Blockchain update data notification

Question

My smart contract looks like this

contract C {
    uint256 a=50;  //

    function setA(uint256 aval) {
        a = aval;
    }
    function getA() constant returns (uint256) { 
        return a;
    }
 }

I am calling the function asynchronously like this

var a = contract.setA.call(100, function(error, value) {
    .....
});

After that if I call this immediately

var a = contract.getA.call(function(error, value) {
    .....
});

I am getting the same value(50) before the transaction is mined. This is because it needs some time if I want to get the changed value. Is there any way or event so that I get notification that the changed value is updated in blockchain when I can getA to be ensured the current exact value?

Answer 1

State is not mutated by calls. You need to send a transaction to actually change the value. Then you need to wait for the transaction to be mined before you can read the updated value.

Something like this should work:

function waitForReceipt(hash, cb) {
  web3.eth.getTransactionReceipt(hash, function (err, receipt) {
    if (err) {
      alert(err);
    }

    if (receipt !== null) {
      // Transaction went through
      if (cb) {
        cb(receipt);
      }
    } else {
      // Try again in 1 second
      window.setTimeout(function () {
        waitForReceipt(hash, cb);
      }, 1000);
    }
  });
}

contract.setA.sendTransaction(100, function (error, transactionHash) {
  waitForReceipt(transactionHash, function () {
    contract.getA.call(function (error, value) {
      console.log("New value: " + value.toString());
    });
  });
});

That code is adapted from https://programtheblockchain.com/posts/2017/12/13/building-decentralized-apps-with-ethereum-and-javascript/.