Smart Contracts have the ability to expose variables and functions for sharing their stored data publicly. In order to communicate with a contract, it is essential to have the interface definition, which can be retrieved either from a JSON file or from the function definition in the source code.
Example Data
This example used below will utilize the VTHO contract, which manages VeChain's VTHO Token.
Retrieving information is "calling" a function within a contract, which can be variables, view functions, and even functions that alter the state for simulation purposes.
contracts.executeCall is used to interact with smart contracts by providing: the contract's address as the first argument, the function ABI as the second argument and function data as the third argument.
A fourth parameter is optional and allows user to provide the options for executing a contract call within a blockchain environment.
Without Parameters
To get the value stored for the variable name from the contract, the VTHO token name in this case, you can use the code below:
When calling a function with parameters, the parameters should be passed as a list in the third argument. For instance, to check the balance of a specific address:
If a function could change the state, it would require a transaction. To check the success of a transaction, you can invoke the function first and examine the output or handle any potential errors. For example, simulating a transfer that returns true if the caller has at least 1 VTHO:
If the transaction encounters an error, the method call will also throw an error which needs to be handled appropriately.
Example Project
contracts.load(address, abi)
To simplify interaction a dynamic object can be created that can interact with passing less of the repeating arguments.
For example contracts.read.name() can load the name without the need to pass function signature, address and thor client every time.
Create Contract Object
To create a contract object it needs to be created from the thor client:
const thor = ThorClient.at('https://mainnet.vechain.org');
// contract ABI is same as in example above
const vtho = thor.contracts.load(
'0x0000000000000000000000000000456e65726779',
contractABI
);
The Contract-Loader always requires a JSON ABI Definition.
Fragments are not supported.
Read Functions
Function calls are encapsulated within a sub-object named read. This enables calling the contract for variable content, viewing functions, or performing simple transaction simulations.
await vtho.read.name() // returns the name
await vtho.read.balanceOf(address) // returns balance of address
await vtho.read.transfer(recipient, amount) // simulates a transfer
Read Options
Custom parameters, such as revision or specifying the caller of a function call, can be set for all requests using setContractReadOptions.
// read balance of an address
vtho.setContractReadOptions({ revision: "12345678" });
const balancePast = await vtho.read.balanceOf(
'0x0000000000000000000000000000000000000000'
);
console.log('Balance Past', balancePast);
