Michael.W基于Foundry精读Openzeppelin第21期——ERC165.sol

0. 版本

[openzeppelin]：v4.8.3，[forge-std]：v1.5.6

0.1 ERC165.sol

Github: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v4.8.3/contracts/utils/introspection/ERC165.sol

ERC165合约是IERC165的标准实现。ERC165提供了本合约是否实现了IERC165接口的查询。如果需要额外支持其他interface，可在目标合约内重写supportsInterface(bytes4)方法。

1. 目标合约

继承ERC165成为一个可调用合约：

Github: https://github.com/RevelationOfTuring/foundry-openzeppelin-contracts/blob/master/src/utils/introspection/MockERC165.sol

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

import "openzeppelin-contracts/contracts/utils/introspection/ERC165.sol";

interface ICustomizedInterface {
    error CustomizedError();

    event CustomizedEvent(uint n);

    function viewFunction(address addr) external view returns (address);

    function pureFunction(uint n) external pure returns (uint);

    function externalFunction(uint[] calldata nums, address[] calldata addrs) external;
}

contract MockERC165 is ERC165, ICustomizedInterface {
    uint _totalSum;
    uint _totalNumberOfAddress;

    // implementation of ICustomizedInterface
    function viewFunction(address addr) external view returns (address){
        if (addr == msg.sender) {
            return addr;
        }

        return address(1024);
    }

    function pureFunction(uint n) external pure returns (uint){
        return n + 1;
    }

    function externalFunction(uint[] calldata nums, address[] calldata addrs) external {
        for (uint i; i < nums.length; ++i) {
            _totalSum += nums[i];
        }

        _totalNumberOfAddress = addrs.length;
    }

    // override supportsInterface(bytes4) of IERC165
    function supportsInterface(bytes4 interfaceId) public view override returns (bool) {
        return interfaceId == type(ICustomizedInterface).interfaceId || super.supportsInterface(interfaceId);
    }
}

同时该合约实现了一个自定义interface——ICustomizedInterface。

全部foundry测试合约：

Github: https://github.com/RevelationOfTuring/foundry-openzeppelin-contracts/blob/master/test/utils/introspection/ERC165.t.sol

2. 代码精读

2.1 IERC165

通过定义interfaceId作为interface的唯一标记。IERC165主要解决了目标合约已实现接口的检测问题——IERC165的实现合约提供了自身已实现interface的查询功能并且可以被其他继承ERC165Checker的合约识别。

具体详情参见：https://eips.ethereum.org/EIPS/eip-165

注：IERC165中要求方法supportsInterface(bytes4)的gas消耗必须小于30000 gas。

2.1.1 interfaceId的计算方式

interfaceId为interface的标识，有3种方法可以求得一个interface的interfaceId：

1. solidity内置的type(interface).interfaceId。以ICustomizedInterface为例：

bytes4 interfaceIdICustomizedInterface = type(ICustomizedInterface).interfaceId;

2. interface内包含的所有函数的selector的异或结果。以ICustomizedInterface为例：

bytes4 interfaceIdICustomizedInterface = ICustomizedInterface.viewFunction.selector
        ^ ICustomizedInterface.pureFunction.selector
        ^ ICustomizedInterface.externalFunction.selector;

3. interface内包含的所有函数的函数签名的异或结果。以ICustomizedInterface为例：

bytes4 interfaceIdICustomizedInterface = bytes4(keccak256('viewFunction(address)'))
        ^ bytes4(keccak256('pureFunction(uint256)'))
        ^ bytes4(keccak256('externalFunction(uint256[],address[])'));

2.2 ERC165

2.2.1 supportsInterface(bytes4 interfaceId)

对外提供本合约是否实现了传入interfaceId标识的interface的查询功能。

    function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
        // 如果传入的interfaceId为IERC165的interfaceId返回true，否则返回false
        return interfaceId == type(IERC165).interfaceId;
    }

注：ERC165只支持对interface IERC165的确认查询，如果需要额外支持其他interface，可在目标合约内重写该方法。具体例子可见后面的foundry测试代码。

2.2.2 foundry代码验证

contract ERC165Test is Test {
    MockERC165 me = new MockERC165();

    function test_SupportsInterface_IERC165() external {
        // way 1: from function selector
        bytes4 interfaceIdIERC165 = IERC165.supportsInterface.selector;
        assertTrue(me.supportsInterface(interfaceIdIERC165));
        // way 2: from hash result of function signature
        interfaceIdIERC165 = bytes4(keccak256('supportsInterface(bytes4)'));
        assertTrue(me.supportsInterface(interfaceIdIERC165));
        // way 3: from type(interface).interfaceId
        interfaceIdIERC165 = type(IERC165).interfaceId;
        assertTrue(me.supportsInterface(interfaceIdIERC165));

        // not support
        bytes4 interfaceIdNotSupport = 0xffffffff;
        assertFalse(me.supportsInterface(interfaceIdNotSupport));
    }

    function test_SupportsInterface_ICustomizedInterface() external {
        // way 1: from XOR of all function selectors in the interface
        bytes4 interfaceIdICustomizedInterface = ICustomizedInterface.viewFunction.selector
        ^ ICustomizedInterface.pureFunction.selector
        ^ ICustomizedInterface.externalFunction.selector;
        assertTrue(me.supportsInterface(interfaceIdICustomizedInterface));
        // way 2: from XOR of all hash result of function signature
        interfaceIdICustomizedInterface = bytes4(keccak256('viewFunction(address)'))
        ^ bytes4(keccak256('pureFunction(uint256)'))
        ^ bytes4(keccak256('externalFunction(uint256[],address[])'));
        assertTrue(me.supportsInterface(interfaceIdICustomizedInterface));
        // way 3: from type(interface).interfaceId
        interfaceIdICustomizedInterface = type(ICustomizedInterface).interfaceId;
        assertTrue(me.supportsInterface(interfaceIdICustomizedInterface));

        // not support
        bytes4 interfaceIdNotSupport = ICustomizedInterface.pureFunction.selector
        ^ ICustomizedInterface.externalFunction.selector;
        assertFalse(me.supportsInterface(interfaceIdNotSupport));
    }

    function test_SupportsInterface_Gas() external {
        // for type(IERC165).interfaceId
        uint startGas = gasleft();
        me.supportsInterface(type(IERC165).interfaceId);
        uint endGas = gasleft();
        assertLt(startGas - endGas, 30000);

        // for type(ICustomizedInterface).interfaceId
        startGas = gasleft();
        me.supportsInterface(type(ICustomizedInterface).interfaceId);
        endGas = gasleft();
        assertLt(startGas - endGas, 30000);
    }
}

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