Fix #156 & #157: Continue/End Exploration player buttons- low-fi (#251)

* Button test implementation

* Added test cases

* Nit changes

* Added 1 more test case

* Optimised test cases

* Nit changes and KDocs

* Changes to StateFragmentPresenter

* Nit changes

* Optimised test-cases

app/src/main/java/org/oppia/app/player/state/StateAdapter.kt

@@ -0,0 +1,92 @@
+package org.oppia.app.player.state
+
+import android.view.LayoutInflater
+import android.view.ViewGroup
+import androidx.databinding.DataBindingUtil
+import androidx.databinding.ViewDataBinding
+import androidx.recyclerview.widget.RecyclerView
+import androidx.databinding.library.baseAdapters.BR
+import kotlinx.android.synthetic.main.state_button_item.view.*
+import org.oppia.app.R
+import org.oppia.app.player.state.itemviewmodel.StateButtonViewModel
+import org.oppia.app.player.state.listener.ButtonInteractionListener
+import org.oppia.app.databinding.StateButtonItemBinding
+
+@Suppress("unused")
+private const val VIEW_TYPE_CONTENT = 1
+@Suppress("unused")
+private const val VIEW_TYPE_INTERACTION_READ_ONLY = 2
+@Suppress("unused")
+private const val VIEW_TYPE_NUMERIC_INPUT_INTERACTION = 3
+@Suppress("unused")
+private const val VIEW_TYPE_TEXT_INPUT_INTERACTION = 4
+private const val VIEW_TYPE_STATE_BUTTON = 5
+
+/** Adapter to inflate different items/views inside [RecyclerView]. The itemList consists of various ViewModels. */
+class StateAdapter(
+  private val itemList: MutableList<Any>,
+  private val buttonInteractionListener: ButtonInteractionListener
+) :
+  RecyclerView.Adapter<RecyclerView.ViewHolder>() {
+
+  lateinit var stateButtonViewModel: StateButtonViewModel
+
+  override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
+    return when (viewType) {
+      // TODO(#249): Generalize this binding to make adding future interactions easier.
+      VIEW_TYPE_STATE_BUTTON -> {
+        val inflater = LayoutInflater.from(parent.context)
+        val binding =
+          DataBindingUtil.inflate<StateButtonItemBinding>(
+            inflater,
+            R.layout.state_button_item,
+            parent,
+            /* attachToParent= */false
+          )
+        StateButtonViewHolder(binding, buttonInteractionListener)
+      }
+      else -> throw IllegalArgumentException("Invalid view type") as Throwable
+    }
+  }
+
+  override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
+    when (holder.itemViewType) {
+      VIEW_TYPE_STATE_BUTTON -> {
+        (holder as StateButtonViewHolder).bind(itemList[position] as StateButtonViewModel)
+      }
+    }
+  }
+
+  override fun getItemViewType(position: Int): Int {
+    return when (itemList[position]) {
+      is StateButtonViewModel -> {
+        stateButtonViewModel = itemList[position] as StateButtonViewModel
+        VIEW_TYPE_STATE_BUTTON
+      }
+      else -> throw IllegalArgumentException("Invalid type of data $position")
+    }
+  }
+
+  override fun getItemCount(): Int {
+    return itemList.size
+  }
+
+  private class StateButtonViewHolder(
+    val binding: ViewDataBinding,
+    private val buttonInteractionListener: ButtonInteractionListener
+  ) : RecyclerView.ViewHolder(binding.root) {
+    internal fun bind(stateButtonViewModel: StateButtonViewModel) {
+      binding.setVariable(BR.buttonViewModel, stateButtonViewModel)
+      binding.root.interaction_button.setOnClickListener {
+        buttonInteractionListener.onInteractionButtonClicked()
+      }
+      binding.root.next_state_image_view.setOnClickListener {
+        buttonInteractionListener.onNextButtonClicked()
+      }
+      binding.root.previous_state_image_view.setOnClickListener {
+        buttonInteractionListener.onPreviousButtonClicked()
+      }
+      binding.executePendingBindings()
+    }
+  }
+}