如何接收可靠智能合同处理函数返回的值？

Question

我正在编写一个NFT智能契约，我将通过硬帽进行测试并部署到RSK上。

//SPDX-License-Identifier: Unlicense
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";

contract MyNFT is ERC721URIStorage {
    uint private _counter;
    address private _owner;

    constructor() ERC721("My NFT", "MNFT") {
      _owner = msg.sender;
    }

    function owner() public view returns (address) {
      return _owner;
    }

    function mintNFT(address recipient, string memory tokenURI)
        public returns (uint256)
    {
        require(msg.sender == owner(), "Only owner is allowed to mint");
        uint newItemId = ++_counter;
        ERC721._mint(recipient, newItemId);
        ERC721URIStorage._setTokenURI(newItemId, tokenURI);

        return newItemId;
    }
}

这里我有两个公共函数：owner和mintNFT，它们都返回一些值。在我的测试中，我想读取来自这两个函数的返回值。以下是我在“硬帽子”上进行的测试：

const { expect } = require("chai");
const { ethers } = require("hardhat");

describe("My NFT", () => {
  let deployer;
  let myNFT;

  // deploy NFT before the tests
  before(async () => {
    [deployer] = await ethers.getSigners();
    const MyNFT = await ethers.getContractFactory('MyNFT');
    myNFT = await MyNFT.deploy();
    await myNFT.deployed();
  });

  describe('Receiving a value returned by a view function', () => {
    it('The deployer should be the s/c owner', async  () => {
      const owner = await myNFT.owner();
      expect(owner).to.equal(deployer.address);
    });
  });
  
  describe('Receiving a value returned by a transacting function', () => {
    it('Should return a correct ID of the newly minted item', async () => {
      const newMintItem = {
        id: 1,
        uri: 'ipfs://Qme3QxqsJih5psasse4d2FFLFLwaKx7wHXW3Topk3Q8b14',
      };
      const newItemId = await myNFT.mintNFT(deployer.address, newMintItem.uri);
      expect(newItemId).to.equal(newMintItem.id);
    });
  });
});

对于owner函数，我得到了我期望的结果:它返回我的帐户地址，第一个测试成功通过。但是，当涉及到mintNFT函数时，我没有得到我所期望的:而不是新创建的项ID，而是得到了完全不同的东西，第二个测试失败了。

为什么两个非常相似的测试给我不同的结果？如何从发送事务的函数中获得返回值？作为参考，这是我使用的hardhat.config.js文件：

require("@nomiclabs/hardhat-waffle");

module.exports = {
  solidity: "0.8.4",
  defaultNetwork: 'rskregtest',
  networks: {
    rskregtest: {
      chainId: 33,
      url: 'http://localhost:4444',
    },
  },
};

Answer 1

从事务返回的值在EVM之外不可用。

您可以发出事件，也可以创建值的公共view getter函数。

contract MyNFT is ERC721URIStorage {
    // `public` visibility autogenerates view function named `_counter()`
    uint public _counter;
    event NFTMinted(uint256 indexed _id);

    function mintNFT(address recipient, string memory tokenURI)
        public returns (uint256)
    {
        // ...
        emit NFTMinted(newItemId);
        return newItemId;
    }
}

it('Should return a correct ID of the newly minted item', async () => {
    const newMintItem = {
        id: 1,
        uri: 'ipfs://Qme3QxqsJih5psasse4d2FFLFLwaKx7wHXW3Topk3Q8b14',
    };

    // testing the emitted event
    await expect(myNFT.mintNFT(deployer.address, newMintItem.uri))
        .to.emit(myNFT, "NFTMinted")
        .withArgs(newMintItem.id);

    // testing the getter value
    const counter = await myNFT._counter();
    expect(counter).to.equal(newMintItem.id);
});

博士：https://ethereum-waffle.readthedocs.io/en/latest/matchers.html#emitting-events

Answer 2

您不能直接接收来自要将事务发送到脱机的函数的返回值。您只能在链上这样做，即当一个SC函数调用另一个SC函数时。

但是，在这种情况下，您可以通过事件获得值，因为ERC721规范包括一个Transfer事件：

    /// @dev This emits when ownership of any NFT changes by any mechanism.
    ///  This event emits when NFTs are created (`from` == 0) and destroyed
    ///  (`to` == 0). Exception: during contract creation, any number of NFTs
    ///  may be created and assigned without emitting Transfer. At the time of
    ///  any transfer, the approved address for that NFT (if any) is reset to none.
    event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);

在Ethers.js中，您已经在硬件测试中使用了它，而不是智能契约函数返回值，您将在这一行中得到一个返回的事务响应对象：

const txResponse = await myNFT.mintNFT(deployer.address, newMintItem.uri);

接下来，您需要通过调用交易收据上的wait来获得txResponse

const txReceipt = await txResponse.wait();

反过来，txReceipt包含一个events数组，该数组包含事务发出的所有事件。在这种情况下，期望它包含一个Transfer事件，指示从零地址到您的帐户地址的第一个薄荷项转移。通过从events数组中提取第一个元素来获取此事件：

const [transferEvent] = txReceipt.events;

接下来，从tokenId参数中获取transferEvent属性，这将是您新创建的NFT的Id：

const { tokenId } = transferEvent.args;

整个测试结果如下：

describe('Receiving a value returned by a transacting function', () => {
    it('Should return a correct ID of the newly minted item', async () => {
      const newMintItem = {
        id: 1,
        uri: 'ipfs://QmY6KX35Rg25rnaffmZzGUFb3raRhtPA5JEFeSSWQA4GHL',
      };
      const txResponse = await myNFT.mintNFT(deployer.address, newMintItem.uri);
      const txReceipt = await txResponse.wait();
      const [transferEvent] = txReceipt.events;
      const { tokenId } = transferEvent.args;
      expect(tokenId).to.equal(newMintItem.id);
    });
  });