Smock API Example

Mocks.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
import {MyContract} from "./MyContract.sol";

library MockMyContract {
    function mockCall(
        function(address) external view returns (uint256),
        uint256 mockReturnValue
    ) internal {
        // do the thing to return mockReturnValue
    }

    function mockCall(
        function(address, address) external view returns (uint256),
        uint256 mockReturnValue
    ) internal {
        // do the thing to return mockReturnValue when called
    }

    function mockCall(
        function(MyContract.Foo memory)
            external
            pure
            returns (MyContract.Foo memory),
        MyContract.Foo memory mockReturnValue
    ) internal {
        // do the thing to return mockReturnValue when calling bar
    }
}

MyContract.sol

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

contract MyContract {
    struct Foo {
        uint256 a;
        uint256 b;
    }

    function balanceOf(address a) external view returns (uint256) {
        return 0;
    }

    function allowance(address a, address b) external view returns (uint256) {
        return 0;
    }

    function bar(Foo memory foo) external pure returns (Foo memory) {
        return foo;
    }
}

Test.t.sol

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

import {MockMyContract} from "./Mocks.sol";
import {MyContract} from "./MyContract.sol";
import {Test} from "forge-std/Test.sol";

contract MyContractTest is Test {
    using MockMyContract for *;

    MyContract internal myContract;

    function setup() public {
        myContract = new MyContract();
        myContract.balanceOf.mockCall(1 ether);
    }

    function testBalanceOf() public {
        assertEq(myContract.balanceOf(address(0)), 1 ether);
    }
}