Can an erc20 token contract decide how much of an investors tokens are claimable based on an external event?

Question

Post ICO, I plan to return a specific amount of investors' fiat funds if an event doesn't occur before the initial claimable event. Is it possible to add a clause into the claimability of tokens based on a call?

Vesting period reached --> check to see if event has occurred (if true) --> release all tokens (if false) --> release percentage of tokens and burn remaining.

Answer 1

In general Ethereum does not support the event-driven paradigm. There needs to be a transactional trigger (most likely you calling a function) that releases the tokens. Or in a withdraw function called by the investors

function withdraw() external {
    if(block.timestamp < vestingPeriodEnd) return; // do nothing
    if(eventOccurred) // return amount A
    else // return amount B
}