How does solidity detect that Ether has been sent from one address to the other address?

Question

I have learned solidity. And I have one question.

How does contract detect that Ether has been sent from one address to the other address?

Answer 1

Each contract has an implicit 'default function' function (). If this function is modified as payable then the contract can receive fund through that function from a trivial send transaction.

If the default function implements code, then that code will be also be run. However the default amount of gas in a send transaction can do little more that trigger an event. Anything more and the sender needs to be aware of the higher amount of gas required.

contract SimplySend {
    event Paid(uint);
    function () payable {
        Paid(msg.value);
    }
}

contract NeedExtraGas {
    event Paid(uint);
    uint total;
    function () payable {
        total += msg.send;
        Paid(msg.value);
    }
}