fabric-samples/asset-transfer-events/chaincode-go/chaincode/smartcontract.go

package chaincode

import (
	"encoding/json"
	"fmt"

	"github.com/hyperledger/fabric-contract-api-go/v2/contractapi"
)

// SmartContract provides functions for managing an Asset
type SmartContract struct {
	contractapi.Contract
}

// Asset describes basic details of what makes up a simple asset
// Insert struct field in alphabetic order => to achieve determinism across languages
// golang keeps the order when marshal to json but doesn't order automatically
type Asset struct {
	AppraisedValue int    `json:"AppraisedValue"`
	Color          string `json:"Color"`
	ID             string `json:"ID"`
	Owner          string `json:"Owner"`
	Size           int    `json:"Size"`
}

// CreateAsset issues a new asset to the world state with given details.
func (s *SmartContract) CreateAsset(ctx contractapi.TransactionContextInterface, id string, color string, size int, owner string, appraisedValue int) error {
	existing, err := s.readState(ctx, id)
	if err == nil && existing != nil {
		return fmt.Errorf("the asset %s already exists", id)
	}

	asset := Asset{
		ID:             id,
		Color:          color,
		Size:           size,
		Owner:          owner,
		AppraisedValue: appraisedValue,
	}
	assetJSON, err := json.Marshal(asset)
	if err != nil {
		return err
	}

	ctx.GetStub().SetEvent("CreateAsset", assetJSON)
	return ctx.GetStub().PutState(id, assetJSON)
}

func (s *SmartContract) readState(ctx contractapi.TransactionContextInterface, id string) ([]byte, error) {
	assetJSON, err := ctx.GetStub().GetState(id)
	if err != nil {
		return nil, fmt.Errorf("failed to read from world state: %w", err)
	}
	if assetJSON == nil {
		return nil, fmt.Errorf("the asset %s does not exist", id)
	}

	return assetJSON, nil
}

// ReadAsset returns the asset stored in the world state with given id.
func (s *SmartContract) ReadAsset(ctx contractapi.TransactionContextInterface, id string) (*Asset, error) {
	assetJSON, err := s.readState(ctx, id)
	if err != nil {
		return nil, err
	}

	var asset Asset
	err = json.Unmarshal(assetJSON, &asset)
	if err != nil {
		return nil, err
	}

	return &asset, nil
}

// UpdateAsset updates an existing asset in the world state with provided parameters.
func (s *SmartContract) UpdateAsset(ctx contractapi.TransactionContextInterface, id string, color string, size int, owner string, appraisedValue int) error {
	_, err := s.readState(ctx, id)
	if err != nil {
		return err
	}

	// overwriting original asset with new asset
	asset := Asset{
		ID:             id,
		Color:          color,
		Size:           size,
		Owner:          owner,
		AppraisedValue: appraisedValue,
	}
	assetJSON, err := json.Marshal(asset)
	if err != nil {
		return err
	}

	ctx.GetStub().SetEvent("UpdateAsset", assetJSON)
	return ctx.GetStub().PutState(id, assetJSON)
}

// DeleteAsset deletes an given asset from the world state.
func (s *SmartContract) DeleteAsset(ctx contractapi.TransactionContextInterface, id string) error {
	assetJSON, err := s.readState(ctx, id)
	if err != nil {
		return err
	}

	ctx.GetStub().SetEvent("DeleteAsset", assetJSON)
	return ctx.GetStub().DelState(id)
}

// TransferAsset updates the owner field of asset with given id in world state, and returns the old owner.
func (s *SmartContract) TransferAsset(ctx contractapi.TransactionContextInterface, id string, newOwner string) (string, error) {
	asset, err := s.ReadAsset(ctx, id)
	if err != nil {
		return "", err
	}

	oldOwner := asset.Owner
	asset.Owner = newOwner

	assetJSON, err := json.Marshal(asset)
	if err != nil {
		return "", err
	}

	ctx.GetStub().SetEvent("TransferAsset", assetJSON)
	err = ctx.GetStub().PutState(id, assetJSON)
	if err != nil {
		return "", err
	}

	return oldOwner, nil
}