Add support for pre-encoded things in AttributeAccessInterface.

src/app/BUILD.gn

@@ -201,6 +201,10 @@ static_library("app") {
     "TimerDelegates.h",
     "WriteClient.cpp",
     "WriteHandler.cpp",
+    "data-model/FabricScopedPreEncodedValue.cpp",
+    "data-model/FabricScopedPreEncodedValue.h",
+    "data-model/PreEncodedValue.cpp",
+    "data-model/PreEncodedValue.h",
     "reporting/Engine.cpp",
     "reporting/Engine.h",
     "reporting/ReportScheduler.h",

src/app/data-model/FabricScopedPreEncodedValue.cpp

@@ -0,0 +1,106 @@
+/*
+ *    Copyright (c) 2024 Project CHIP Authors
+ *    All rights reserved.
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *        http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+#include "FabricScopedPreEncodedValue.h"
+#include <lib/support/CodeUtils.h>
+
+#include <optional>
+
+namespace chip {
+namespace app {
+namespace DataModel {
+
+FabricScopedPreEncodedValue::FabricScopedPreEncodedValue(const ByteSpan & aData)
+{
+    mReader.Init(aData);
+}
+
+CHIP_ERROR FabricScopedPreEncodedValue::EncodeForRead(TLV::TLVWriter & aWriter, TLV::Tag aTag, FabricIndex aFabricIndex) const
+{
+    // Clone our reader, since we need to do Next() to move to its first element
+    // but not change our state.  And CopyElement needs a non-const reader
+    // anyway since it also changes reader state.
+    TLV::TLVReader reader(mReader);
+
+    ReturnErrorOnFailure(reader.Next());
+
+    return aWriter.CopyElement(aTag, reader);
+}
+
+FabricIndex FabricScopedPreEncodedValue::GetFabricIndex() const
+{
+    TLV::TLVReader reader(mReader);
+    CHIP_ERROR err = reader.Next();
+    if (err != CHIP_NO_ERROR)
+    {
+        return kUndefinedFabricIndex;
+    }
+
+    // We must have a struct here.
+    if (reader.GetType() != TLV::kTLVType_Structure)
+    {
+        return kUndefinedFabricIndex;
+    }
+
+    TLV::TLVType structType;
+    err = reader.EnterContainer(structType);
+    if (err != CHIP_NO_ERROR)
+    {
+        return kUndefinedFabricIndex;
+    }
+
+    // Now look for a single field with the right tag.
+    std::optional<FabricIndex> foundFabricIndex;
+    constexpr TLV::Tag kFabricIndexTag = TLV::ContextTag(254);
+    while ((err = reader.Next()) == CHIP_NO_ERROR)
+    {
+        if (reader.GetTag() != kFabricIndexTag)
+        {
+            continue;
+        }
+
+        if (foundFabricIndex.has_value())
+        {
+            // Two fabric indices?  Just give up.  Note that this will lead to
+            // errors encoding our value.
+            return kUndefinedFabricIndex;
+        }
+
+        err = reader.Get(foundFabricIndex.emplace());
+        if (err != CHIP_NO_ERROR)
+        {
+            return kUndefinedFabricIndex;
+        }
+    }
+
+    if (err != CHIP_ERROR_END_OF_TLV)
+    {
+        return kUndefinedFabricIndex;
+    }
+
+    err = reader.ExitContainer(structType);
+    if (err != CHIP_NO_ERROR)
+    {
+        return kUndefinedFabricIndex;
+    }
+
+    return foundFabricIndex.value_or(kUndefinedFabricIndex);
+}
+
+} // namespace DataModel
+} // namespace app
+} // namespace chip

src/app/data-model/FabricScopedPreEncodedValue.h

@@ -0,0 +1,59 @@
+/*
+ *    Copyright (c) 2024 Project CHIP Authors
+ *    All rights reserved.
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *        http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+#include <lib/core/CHIPError.h>
+#include <lib/core/DataModelTypes.h>
+#include <lib/core/TLVReader.h>
+#include <lib/core/TLVTags.h>
+#include <lib/core/TLVWriter.h>
+
+namespace chip {
+namespace app {
+namespace DataModel {
+
+/**
+ * FabridScopedPreEncodedValue represents the value of a single item in a list
+ * of fabric-scoped structs that has already been encoded as TLV.  To enable
+ * reading these values successfully, the struct must have the fabric index it
+ * corresponds to encoded with the right tag.
+ *
+ * Note: Fabric-sensitive fields are currently not supported; there is no way to
+ * specify which fields are fabric-sensitive.
+ */
+class FabricScopedPreEncodedValue
+{
+public:
+    /**
+     * The data buffer backing the ByteSpan must outlive the
+     * FabricScopedPreEncodedValue instance.
+     */
+    FabricScopedPreEncodedValue(const ByteSpan & aData);
+
+    /**
+     * Encodable object API implementation.
+     */
+    static constexpr bool kIsFabricScoped = true;
+    CHIP_ERROR EncodeForRead(TLV::TLVWriter & aWriter, TLV::Tag aTag, FabricIndex aFabricIndex) const;
+    FabricIndex GetFabricIndex() const;
+
+private:
+    TLV::TLVReader mReader;
+};
+
+} // namespace DataModel
+} // namespace app
+} // namespace chip

src/app/data-model/PreEncodedValue.cpp

@@ -0,0 +1,44 @@
+/*
+ *    Copyright (c) 2024 Project CHIP Authors
+ *    All rights reserved.
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *        http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+#include "PreEncodedValue.h"
+#include <lib/support/CodeUtils.h>
+
+namespace chip {
+namespace app {
+namespace DataModel {
+
+PreEncodedValue::PreEncodedValue(const ByteSpan & aData)
+{
+    mReader.Init(aData);
+}
+
+CHIP_ERROR PreEncodedValue::Encode(TLV::TLVWriter & aWriter, TLV::Tag aTag) const
+{
+    // Clone our reader, since we need to do Next() to move to its first element
+    // but not change our state.  And CopyElement needs a non-const reader
+    // anyway since it also changes reader state.
+    TLV::TLVReader reader(mReader);
+
+    ReturnErrorOnFailure(reader.Next());
+
+    return aWriter.CopyElement(aTag, reader);
+}
+
+} // namespace DataModel
+} // namespace app
+} // namespace chip

src/app/data-model/PreEncodedValue.h

@@ -0,0 +1,53 @@
+/*
+ *    Copyright (c) 2024 Project CHIP Authors
+ *    All rights reserved.
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *        http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+#include <lib/core/CHIPError.h>
+#include <lib/core/TLVReader.h>
+#include <lib/core/TLVTags.h>
+#include <lib/core/TLVWriter.h>
+
+namespace chip {
+namespace app {
+namespace DataModel {
+
+/**
+ * PreEncodedValue represents the value of an attribute or the value of a single
+ * item in a list of non-fabric-scoped structs that has already been
+ * encoded as TLV.
+ */
+class PreEncodedValue
+{
+public:
+    /**
+     * The data buffer backing the ByteSpan must outlive the PreEncodedValue
+     * instance.
+     */
+    PreEncodedValue(const ByteSpan & aData);
+
+    /**
+     * Encodable object API implementation.
+     */
+    static constexpr bool kIsFabricScoped = false;
+    CHIP_ERROR Encode(TLV::TLVWriter & aWriter, TLV::Tag aTag) const;
+
+private:
+    TLV::TLVReader mReader;
+};
+
+} // namespace DataModel
+} // namespace app
+} // namespace chip