Fix #235: Change fragment implementation from directly xml code to better kotlin code.

app/src/main/java/org/oppia/app/player/exploration/ExplorationActivity.kt

@@ -12,11 +12,13 @@ const val EXPLORATION_ACTIVITY_TOPIC_ID_ARGUMENT_KEY = "ExplorationActivity.expl
 class ExplorationActivity : InjectableAppCompatActivity() {
   @Inject
   lateinit var explorationActivityPresenter: ExplorationActivityPresenter
+  var explorationId: String = ""
 
   override fun onCreate(savedInstanceState: Bundle?) {
     super.onCreate(savedInstanceState)
     activityComponent.inject(this)
-    explorationActivityPresenter.handleOnCreate()
+    explorationId = intent.getStringExtra(EXPLORATION_ACTIVITY_TOPIC_ID_ARGUMENT_KEY)
+    explorationActivityPresenter.handleOnCreate(explorationId)
   }
 
   companion object {

app/src/main/java/org/oppia/app/player/exploration/ExplorationActivityPresenter.kt

@@ -1,20 +1,24 @@
 package org.oppia.app.player.exploration
 
+import android.os.Bundle
 import androidx.appcompat.app.AppCompatActivity
 import org.oppia.app.R
 import org.oppia.app.activity.ActivityScope
-import org.oppia.app.home.HomeFragment
 import javax.inject.Inject
 
 /** The controller for [ExplorationActivity]. */
 @ActivityScope
 class ExplorationActivityPresenter @Inject constructor(private val activity: AppCompatActivity) {
-  fun handleOnCreate() {
+  fun handleOnCreate(explorationId: String) {
     activity.setContentView(R.layout.exploration_activity)
     if (getExplorationFragment() == null) {
+      val explorationFragment = ExplorationFragment()
+      val args = Bundle()
+      args.putString(EXPLORATION_ACTIVITY_TOPIC_ID_ARGUMENT_KEY, explorationId)
+      explorationFragment.arguments = args
       activity.supportFragmentManager.beginTransaction().add(
         R.id.exploration_fragment_placeholder,
-        ExplorationFragment()
+        explorationFragment
       ).commitNow()
     }
   }

app/src/main/java/org/oppia/app/player/exploration/ExplorationFragmentPresenter.kt

@@ -1,11 +1,14 @@
 package org.oppia.app.player.exploration
 
+import android.os.Bundle
 import android.view.LayoutInflater
 import android.view.View
 import android.view.ViewGroup
 import androidx.fragment.app.Fragment
+import org.oppia.app.R
 import org.oppia.app.databinding.ExplorationFragmentBinding
 import org.oppia.app.fragment.FragmentScope
+import org.oppia.app.player.state.StateFragment
 import javax.inject.Inject
 
 /** The controller for [ExplorationFragment]. */
@@ -14,6 +17,22 @@ class ExplorationFragmentPresenter @Inject constructor(
   private val fragment: Fragment
 ) {
   fun handleCreateView(inflater: LayoutInflater, container: ViewGroup?): View? {
-    return ExplorationFragmentBinding.inflate(inflater, container, /* attachToRoot= */ false).root
+    val binding = ExplorationFragmentBinding.inflate(inflater, container, /* attachToRoot= */ false).root
+
+    if (getStateFragment() == null) {
+      val stateFragment = StateFragment()
+      val args = Bundle()
+      args.putString(EXPLORATION_ACTIVITY_TOPIC_ID_ARGUMENT_KEY, fragment.arguments!!.getString(EXPLORATION_ACTIVITY_TOPIC_ID_ARGUMENT_KEY))
+      stateFragment.arguments = args
+      fragment.childFragmentManager.beginTransaction().add(
+        R.id.state_fragment_placeholder,
+        stateFragment
+      ).commitNow()
+    }
+    return binding
+  }
+
+  private fun getStateFragment(): StateFragment? {
+    return fragment.childFragmentManager.findFragmentById(R.id.state_fragment_placeholder) as StateFragment?
   }
 }

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

@@ -1,17 +1,21 @@
 package org.oppia.app.player.state
 
+import android.os.Bundle
 import android.view.LayoutInflater
 import android.view.View
 import android.view.ViewGroup
 import androidx.fragment.app.Fragment
 import androidx.lifecycle.LiveData
 import androidx.lifecycle.Observer
 import androidx.lifecycle.Transformations
+import org.oppia.app.R
 import org.oppia.app.databinding.StateFragmentBinding
 import org.oppia.app.fragment.FragmentScope
 import org.oppia.app.model.CellularDataPreference
 import org.oppia.app.model.EphemeralState
+import org.oppia.app.player.audio.AudioFragment
 import org.oppia.app.player.audio.CellularDataDialogFragment
+import org.oppia.app.player.exploration.EXPLORATION_ACTIVITY_TOPIC_ID_ARGUMENT_KEY
 import org.oppia.app.viewmodel.ViewModelProvider
 import org.oppia.domain.audio.CellularDialogController
 import org.oppia.domain.exploration.ExplorationProgressController
@@ -33,6 +37,7 @@ class StateFragmentPresenter @Inject constructor(
 
   private var showCellularDataDialog = true
   private var useCellularData = false
+  private var explorationId: String = ""
 
   fun handleCreateView(inflater: LayoutInflater, container: ViewGroup?): View? {
     cellularDialogController.getCellularDataPreference()
@@ -49,12 +54,17 @@ class StateFragmentPresenter @Inject constructor(
       it.stateFragment = fragment as StateFragment
       it.viewModel = getStateViewModel()
     }
-
+    explorationId = fragment.arguments!!.getString(EXPLORATION_ACTIVITY_TOPIC_ID_ARGUMENT_KEY)
+    getAudioFragment()
     subscribeToCurrentState()
 
     return binding.root
   }
 
+  private fun getAudioFragment(): AudioFragment? {
+    return fragment.childFragmentManager.findFragmentById(R.id.audio_fragment_placeholder) as AudioFragment?
+  }
+
   fun handleAudioClick() {
     if (showCellularDataDialog) {
       setAudioFragmentVisible(false)

app/src/main/res/layout/exploration_fragment.xml

@@ -5,16 +5,13 @@
   <androidx.constraintlayout.widget.ConstraintLayout
     android:layout_width="match_parent"
     android:layout_height="match_parent">
-
-    <fragment
-      android:id="@+id/state_fragment"
-      class="org.oppia.app.player.state.StateFragment"
+    <FrameLayout
+      android:id="@+id/state_fragment_placeholder"
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
       app:layout_constraintBottom_toBottomOf="parent"
       app:layout_constraintEnd_toEndOf="parent"
       app:layout_constraintStart_toStartOf="parent"
       app:layout_constraintTop_toTopOf="parent" />
-
   </androidx.constraintlayout.widget.ConstraintLayout>
 </layout>

app/src/main/res/layout/state_fragment.xml

@@ -14,7 +14,8 @@
   <androidx.constraintlayout.widget.ConstraintLayout
     android:layout_width="match_parent"
     android:layout_height="match_parent">
-    <LinearLayout
+    <FrameLayout
+      android:id="@+id/audio_fragment_placeholder"
       android:layout_width="0dp"
       android:layout_height="wrap_content"
       android:layout_marginStart="24dp"
@@ -23,12 +24,7 @@
       app:layout_constraintEnd_toEndOf="parent"
       app:layout_constraintStart_toStartOf="parent"
       app:layout_constraintTop_toTopOf="parent">
-      <fragment
-        android:id="@+id/audio_fragment"
-        class="org.oppia.app.player.audio.AudioFragment"
-        android:layout_width="match_parent"
-        android:layout_height="wrap_content"/>
-    </LinearLayout>
+    </FrameLayout>
     <Button
       android:id="@+id/dummy_audio_button"
       android:layout_width="wrap_content"