How do we use watch with an event in geth?

Question

I have a geth client running (private) and I have a simple test contract to test events.

> con4.changedAge().watch()
{
  callbacks: [undefined],
  filterId: "0x3cf5f07c90dd95c5d97276659bf78d0f",
  getLogsCallbacks: [],
  implementation: {
    getLogs: function(),
    newFilter: function(),
    poll: function(),
    uninstallFilter: function()
  },
  options: {
    address: "0xa6b4017e8437ace3301f9751735e84836cfc0b21",
    from: undefined,
    fromBlock: undefined,
    to: undefined,
    toBlock: undefined,
    topics:     
["0x25d30a71a6b93533225c97277375c3d49d3451741fad4852f106d04a5f083b5a"]
  },
  pollFilters: [],
  requestManager: {
    polls: {
      0x3cf5f07c90dd95c5d97276659bf78d0f: {
        data: {...},
        id: "0x3cf5f07c90dd95c5d97276659bf78d0f",
        callback: function(error, messages),
        uninstall: function()
      },
      0x49268254732d941a071b3c422d35104e: {
        data: {...},
        id: "0x49268254732d941a071b3c422d35104e",
        callback: function(error, messages),
        uninstall: function()
      }
    },
    provider: {
      newAccount: function(),
      openWallet: function(),
      send: function github.com/ethereum/go-ethereum/console.    (*bridge).Send-fm(),
      sendAsync: function github.com/ethereum/go-ethereum/console.    (*bridge).Send-fm(),
      sign: function(),
      unlockAccount: function()
    },
    timeout: {},
    poll: function(),
    reset: function(keepIsSyncing),
    send: function(data),
    sendAsync: function(data, callback),
    sendBatch: function(data, callback),
    setProvider: function(p),
    startPolling: function(data, pollId, callback, uninstall),
    stopPolling: function(pollId)
  },
  formatter: function(),
  get: function(callback),
  stopWatching: function(callback),
  watch: function(callback)

}

Here is the contract -

pragma solidity ^0.4.18;

contract CollegeAdmin {
uint public studentAge=18;
event changedAge(uint age);
function setAge(uint age) public {
studentAge = age;
changedAge(age);
}

} 

I just want to test when the event is triggered. I thought something like con4.setAge(19) could then trigger the watch. Has anyone any ideas?

Answer 1

You can call this:

var changedAgeEvent = contract.changedAge();

then:

changedAgeEvent.watch(function(error, result){
            if (!error)
                {
                    console.log(result);
                } else {
                    console.log(error);
                }
        });

it will trigger and log the event variables to the console each time changedAge is triggered.

Answer 2

watch is a async method. Checkout below code for more details.

$> ageCallback = con4.changedAge()

Will save ageCallback method ref.Which will have a watch() method. Watch method will take a function as a callback.

$> ageCallback.watch(function(error, result){

     if(error){
       console.log("Error", error);
       return;
     }
    console.log("Result", result);
});

So final code without 2nd variable:

$> con4.changedAge().watch(function(error, result){
     if(error){
       console.log("Error", error);
       return;
     }
    console.log("Result", result);
});

Answer 3

This worked for me -

var myEvent = con.changedAge({}, {fromBlock: 0, toBlock: 'latest'});
r=1
myEvent.watch(function(error, result){
 console.log(result)
r=result.args.age
console.log(r)
});

docs here - https://github.com/ethereum/wiki/wiki/JavaScript-API#callback-return