Fix part of #5070: Display empty answer message in ratio input interaction

app/src/main/java/org/oppia/android/app/parser/StringToRatioParser.kt

@@ -29,6 +29,7 @@ class StringToRatioParser {
     val normalized = text.normalizeWhitespace()
     val ratio = parseRatioOrNull(normalized)
     return when {
+      normalized.isBlank() -> RatioParsingError.EMPTY_INPUT
       !normalized.matches(invalidRatioRegex) || ratio == null -> RatioParsingError.INVALID_FORMAT
       numberOfTerms != 0 && ratio.ratioComponentCount != numberOfTerms -> {
         RatioParsingError.INVALID_SIZE
@@ -77,7 +78,8 @@ class StringToRatioParser {
     INVALID_FORMAT(error = R.string.ratio_error_invalid_format),
     INVALID_COLONS(error = R.string.ratio_error_invalid_colons),
     INVALID_SIZE(error = R.string.ratio_error_invalid_size),
-    INCLUDES_ZERO(error = R.string.ratio_error_includes_zero);
+    INCLUDES_ZERO(error = R.string.ratio_error_includes_zero),
+    EMPTY_INPUT(error = R.string.ratio_error_empty_input);
 
     /**
      * Returns the string corresponding to this error's string resources, or null if there is none.

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

@@ -46,12 +46,17 @@ class RatioExpressionInputInteractionViewModel private constructor(
         override fun onPropertyChanged(sender: Observable, propertyId: Int) {
           errorOrAvailabilityCheckReceiver.onPendingAnswerErrorOrAvailabilityCheck(
             pendingAnswerError,
-            answerText.isNotEmpty()
+            true
           )
         }
       }
     errorMessage.addOnPropertyChangedCallback(callback)
     isAnswerAvailable.addOnPropertyChangedCallback(callback)
+
+    errorOrAvailabilityCheckReceiver.onPendingAnswerErrorOrAvailabilityCheck(
+      null,
+      true
+    )
   }
 
   override fun getPendingAnswer(): UserAnswer = UserAnswer.newBuilder().apply {
@@ -67,23 +72,24 @@ class RatioExpressionInputInteractionViewModel private constructor(
     }
   }.build()
 
-  /** It checks the pending error for the current ratio input, and correspondingly updates the error string based on the specified error category. */
+  /**
+   * It checks the pending error for the current ratio input, and correspondingly
+   * updates the error string based on the specified error category.
+   */
   override fun checkPendingAnswerError(category: AnswerErrorCategory): String? {
-    if (answerText.isNotEmpty()) {
-      when (category) {
-        AnswerErrorCategory.REAL_TIME ->
-          pendingAnswerError =
-            stringToRatioParser.getRealTimeAnswerError(answerText.toString())
-              .getErrorMessageFromStringRes(resourceHandler)
-        AnswerErrorCategory.SUBMIT_TIME ->
-          pendingAnswerError =
-            stringToRatioParser.getSubmitTimeError(
-              answerText.toString(),
-              numberOfTerms = numberOfTerms
-            ).getErrorMessageFromStringRes(resourceHandler)
-      }
-      errorMessage.set(pendingAnswerError)
+    pendingAnswerError = when (category) {
+      AnswerErrorCategory.REAL_TIME ->
+        if (answerText.isNotEmpty())
+          stringToRatioParser.getRealTimeAnswerError(answerText.toString())
+            .getErrorMessageFromStringRes(resourceHandler)
+        else null
+      AnswerErrorCategory.SUBMIT_TIME ->
+        stringToRatioParser.getSubmitTimeError(
+          answerText.toString(),
+          numberOfTerms = numberOfTerms
+        ).getErrorMessageFromStringRes(resourceHandler)
     }
+    errorMessage.set(pendingAnswerError)
     return pendingAnswerError
   }
 

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

@@ -174,6 +174,7 @@
   <string name="ratio_error_invalid_colons" description="The error message for ratio when there is colons next to each other.">Your answer has two colons (:) next to each other.</string>
   <string name="ratio_error_invalid_size" description="The error message for ratio when there is less number of terms than the creator has specified in customization arg.">Number of terms is not equal to the required terms.</string>
   <string name="ratio_error_includes_zero" description="The error message for ratio when there is 0 term in the ratio.">Ratios cannot have 0 as an element.</string>
+  <string name="ratio_error_empty_input">Enter a ratio to continue</string>
   <string name="unknown_size">Unknown size</string>
   <string name="size_bytes">%s Bytes</string>
   <string name="size_kb">%s KB</string>