get the return value of payable function? [duplicate]

Question

Let's say there's a payable Solidity function

function doSomething() 
  payable
  returns(uint)
{
  require(msg.value == 1 ether);
  return 1;
}

And you call this function

instance.doSomething({
      from:_account,
      value:1 ether
    }).then(function(value) {
    }).catch(function(err) {
    });

This will send ether to the function, make transaction and will return the transaction result. Cool.

But how can I get the return value of that Solidity function? Transaction record does not have anything about it.

'instance.doSomething.sendTransaction()' also the same situation.

Because the function is payable, it should receive ethereum.

Many thanks, coindevbw

Answer 1

Since transactions don't return values outside Solidity, what you need to do inside the function is to fire an event. Then, in your front-end you can watch events and filter them accordingly to retrieve whatever data you provided to that event.

If you use truffle, here's a simple way to do it form their docs:

http://truffleframework.com/docs/getting_started/contracts

var account_one = "0x1234..."; // an address
var account_two = "0xabcd..."; // another address

var meta;
MetaCoin.deployed().then(function(instance) {
  meta = instance;  
  return meta.sendCoin(account_two, 10, {from: account_one});
}).then(function(result) {
  // result is an object with the following values:
  //
  // result.tx      => transaction hash, string
  // result.logs    => array of decoded events that were triggered within this transaction
  // result.receipt => transaction receipt object, which includes gas used

  // We can loop through result.logs to see if we triggered the Transfer event.
  for (var i = 0; i < result.logs.length; i++) {
    var log = result.logs[i];

    if (log.event == "Transfer") {
      // We found the event!
      break;
    }
  }
}).catch(function(err) {
  // There was an error! Handle it.
});

Answer 2

I have no points to comment yet, so I will paste a link to a very simple solution from Force Hero.

In a nutshell:

const tx = await contract.transfer(...args); // 100ms
const rc = await tx.wait(); // 0ms, as tx is already confirmed
const event = rc.events.find(event => event.event === 'Transfer');
const [from, to, value] = event.args;
console.log(from, to, value);

Hope it helps someone.