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Interact with a Contract

Introduction

If you were writing raw requests to the TRON network yourself in order to interact with your contracts or conduct tests, you'd soon realize that writing these requests is clunky and cumbersome. As well, you might find that managing the state for each request you've made is complicated. Fortunately, TronBox takes care of this complexity for you to make interacting with your contracts a breeze.

Read and write data

The TRON network distinguishes between writing data to the network and reading data from it, which plays a significant part in how you write your application. In general, writing data is called a transaction whereas reading data is called a call. Transactions and calls are treated very differently and have the following characteristics.

Transaction

Transactions fundamentally change the state of the network. A transaction can be as simple as sending TRX to another account, or as complicated as executing a contract function or adding a new contract to the network. The defining characteristic of a transaction is that it writes (or changes) data. Transactions cost "energy" to run and take time to process. When you execute a contract's function via a transaction, you cannot receive that function's return value because the transaction isn't processed immediately. In general, functions meant to be executed via a transaction will not return a value, but a transaction id instead. So in summary, transactions:

  • Cost Energy and Bandwidth, which can be obtained by freezing TRX
  • Change the state of the network
  • Aren't processed immediately (need to wait for confirmations by Super Representatives)
  • Won't expose a return value (only a transaction id)

Call

Calls, on the other hand, are very different. Calls can be used to execute code on the network, though no data (e.g., state variables) will be permanently changed. The defining characteristic of calls is that they read data. When you execute a contract function via a call, you will receive the return value immediately. In summary, calls:

  • Are free (cost no Energy or Bandwidth)
  • Do not change the state of the network
  • Are executed immediately
  • Will expose a return value

Choosing between a transaction and a call is as simple as deciding whether you want to read data, or write it.

Introducing contract abstractions

Contract abstractions are the bread and butter of interacting with TRON contracts from Javascript. In short, contract abstractions are wrapper code that makes interaction with your contracts easy, in a way that lets you forget about the many engines and gears executing under the hood. TronBox uses its own contract abstraction via the tronbox-contract module, and this contract abstraction is described below.

Here we use the MetaCoin contract available to you through TronBox via tronbox unbox metacoin to help you appreciate the usefulness of a contract abstraction.

pragma solidity >=0.4.25 <0.6.0;
 
import "./ConvertLib.sol";
 
 
 
//This is just a simple example of a coin-like contract. It is not standards-compliant.
//If you want to create a standards-compliant token, see: https://github.com/ConsenSys/Tokens.
 
contract MetaCoin {
    mapping (address => uint) balances;
 
    event Transfer(address indexed _from, address indexed _to, uint256 _value);
 
    constructor() public {
        balances[tx.origin] = 10000;
    }
 
    function sendCoin(address receiver, uint amount) public returns(bool sufficient) {
        if (balances[msg.sender] < amount) return false;
        balances[msg.sender] -= amount;
        balances[receiver] += amount;
        emit Transfer(msg.sender, receiver, amount);
        return true;
}
 
    function getBalanceInEth(address addr) public view returns(uint){
        return ConvertLib.convert(getBalance(addr),2);
}
 
    function getBalance(address addr) public view returns(uint) {
        return balances[addr];
}
}

This contract has three methods aside from the constructor (sendCoin, getBalanceInEth, and getBalance). All three methods can be executed as either a transaction or a call.

Now let's look at the Javascript object called MetaCoin provided for us by TronBox, as made available in the TronBox console:

tronbox(development) > MetaCoin.deployed().then(instance => console.log(instance));
 
// outputs:
//
// Contract
// - address: "0xa9f441a487754e6b27ba044a5a8eb2eec77f6b92"
// - allEvents: ()
// - getBalance: ()
// - getBalanceInEth: ()
// - sendCoin: ()
// ...

NOTE that the abstraction contains the exact same functions that exist within our contract. It also contains an address that points to the deployed version of the MetaCoin contract.

Execute contract functions

With the abstraction, you can easily execute contract functions on the TRON network.

Make a transaction

There are three functions on the MetaCoin contract that we can execute. If you analyze each of them, you'll see that sendCoin is the only function that aims to make changes to the network. The goal of sendCoin is to "send" some Meta coins from one account to another, and these changes should persist.

When calling sendCoin, we'll execute it as a transaction. In the following example, we'll send 10 Meta coins from one account to another in a way that is making a transaction:

tronbox(development) > MetaCoin.deployed().then(res => res.sendCoin(tronWeb._accounts[1], 500, { from: tronWeb._accounts[0] }));

A few things are interesting about the above code:

  • We called the abstraction's sendCoin function directly. This will result in a transaction by default (i.e, writing data) instead of call.
  • We passed an object as the third parameter to sendCoin. Note that the sendCoin function in our Solidity contract doesn't have a third parameter. What you see above is a special object that can always be passed as the last parameter to a function that lets you edit specific details about the transaction. Here, we set the from address ensuring this transaction came from accounts[0].

Make a call

Continuing with MetaCoin, notice the getBalance function is a great candidate for reading data from the network. It doesn't need to make any changes, as it just returns the MetaCoin balance of the address passed to it. Let's give it a shot:

tronbox(development) > MetaCoin.deployed().then(res => res.getBalance(tronWeb._accounts[0]));

Process transaction results

When you make a transaction, you're given a result object that gives you a wealth of information about the transaction.

tronbox(development) > MetaCoin.deployed().then(res => res.sendCoin(tronWeb._accounts[1], 500, { from: tronWeb._accounts[0] }));
 
// outputs:
// 475d5006b56685cc40e07b36498f4d4809f2ce4fa9c678c108476dfe4e02c59e

Specifically, you get the following via the transaction id:

tronbox(development) > tronWeb.trx.getTransactionInfo('475d5006b56685cc40e07b36498f4d4809f2ce4fa9c678c108476dfe4e02c59e');
 
//outputs:
// {
//   id: '475d5006b56685cc40e07b36498f4d4809f2ce4fa9c678c108476dfe4e02c59e',
//   fee: 1253000,
//   blockNumber: 64403,
//   blockTimeStamp: 1667550996000,
//   contractResult: [
//     '0000000000000000000000000000000000000000000000000000000000000001'
//   ],
//   contract_address: '419c1aad134258c42af129822814098333ba251e52',
//   receipt: {
//     energy_fee: 1253000,
//     energy_usage_total: 12530,
//     net_usage: 346,
//     result: 'SUCCESS'
//   },
//   log: [
//     {
//       address: '9c1aad134258c42af129822814098333ba251e52',
//       topics: [Array],
//       data: '000000000000000000000000989be99ce2c707c16b3d390e943b85c76381718600000000000000000000000093668cda1f0dce82c808d7c1134c3e85d7fc71d200000000000000000000000000000000000000000000000000000000000001f4'
//     }
//   ]
// }
  • id (string) - Transaction hash
  • log (array) - Decoded events (logs)
  • receipt (object) - Transaction receipt (includes the amount of Energy used)

Catch events

Your contracts can fire events that you can catch to gain more insight into what your contracts are doing. Solidity uses the LOG command to record event information in the log field TransactionInfo. The structure of an event is shown below by using the gettransactioninfobyid API to fetch TransactionInfo.
The event is triggered as part of the sendCoin call (Transfer(msg.sender, receiver, amount);).

{
    "id": "475d5006b56685cc40e07b36498f4d4809f2ce4fa9c678c108476dfe4e02c59e",
 
    ......
 
    "log": [
    {
      "address": "FEQNQt6AQsPiaFJGhrWbss6MjSXC9Prq2",
      "topics": [
        "ddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"
      ],
      "data": "000000000000000000000000989be99ce2c707c16b3d390e943b85c76381718600000000000000000000000093668cda1f0dce82c808d7c1134c3e85d7fc71d200000000000000000000000000000000000000000000000000000000000001f4"
    }
  ]
}
 
log: [
    {
      address: '9c1aad134258c42af129822814098333ba251e52',
      topics: [Array],
      data: '000000000000000000000000989be99ce2c707c16b3d390e943b85c76381718600000000000000000000000093668cda1f0dce82c808d7c1134c3e85d7fc71d200000000000000000000000000000000000000000000000000000000000001f4'
    }
  ]

Add a new contract to the network

We can deploy our own version to the network using the .new() function:

tronbox(development)> MetaCoin.new('50').then(res => {console.log(res.address)})
 
41449ca55809f15910db70e67f63c875fcc33e04ac