Payable method that don’t know at once how much amount it worth

Smart Contracts
9

It is possible to create a contract method that can be paid for, but the amount of the payment needs to be specified at the time the method is called. This is because transactions on the Ethereum network, are used to execute contract methods, include a "value" field that specifies the amount of Ether being sent along with the transaction. If you want to create a contract method that generates a random amount and requires payment of that amount, you can do so by having the method generate the random amount, store it in a contract state variable, and then have the method require the caller to send the correct amount of Ether as part of the transaction.

sample code:

pragma solidity ^0.7.0;

contract RandomPayment {
    uint public randomAmount;

    function generateRandomAmount() public {
        randomAmount = uint(keccak256(abi.encodePacked(now, msg.sender))) % 1000;
    }

    function payRandomAmount() public payable {
        require(msg.value == randomAmount, "Incorrect payment amount");
        // Do something with the payment
    }
}

To call these functions using web3.js, you can use the following code:

const Web3 = require('web3');
const web3 = new Web3(/* your provider */);

const contractAddress = /* your contract address */;
const contract = new web3.eth.Contract(/* your contract ABI */, contractAddress);

// Generate a random amount
await contract.methods.generateRandomAmount().send({ from: /* your address */ });

// Get the random amount generated by the contract
const randomAmount = await contract.methods.randomAmount().call();

// Make a payment of the random amount
await contract.methods.payRandomAmount().send({
    from: /* your address */,
    value: randomAmount,
});

Goodluck

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