Some interface cleanup on AttributeValueEncoder. (#12661)

1) Address unaddressed or deferred review comments from #12019.
2) Add a simpler way to encode a null value.

src/app/AttributeAccessInterface.cpp

@@ -69,6 +69,7 @@ CHIP_ERROR AttributeValueEncoder::EncodeEmptyList()
             // Put an empty array before encoding the first array element for list chunking.
             AttributeReportBuilder builder;
 
+            mPath.mListOp = ConcreteDataAttributePath::ListOperation::ReplaceAll;
             ReturnErrorOnFailure(builder.PrepareAttribute(mAttributeReportIBsBuilder, mPath, mDataVersion));
             ReturnErrorOnFailure(builder.EncodeValue(mAttributeReportIBsBuilder, DataModel::List<uint8_t>()));
 
@@ -82,6 +83,9 @@ CHIP_ERROR AttributeValueEncoder::EncodeEmptyList()
     // revert partial data.
     mEncodeState.mAllowPartialData = true;
 
+    // For all elements in the list, a report with append operation will be generated. This will not be changed during encoding
+    // of each report since the users cannot access mPath.
+    mPath.mListOp = ConcreteDataAttributePath::ListOperation::AppendItem;
     return CHIP_NO_ERROR;
 }
 

src/app/AttributeAccessInterface.h

@@ -147,9 +147,9 @@ class AttributeValueEncoder
     {}
 
     /**
-     * Encode builds a single AttributeReportIB in AttributeReportIBs.
-     * When we are encoding a single element in the list, the actual path in the report contains a null list index as "append"
-     * operation.
+     * Encode a single value.  This value will not be chunked; it will either be
+     * entirely encoded or fail to be encoded.  Consumers are allowed to make
+     * either one call to Encode or one call to EncodeList to handle a read.
      */
     template <typename... Ts>
     CHIP_ERROR Encode(Ts &&... aArgs)
@@ -158,6 +158,15 @@ class AttributeValueEncoder
         return EncodeAttributeReportIB(std::forward<Ts>(aArgs)...);
     }
 
+    /**
+     * Encode an explicit null value.
+     */
+    CHIP_ERROR EncodeNull()
+    {
+        // Doesn't matter what type Nullable we use here.
+        return Encode(DataModel::Nullable<uint8_t>());
+    }
+
     /**
      * aCallback is expected to take a const auto & argument and Encode() on it as many times as needed to encode all the list
      * elements one by one.  If any of those Encode() calls returns failure, aCallback must stop encoding and return failure.  When
@@ -181,11 +190,7 @@ class AttributeValueEncoder
         // EmptyList acts as the beginning of the whole array type attribute report.
         // An empty list is encoded iff both mCurrentEncodingListIndex and mEncodeState.mCurrentEncodingListIndex are invalid
         // values. After encoding the empty list, mEncodeState.mCurrentEncodingListIndex and mCurrentEncodingListIndex are set to 0.
-        mPath.mListOp = ConcreteDataAttributePath::ListOperation::ReplaceAll;
         ReturnErrorOnFailure(EncodeEmptyList());
-        // For all elements in the list, a report with append operation will be generated. This will not be changed during encoding
-        // of each report since the users cannot access mPath.
-        mPath.mListOp = ConcreteDataAttributePath::ListOperation::AppendItem;
         ReturnErrorOnFailure(aCallback(ListEncodeHelper(*this)));
         // The Encode procedure finished without any error, clear the state.
         mEncodeState = AttributeEncodeState();
@@ -239,12 +244,16 @@ class AttributeValueEncoder
     }
 
     /**
-     * Actual logic for encoding a single AttributeReportIB in AttributeReportIBs.
+     * Builds a single AttributeReportIB in AttributeReportIBs.  The caller is
+     * responsible for setting up mPath correctly.
+     *
+     * In particular, when we are encoding a single element in the list, mPath
+     * must indicate a null list index to represent an "append" operation.
+     * operation.
      */
     template <typename... Ts>
     CHIP_ERROR EncodeAttributeReportIB(Ts &&... aArgs)
     {
-        mTriedEncode = true;
         AttributeReportBuilder builder;
 
         ReturnErrorOnFailure(builder.PrepareAttribute(mAttributeReportIBsBuilder, mPath, mDataVersion));
@@ -258,6 +267,10 @@ class AttributeValueEncoder
      *
      * If internal state indicates we have already encoded the empty list, this function will encode nothing, set
      * mCurrentEncodingListIndex to 0 and return CHIP_NO_ERROR.
+     *
+     * In all cases this function guarantees that mPath.mListOp is AppendItem
+     * after it returns, because at that point we will be encoding the list
+     * items.
      */
     CHIP_ERROR EncodeEmptyList();