Improve DecodableList when not in list

src/app/data-model/DecodableList.h

@@ -32,36 +32,40 @@ namespace DataModel {
  * This class provides an iteratable decoder of list items within TLV payloads
  * such that no memory has to be provided ahead of time to store the entirety of the decoded
  * list contents.
- *
  */
 template <typename T>
 class DecodableList
 {
 public:
-    DecodableList()
-    {
-        // Init to an empty list.
-        mReader.Init(nullptr, 0);
-    }
+    DecodableList() { ClearReader(); }
 
     /*
      * @brief
      *
      * This call stores a TLV reader positioned on the list this class is to manage.
      *
-     * Specifically, the passed-in TLV reader should be pointing into the list just after
+     * Specifically, the passed-in reader should be pointing into the list just after
      * having called `OpenContainer` on the list element.
      */
     void SetReader(const TLV::TLVReader & reader) { mReader = reader; }
 
+    /*
+     * @brief
+     *
+     * This call clears the TLV reader managed by this class, so it can be reused.
+     */
+    void ClearReader() { mReader.Init(nullptr, 0); }
+
     class Iterator
     {
     public:
         /*
          * Initialize the iterator with a reference to a reader.
          *
          * This reader should be pointing into the list just after
-         * having called `OpenContainer` on the list element.
+         * having called `OpenContainer` on the list element, or should
+         * have a `kTLVType_NotSpecified` container type if there is
+         * no list.
          */
         Iterator(const TLV::TLVReader & reader)
         {
@@ -77,16 +81,20 @@ class DecodableList
          * If an element does exist and was successfully decoded, this
          * shall return true.
          *
-         * Otherwise, if the end of list is reached, this call shall return
-         * false.
+         * Otherwise, if the end of list is reached, or there was no list,
+         * this call shall return false.
          *
          * If an error was encountered at any point during the iteration or decode,
          * this shall return false as well. The caller is expected to invoke GetStatus()
          * to retrieve the status of the operation.
-         *
          */
         bool Next()
         {
+            if (mReader.GetContainerType() == TLV::kTLVType_NotSpecified)
+            {
+                return false;
+            }
+
             if (mStatus == CHIP_NO_ERROR)
             {
                 mStatus = mReader.Next();
@@ -111,7 +119,6 @@ class DecodableList
          *
          * Notably, if the end-of-list was encountered in a previous call to Next,
          * the status returned shall be CHIP_NO_ERROR.
-         *
          */
         CHIP_ERROR GetStatus() const
         {
@@ -134,12 +141,24 @@ class DecodableList
     Iterator begin() const { return Iterator(mReader); }
 
     /*
-     * Compute the size of the list.  This can fail if the TLV is malformed.  If
+     * Compute the size of the list. This can fail if the TLV is malformed. If
      * this succeeds, that does not guarantee that the individual items can be
      * successfully decoded; consumers should check Iterator::GetStatus() when
-     * actually decoding them.
+     * actually decoding them. If there is no list then the size is considered
+     * to be zero.
      */
-    CHIP_ERROR ComputeSize(size_t * size) const { return mReader.CountRemainingInContainer(size); }
+    CHIP_ERROR ComputeSize(size_t * size) const
+    {
+        if (mReader.GetContainerType() == TLV::kTLVType_NotSpecified)
+        {
+            *size = 0;
+            return CHIP_NO_ERROR;
+        }
+        else
+        {
+            return mReader.CountRemainingInContainer(size);
+        }
+    }
 
 private:
     TLV::TLVReader mReader;
@@ -149,13 +168,10 @@ template <typename X>
 CHIP_ERROR Decode(TLV::TLVReader & reader, DecodableList<X> & x)
 {
     VerifyOrReturnError(reader.GetType() == TLV::kTLVType_Array, CHIP_ERROR_SCHEMA_MISMATCH);
-
     TLV::TLVType type;
-
     ReturnErrorOnFailure(reader.EnterContainer(type));
     x.SetReader(reader);
     ReturnErrorOnFailure(reader.ExitContainer(type));
-
     return CHIP_NO_ERROR;
 }