src/app/AttributeAccessInterface.h

/*
 *
 *    Copyright (c) 2021 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.
 */

#pragma once

#include <app/ConcreteAttributePath.h>
#include <app/MessageDef/AttributeDataElement.h>
#include <app/data-model/Encode.h>
#include <app/data-model/List.h> // So we can encode lists
#include <app/data-model/TagBoundEncoder.h>
#include <app/util/basic-types.h>
#include <lib/core/CHIPTLV.h>
#include <lib/core/Optional.h>

/**
 * Callback class that clusters can implement in order to interpose custom
 * attribute-handling logic.  An AttributeAccessInterface instance is associated
 * with some specific cluster.  A single instance may be used for a specific
 * endpoint or for all endpoints.
 *
 * Instances of AttributeAccessInterface that are registered via
 * registerAttributeAccessOverride will be consulted before taking the normal
 * attribute access codepath and can use that codepath as a fallback if desired.
 */
namespace chip {
namespace app {

class AttributeValueEncoder : protected TagBoundEncoder
{
public:
    AttributeValueEncoder(TLV::TLVWriter * aWriter, FabricIndex aAccessingFabricIndex) :
        TagBoundEncoder(aWriter, TLV::ContextTag(AttributeDataElement::kCsTag_Data)), mAccessingFabricIndex(aAccessingFabricIndex)
    {}

    template <typename... Ts>
    CHIP_ERROR Encode(Ts... aArgs)
    {
        mTriedEncode = true;
        if (mWriter == nullptr)
        {
            return CHIP_NO_ERROR;
        }
        return TagBoundEncoder::Encode(std::forward<Ts>(aArgs)...);
    }

    /**
     * aCallback is expected to take a const TagBoundEncoder& 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 all items are encoded
     * aCallback is expected to return success.
     *
     * aCallback may not be called.  Consumers must not assume it will be
     * called.
     */
    template <typename ListGenerator>
    CHIP_ERROR EncodeList(ListGenerator aCallback)
    {
        mTriedEncode = true;
        if (mWriter == nullptr)
        {
            return CHIP_NO_ERROR;
        }
        return TagBoundEncoder::EncodeList(aCallback);
    }

    bool TriedEncode() const { return mTriedEncode; }

    /**
     * The accessing fabric index for this read or subscribe interaction.
     */
    FabricIndex AccessingFabricIndex() const { return mAccessingFabricIndex; }

    // For consumers that can't just do a single Encode call for some reason
    // (e.g. they're encoding a list a bit at a time).
    TLV::TLVWriter * PrepareManualEncode()
    {
        // If this is called, the consumer is trying to encode a value.
        mTriedEncode = true;
        return mWriter;
    }

private:
    bool mTriedEncode = false;
    const FabricIndex mAccessingFabricIndex;
};

class AttributeAccessInterface
{
public:
    /**
     * aEndpointId can be Missing to indicate that this object is meant to be
     * used with all endpoints.
     */
    AttributeAccessInterface(Optional<EndpointId> aEndpointId, ClusterId aClusterId) :
        mEndpointId(aEndpointId), mClusterId(aClusterId)
    {}
    virtual ~AttributeAccessInterface() {}

    /**
     * Callback for reading attributes.
     *
     * @param [in] aPath indicates which exact data is being read.
     * @param [in] aEncoder the AttributeValueEncoder to use for encoding the
     *             data.  If this function returns scucess and no attempt is
     *             made to encode data using aEncoder, the
     *             AttributeAccessInterface did not try to provide any data.  In
     *             this case, normal attribute access will happen for the read.
     *             This may involve reading from the attribute store or external
     *             attribute callbacks.
     */
    virtual CHIP_ERROR Read(const ConcreteAttributePath & aPath, AttributeValueEncoder & aEncoder) = 0;

    /**
     * Mechanism for keeping track of a chain of AttributeAccessInterfaces.
     */
    void SetNext(AttributeAccessInterface * aNext) { mNext = aNext; }
    AttributeAccessInterface * GetNext() const { return mNext; }

    /**
     * Check whether a this AttributeAccessInterface is relevant for a
     * particular endpoint+cluster.  An AttributeAccessInterface will be used
     * for a read from a particular cluster only when this function returns
     * true.
     */
    bool Matches(EndpointId aEndpointId, ClusterId aClusterId) const
    {
        return (!mEndpointId.HasValue() || mEndpointId.Value() == aEndpointId) && mClusterId == aClusterId;
    }

    /**
     * Check whether an AttributeAccessInterface is relevant for a particular
     * specific endpoint.  This is used to clean up overrides registered for an
     * endpoint that becomes disabled.
     */
    bool MatchesExactly(EndpointId aEndpointId) const { return mEndpointId.HasValue() && mEndpointId.Value() == aEndpointId; }

    /**
     * Check whether another AttributeAccessInterface wants to handle the same set of
     * attributes as we do.
     */
    bool Matches(const AttributeAccessInterface & aOther) const
    {
        return mClusterId == aOther.mClusterId &&
            (!mEndpointId.HasValue() || !aOther.mEndpointId.HasValue() || mEndpointId.Value() == aOther.mEndpointId.Value());
    }

private:
    Optional<EndpointId> mEndpointId;
    ClusterId mClusterId;
    AttributeAccessInterface * mNext = nullptr;
};

} // namespace app
} // namespace chip