Transfer From And Transfer

In ERC20 token sepcification, we know that there are two ways to transfer token from one account to another. That are transfer and transfer from methods. And we know that the transfer method just transfers token from the msg.sender to another. And transfer from methods is transfer from the owner of the token to another account. And there is a pre-condition before transfer from frunction be called. That is the owner of the token should approve the msg.sender the apporopriate amount tokens. I come across an user case when i exercises writing CrownFund smart contract.

    function pledge(uint256 _id, uint256 _amount) external {
        Campaign storage campaign = campaigns[_id];
        require(block.timestamp > campaign.startAt, "campaign not started");
        require(block.timestamp <= campaign.endAt, "campaign ended");
        campaign.pledged += _amount;
        pledgeAmount[_id][msg.sender] += _amount;
        token.transferFrom(msg.sender, address(this), _amount);
        emit Pledge(_id, msg.sender, _amount);
    }

When people are pledging a campaign. They should transfer some amount of tokens to the contract. In this line code token.transferFrom(msg.sender, address(this), _amount);, They call the function pledge in CrownFund contract. Then the CrownFund contract call the transferFrom function through directly call contract. In the transferFrom function the msg.sender should be the address of the CrownFund contract. And the receivor for the token is also the CrownFund. So the pledgor have to approve the contract. And then the contract can call the transferFrom function to transfer to itself.

And in many cases like one Externally Owned Account want to transfer token to a contract. At first the EOA call one function in the contract. And the contract call token contract to execute the token transaction. So we should use the transferFrom function to approve the contract allowance. Then the contract use the algorithm to transfer itself the token.

The whole code is in my github repository CrownFund.sol.