SelectorsAndSignatures.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

contract SelectorsAndSignatures {
    address public s_someAddress;
    uint256 public s_amount; 

    function getSelectorOne() public pure returns(bytes4 selector){
        selector = bytes4(keccak256(bytes("transfer(address,uint256)")));
    }

    function getSelectorTwo() public view returns(bytes4 selector){
        bytes memory functionCallData = abi.encodeWithSignature("transfer(address,uint256)", address(this), 123);
        selector = bytes4(bytes.concat(functionCallData[0], functionCallData[1], functionCallData[2], functionCallData[3]));
    }

    function getCallData() public view returns(bytes memory){
        return abi.encodeWithSignature("transfer(address,uint256)", address(this), 123);
    }

    // Pass this:
    // 0xa9059cbb000000000000000000000000d7acd2a9fd159e69bb102a1ca21c9a3e3a5f771b000000000000000000000000000000000000000000000000000000000000007b
    // This is output of `getCallData()`
    function getSelectorThree(bytes calldata functionCallData) public pure returns(bytes4 selector){
        // offset is a special attribute of calldata
        assembly {
            selector := calldataload(functionCallData.offset)
        }
    }

    function transfer(address someAddress, uint256 amount) public {
        // Some code
        s_someAddress = someAddress;
        s_amount = amount;
    }

    function getSelectorFour() public pure returns(bytes4 selector){
        return this.transfer.selector;
    }

    function getSignatureOne() public pure returns(string memory){
        return "transfer(address,uint256)";
    }
}


contract CallFunctionWithoutContract {

    address public s_selectorsAndSignaturesAddress;

    constructor(address selectorsAndSignaturesAddress){
        s_selectorsAndSignaturesAddress = selectorsAndSignaturesAddress;
    }

    // pass in 0xa9059cbb000000000000000000000000d7acd2a9fd159e69bb102a1ca21c9a3e3a5f771b000000000000000000000000000000000000000000000000000000000000007b
    // you could use this to change state
    function callFunctionDirectly(bytes calldata callData) public returns(bytes4, bool) {
        (bool success, bytes memory returnData) = s_selectorsAndSignaturesAddress.call(
            abi.encodeWithSignature("getSelectorThree(bytes)", callData)
        );
        return (bytes4(returnData), success);
    }

    // with a staticcall, we can have this be a view function!
    function staticCallFunctionDirectly() public view returns(bytes4, bool){
        (bool success, bytes memory returnData) = s_selectorsAndSignaturesAddress.staticcall(
            abi.encodeWithSignature("getSelectorOne()")
        );
        return (bytes4(returnData), success);
    }

    function callTransferFunctionDirectly(address someAddress, uint256 amount) public returns(bytes4, bool) {
        (bool success, bytes memory returnData) = s_selectorsAndSignaturesAddress.call(
            abi.encodeWithSignature("transfer(address,uint256)", someAddress, amount)
        );
        return (bytes4(returnData), success);
    }

}