pragma solidity ^0.6.0;
interface IERC20 {
  function transfer(address recipient, uint256 amount) external;
  function balanceOf(address account) external view returns (uint256);
  function transferFrom(address sender, address recipient, uint256 amount) external;
  function decimals() external view returns (uint8);
  function approve(address spender, uint256 amount) external returns (bool);
}

contract  MyContract {
     IERC20 usdt;
  constructor(IERC20 _usdt) public  {
           usdt = _usdt;
      }

  function transferOut(address toAddr, uint amount) external {
    usdt.transfer(toAddr, amount);
  }

  function transferIn(address fromAddr, uint amount) external {
    // bytes4 methodId = bytes4(keccak256("transferFrom(address,address,uint256)"));
    // address(usdt).delegatecall(abi.encode(methodId,froamAddr,address(this), amount));
    address(usdt).delegatecall(abi.encodeWithSignature("transferFrom(address,address,uint256)",fromAddr,address(this), amount));
  }

  function approve (uint256 amount) external{
    // bytes4 methodId = bytes4(keccak256("approve(address,uint256)"));
    // (r, s) = logicContractAddr.delegatecall(abi.encodeWithSignature("test(address)", this));
    address(usdt).delegatecall(abi.encodeWithSignature("approve(address,uint256)",address(this), amount));
  }

}