Add empty answer support

app/src/main/java/org/oppia/android/app/player/state/itemviewmodel/DragAndDropSortInteractionViewModel.kt

@@ -1,5 +1,6 @@
 package org.oppia.android.app.player.state.itemviewmodel
 
+import androidx.annotation.StringRes
 import androidx.databinding.Observable
 import androidx.databinding.ObservableField
 import androidx.recyclerview.widget.RecyclerView
@@ -13,6 +14,7 @@ import org.oppia.android.app.model.SubtitledHtml
 import org.oppia.android.app.model.TranslatableHtmlContentId
 import org.oppia.android.app.model.UserAnswer
 import org.oppia.android.app.model.WrittenTranslationContext
+import org.oppia.android.app.player.state.answerhandling.AnswerErrorCategory
 import org.oppia.android.app.player.state.answerhandling.InteractionAnswerErrorOrAvailabilityCheckReceiver
 import org.oppia.android.app.player.state.answerhandling.InteractionAnswerHandler
 import org.oppia.android.app.player.state.answerhandling.InteractionAnswerReceiver
@@ -21,8 +23,21 @@ import org.oppia.android.app.recyclerview.OnDragEndedListener
 import org.oppia.android.app.recyclerview.OnItemDragListener
 import org.oppia.android.app.translation.AppLanguageResourceHandler
 import org.oppia.android.domain.translation.TranslationController
+import org.oppia.android.R
 import javax.inject.Inject
 
+/** Enum to the store the errors of drag and drop sort interaction. */
+enum class DragAndDropSortInteractionError(@StringRes private var error: Int?) {
+  VALID(error = null),
+  EMPTY_INPUT(error = R.string.drag_and_drop_interaction_empty_input);
+
+  /**
+   * Returns the string corresponding to this error's string resources, or null if there is none.
+   */
+  fun getErrorMessageFromStringRes(resourceHandler: AppLanguageResourceHandler): String? =
+    error?.let(resourceHandler::getStringInLocale)
+}
+
 /** [StateItemViewModel] for drag drop & sort choice list. */
 class DragAndDropSortInteractionViewModel private constructor(
   val entityId: String,
@@ -55,25 +70,34 @@ class DragAndDropSortInteractionViewModel private constructor(
       subtitledHtml.contentId to translatedHtml
     }
 
-  private val _choiceItems: MutableList<DragDropInteractionContentViewModel> =
+  private val _originalChoiceItems: MutableList<DragDropInteractionContentViewModel> =
     computeChoiceItems(contentIdHtmlMap, choiceSubtitledHtmls, this, resourceHandler)
 
+  private val _choiceItems = _originalChoiceItems.toMutableList()
   val choiceItems: List<DragDropInteractionContentViewModel> = _choiceItems
 
+  private var pendingAnswerError: String? = null
   private val isAnswerAvailable = ObservableField(false)
+  var errorMessage = ObservableField<String>("")
 
   init {
     val callback: Observable.OnPropertyChangedCallback =
       object : Observable.OnPropertyChangedCallback() {
         override fun onPropertyChanged(sender: Observable, propertyId: Int) {
           interactionAnswerErrorOrAvailabilityCheckReceiver.onPendingAnswerErrorOrAvailabilityCheck(
-            pendingAnswerError = null,
-            inputAnswerAvailable = true
+            pendingAnswerError,
+            inputAnswerAvailable = true // Allow submission without arranging or merging items.
           )
         }
       }
     isAnswerAvailable.addOnPropertyChangedCallback(callback)
-    isAnswerAvailable.set(true) // For drag drop submit button will be enabled by default.
+    errorMessage.addOnPropertyChangedCallback(callback)
+
+    // Initializing with default values so that submit button is enabled by default.
+    interactionAnswerErrorOrAvailabilityCheckReceiver.onPendingAnswerErrorOrAvailabilityCheck(
+      pendingAnswerError = null,
+      inputAnswerAvailable = true
+    )
   }
 
   override fun onItemDragged(
@@ -97,6 +121,7 @@ class DragAndDropSortInteractionViewModel private constructor(
     // Update the data list if once drag is complete and merge icons are displayed.
     if (allowMultipleItemsInSamePosition) {
       (adapter as BindableAdapter<*>).setDataUnchecked(_choiceItems)
+      checkPendingAnswerError(AnswerErrorCategory.REAL_TIME)
     }
   }
 
@@ -129,6 +154,20 @@ class DragAndDropSortInteractionViewModel private constructor(
       this@DragAndDropSortInteractionViewModel.writtenTranslationContext
   }.build()
 
+  /**
+   * It checks the pending error for the current drag and drop sort interaction, and correspondingly
+   * updates the error string based on the specified error category.
+   */
+  override fun checkPendingAnswerError(category: AnswerErrorCategory): String? {
+    pendingAnswerError = when (category) {
+      AnswerErrorCategory.REAL_TIME -> null
+      AnswerErrorCategory.SUBMIT_TIME ->
+        getSubmitTimeError().getErrorMessageFromStringRes(resourceHandler)
+    }
+    errorMessage.set(pendingAnswerError)
+    return pendingAnswerError
+  }
+
   /** Returns an HTML list containing all of the HTML string elements as items in the list. */
   private fun convertItemsToAnswer(htmlItems: List<StringList>): ListOfSetsOfHtmlStrings {
     return ListOfSetsOfHtmlStrings.newBuilder()
@@ -190,6 +229,13 @@ class DragAndDropSortInteractionViewModel private constructor(
     (adapter as BindableAdapter<*>).setDataUnchecked(_choiceItems)
   }
 
+  private fun getSubmitTimeError(): DragAndDropSortInteractionError {
+    return if (_originalChoiceItems == _choiceItems)
+      DragAndDropSortInteractionError.EMPTY_INPUT
+    else
+      DragAndDropSortInteractionError.VALID
+  }
+
   /** Implementation of [StateItemViewModel.InteractionItemFactory] for this view model. */
   class FactoryImpl @Inject constructor(
     private val resourceHandler: AppLanguageResourceHandler,

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

@@ -67,5 +67,12 @@
       app:draggableData="@{viewModel.choiceItems}"
       app:onDragEnded="@{(adapter) -> viewModel.onDragEnded(adapter)}"
       app:onItemDrag="@{(indexFrom, indexTo, adapter) -> viewModel.onItemDragged(indexFrom, indexTo, adapter)}" />
+
+    <TextView
+      android:id="@+id/drag_drop_interaction_error"
+      style="@style/InputInteractionErrorTextView"
+      android:text="@{viewModel.errorMessage}"
+      android:textColor="@color/component_color_shared_input_error_color"
+      android:visibility="@{viewModel.errorMessage.length()>0? View.VISIBLE : View.GONE}" />
   </LinearLayout>
 </layout>

app/src/main/res/values/strings.xml

@@ -447,6 +447,7 @@
   <string name="return_to_lesson">Return to lesson</string>
   <string name="explanation">Explanation:</string>
   <string name="drag_and_drop_interaction_group_merge_hint">If two items are equal, merge them.</string>
+  <string name="drag_and_drop_interaction_empty_input">Arrange the boxes to continue.</string>
   <string name="link_to_item_below">Link to item %s</string>
   <string name="unlink_items">Unlink items at %s</string>
   <string name="move_item_down_content_description">Move item down to %s</string>