tion that we receive with­out even real­iz­ing it. This allows us to think about things prac­ti­cal­ly; such as under­stand­ing which tasks need to be com­plet­ed in the next hour, day, or week. But, when it comes to dis­play­ing infor­ma­tion in orga­nized groups on a device’s screen, archi­tec­tural­ly it can be a bit of a pain. So, how can we more effec­tive­ly group data in a RecyclerView?

Let’s make it hap­pen #

With a basic RecyclerView, we can cat­e­go­rize data by over­rid­ing func­tions in our imple­men­ta­tion of RecyclerView.Adapter. By over­rid­ing getItemViewType, we can cre­ate a dif­fer­ent type of RecyclerView.ViewHolder in our onCreateViewHolder. This is great! It allows us to eas­i­ly imple­ment many dif­fer­ent types of views/​view hold­ers all with­in the same RecyclerView. But remem­ber, with great pow­er comes great respon­si­bil­i­ty. Man­ag­ing a data struc­ture to han­dle group­ing can quick­ly become error prone as you are like­ly ref­er­enc­ing a Map<Any, List<Any>>. Ref­er­enc­ing an item at posi­tion 101 can­not be done with a sim­ple index, but instead will require iter­at­ing through the groups to count their chil­dren. In the end, you will have a basic struc­ture like this.

class SomeRecyclerAdapter: RecyclerView.Adapter<RecyclerView.ViewHolder>() {
  /*
    * Remember, your logic would likely be more complicated to handle
    * a nested data structure. You would probably want to pull this out into its own
    * function so that it could also be used here, in getItemViewType,
    * and in onBindViewHolder as well.
    */
  override fun getItemCount(): Int {
    return data.size
  }

  override fun getItemViewType(position: Int): Int {
    return when (position) {
      0 -> TYPE_HEADER
      else -> TYPE_ITEM
    }
  }

  override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
    val inflater = parent.context.layoutInflater
    return when (viewType) {
      TYPE_HEADER -> HeaderViewHolder(inflater.inflate(R.layout.header, parent, false))
      else -> ItemViewHolder(inflater.inflate(R.layout.item, parent, false))
    }
  }

  override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
    when (holder) {
      is HeaderViewHolder -> {
        // do some Header things
      }
      is ItemViewHolder -> {
        // do some Item things
      }
    }
  }
}

But wait, I want­ed ani­ma­tions #

You may be think­ing that what has been described so far is pret­ty straight­for­ward. How­ev­er, I chal­lenge you to con­sid­er how you would expand and col­lapse your cat­e­gories. If you’re like me, your answer like­ly requires you to manip­u­late your data struc­ture, pos­si­bly add new vari­ables to man­age vis­i­bil­i­ty, con­vert these changes into notifyItemRangeInserted and notifyItemRangeRemoved events, and in gen­er­al, make things complicated.

At that point, if you’re any­thing like me you will have tried a few libraries or maybe even attempt­ed to roll your own. For me, Groupie was the clear front-run­ner in terms of func­tion­al­i­ty and sim­plic­i­ty but just had one issue; the col­lapse ani­ma­tion was just not right ( full dis­clo­sure, I cre­at­ed an issue only to real­ize the error was in my code ). So, I want­ed to under­stand how it worked and hope­ful­ly, pro­vide the fix.

Let’s recon­sid­er what we would like in our code:

  • We want a data struc­ture that is easy to under­stand and manipulate.
  • We want to avoid han­dling mul­ti­ple types when creating/​binding views
  • We want to remain eas­i­ly manageable.

With that in mind, I ded­i­cat­ed a week­end to craft­ing the per­fect solu­tion. What I end­ed up with was a func­tion­al solu­tion (Mac­Grouper), and the under­stand­ing that I should prob­a­bly have looked at my own code before blam­ing Groupie for my prob­lems. The premise of Groupie, and Mac­Grouper for that mat­ter, is that a Group (or cat­e­go­ry) is real­ly just an Item that con­tains a List<Item>. When the GroupAdapter (dis­play­ing a List<Item>) reports its size, it is able to ask all of the Items for their size (and they report back 1+).

if (isExpanded) { children.size + 1 } else 1

This means your RecyclerView.Adapter only real­ly cares about Groups. A Group knows its num­ber of chil­dren and han­dles vis­i­bil­i­ty changes. This means it can make the appro­pri­ate item range change calls auto­mat­i­cal­ly for you.

The mag­ic of it all is very clean code that is easy to under­stand. For the full exam­ple refer to my gist to cre­ate an Expand­ableList­Frag­ment.

val phones = mapOf(
  "Google" to listOf(
    "Pixel",
    "Pixel XL",
    "Pixel 2",
    "Pixel 2 XL"
  ),
  "HTC" to listOf(
    "Nexus One"
  ),
  "Huawei" to listOf(
    "Nexus 6P"
  )
)

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

  val layoutManager = GridLayoutManager(activity, groupAdapter.spanCount)
  layoutManager.spanSizeLookup = groupAdapter.spanSizeLookup

  phones.forEach { (brand, phones) ->
    val brandItem = BrandItem(brand)
    val expandableGroup = ExpandableGroup(brandItem)
    phones.forEach { phone ->
      expandableGroup.add(PhoneItem(phone))
    }
    groupAdapter.add(expandableGroup)
  }

  recyclerView.layoutManager = layoutManager
  recyclerView.adapter = groupAdapter
}

All this is to say, I high­ly rec­om­mend giv­ing Groupie a shot for any­thing that might require a com­plex RecyclerView lay­out. The pro­vid­ed exam­ples are an excel­lent way to get start­ed or check out my exam­ple gist for an Expand­ableList­Frag­ment.

Scott Schmitz
Scott Schmitz
Software Developer

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