如何接收Solidity智能合约交易函数返回的值?

Question

我正在编写一个NFT智能合约，我将通过Hardhat测试并部署在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Hardhat上运行的测试:

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);
});
});
});

在owner函数的情况下，我得到了我所期望的:它返回我的帐户地址，并且第一次测试成功通过。然而，当涉及到mintNFT功能时，我没有得到我所期望的:而不是新创建的项目ID，我得到了一些非常不同的东西，我的第二次测试失败了。

为什么两个非常相似的测试给我不同的结果?我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

你不能直接接收一个函数的返回值，你正在发送一个交易到链下。你只能在链上这样做——即当一个SC函数调用另一个SC函数时。

但是，在这种特殊情况下，您可以通过事件获得该值，因为ERC721规范包含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);

在您已经在hardhat测试中使用的Ethers.js中，您将获得一行返回的事务响应对象，而不是智能合约函数返回值:

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

接下来，您需要通过在txResponse上调用wait来获得交易收据:

const txReceipt = await txResponse.wait();

反过来，txReceipt包含一个events数组，其中包含事务发出的所有事件。在这种情况下，期望它包含一个单一的Transfer事件，表明第一个铸造的物品从零地址转移到您的帐户地址。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transferEvent参数中获取tokenId属性，这将是新生成的NFT的Id:

const { tokenId } = transferEvent.args;

整个测试应该如下所示:

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);
});
});

Answer 2

从事务返回的值在EVM之外不可用。

你可以触发一个事件，或者为这个值创建一个公共viewgetter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);
});

文档:https://ethereum-waffle.readthedocs.io/en/latest/matchers.html发送事件