Components and Coding¶
Author: A Byte Ahead https://medium.com/@laalaguer/how-to-develop-a-dapp-on-vechain-iii-components-coding-8c4eea965684
Congratulations! You have made to the last episode of this tutorial. Here we will put everything we know together and make a useful program: a VTHO token transfer tool. The official Sync only provides the VET transfer functionality, our VTHO tools is a very good supplement to the status quo.
Preparation¶
Make sure you have the following things ready:
- Sync set up and running on your computer.
- A testing account, which is already presented in Sync.
- Some VET and VTHO in your testing account.
- Sync running on test net. [block:image] { “images”: [ { “image”: [ “https://doc.vechainworld.io/images/e3d2843-1_5sGTMuzQ8gaIiPG9COavjw.jpeg”, “1_5sGTMuzQ8gaIiPG9COavjw.jpeg”, 1600, 1359, “#eceaea” ] } ] } [/block]
If you don’t know how above requirements are done, check the previous episode: https://medium.com/@laalaguer/how-to-develop-a-dapp-on-vechain-ii-setup-walk-around-109a01bf7ae9
Setup in 3 minutes¶
Great! Now we start to build the real web page DApp. Let us set up the project and necessities first. We will use Vue.js and BootstrapVue as our foundation. [block:code] { “codes”: [ { “code”: “> mkdir vtho-transfer\n> cd vtho-transfer/\n> npm install -g @vue/cli # Install vue-cli.\n\n> vue create -b -n my-project # Create a clean Vue project.\n> cd my-project # Now you are in a Vue workspace!\n> npm install vue bootstrap-vue bootstrap # Install BootstrapVue.\n> npm install bignumber.js # Math library.\n> npm run serve # Start development server on background.”, “language”: “text”, “name”: null } ] } [/block] We are all set! Now let us view our precious project in Sync. Open a tab and navigate to http://localhost:8080. It should be like this: [block:image] { “images”: [ { “image”: [ “https://doc.vechainworld.io/images/20a7a0d-1_Sg-YkK-8W-WWbt8WV4F_uA.jpeg”, “1_Sg-YkK-8W-WWbt8WV4F_uA.jpeg”, 1600, 1335, “#f1f2f2” ] } ] } [/block]
Coding¶
Due to the length of this tutorial, we write one core component and demo the key functions. The rest of the code and a more complicated example can be found at the bottom of this tutorial.
The Appearance¶
The app must be stylish! Clean up the default main.js and add a BootstrapVue library, quite simple:
[block:code]
{
“codes”: [
{
“code”: “import Vue from ‘vue’\nimport BootstrapVue from ‘bootstrap-vue’ // Here\nimport ‘bootstrap/dist/css/bootstrap.css’ // Here\nimport ‘bootstrap-vue/dist/bootstrap-vue.css’ // Here\nimport App from ‘./App.vue’\n\nVue.use(BootstrapVue) // Here\n\nVue.config.productionTip = false\n\nnew Vue({\n render: h => h(App),\n}).$mount(‘#app’)”,
“language”: “javascript”
}
]
}
[/block]
Put the component on the radar screen by editing the App.vue:
[block:code]
{
“codes”: [
{
“code”: “\n <div id=”app”>\n My Amount Here <span class=”text-primary”>VTHO
Query the VTHO Balance¶
Okay, let us first create a function queries the VTHO balance from the VTHO contract on test net, actually, it can be used to query any contract that is VIP180 compatible. It should be like this:
[block:code]
{
“codes”: [
{
“code”: “/**\n * Get token amount of holder from a contract.\n * @param {String} addressContract 0x started address.\n * @param {String} addressHolder 0x started address.\n */\nasync function getTokenBalance (addressContract, addressHolder) {\n const balanceOfABI = {\n ‘constant’: true,\n ‘inputs’: [\n {\n ‘name’: ‘_owner’,\n ‘type’: ‘address’\n }\n ],\n ‘name’: ‘balanceOf’,\n ‘outputs’: [\n {\n ‘name’: ‘balance’,\n ‘type’: ‘uint256’\n }\n ],\n ‘payable’: false,\n ‘stateMutability’: ‘view’,\n ‘type’: ‘function’\n }\n // eslint-disable-next-line\n const balanceOfMethod = connex.thor.account(addressContract).method(balanceOfABI)\n const balanceInfo = await balanceOfMethod.call(addressHolder)\n return balanceInfo\n}”,
“language”: “javascript”
}
]
}
[/block]
Quite self-explanatory, this getTokenBalance function queries the contract about how much token the user holds, when we call it, we feed it with the actual contract address and user’s account address. Notice that we used the call() function of connex.js, which we introduced in the previous episode.
Sign Transaction and Send VTHO¶
How to sign the transaction and send it to the other person? Simple, we just use the sign-and-send API embedded in the connex.js. Let’s create a function for that, too. Still, in the operation.js, append this code:
[block:code]
{
“codes”: [
{
“code”: “/**\n * Transfer token from one to another.\n * @param {String} addressContract Contract address.\n * @param {String} signerAddress Enforce who signs the transaction.\n * @param {String} toAddress Receiver of transfer.\n * @param {String} amountEVM Big number in string.\n * @param {Number} amountHuman Normal number in Javascript.\n * @param {String} symbol Symbol of token.\n */\nasync function transferToken (addressContract, signerAddress, toAddress, amountEVM, amountHuman, symbol) {\n const transferABI = {\n ‘constant’: false,\n ‘inputs’: [\n {\n ‘name’: ‘_to’,\n ‘type’: ‘address’\n },\n {\n ‘name’: ‘_value’,\n ‘type’: ‘uint256’\n }\n ],\n ‘name’: ‘transfer’,\n ‘outputs’: [\n {\n ‘name’: ‘’,\n ‘type’: ‘bool’\n }\n ],\n ‘payable’: false,\n ‘stateMutability’: ‘nonpayable’,\n ‘type’: ‘function’\n }\n // eslint-disable-next-line\n const transferMethod = connex.thor.account(addressContract).method(transferABI)\n const transferClause = transferMethod.asClause(toAddress, amountEVM)\n // eslint-disable-next-line\n const signingService = connex.vendor.sign(‘tx’)\n signingService\n .signer(signerAddress) // Enforce signer\n .comment(‘Token transfer: ‘ + amountHuman.toString() + ‘ ‘ + symbol)\n \n let transactionInfo = await signingService.request([\n {\n comment: ‘Hello! Transfer Demo!’,\n …transferClause\n }\n ])\n return transactionInfo\n }”,
“language”: “javascript”
}
]
}
[/block]
The above snippet is mainly about transferToken, which is a general purpose transfer action. It takes the contract address, sender address, receiver address, amount of transfer, then asks the connex to sign the transaction and send out!
Now finish the operations.js file with the export section as usual: [block:code] { “codes”: [ { “code”: “ export {\n getTokenBalance,\n transferToken\n }”, “language”: “javascript” } ] } [/block]
Formatting Numbers¶
In the smart contract, the decimals don’t exist. All numbers are integers. To present the numbers right to the decimal, we power it with 10 times precision. So when we say: 123.456 in human language, it should be presented in:
123.456 * (10 ^ 18) and then send to EVM. And vice versa.
So we need some utilities as a broker to help us with number formatting. Create a new file utils.js and place the following content inside: [block:code] { “codes”: [ { “code”: “const BigNumber = require(‘bignumber.js’)\nconst DECIMALS = function (points) {\n return new BigNumber(10 ** points) // Decimals = 18 on VTHO and most contracts.\n}\n\n/\n * Turn a string to big number.\n * @param {String} aString a number string.\n */\nconst makeBN = function (aString) {\n return BigNumber(aString)\n}\n\n/\n * Turn a BigNumber into a printable string.\n * @param {BigNumer} aBigNumber A big number.\n * @param {Integer} dp An integer of percision, default is 2.\n */\nconst printBN = function (aBigNumber, dp = 2) {\n return aBigNumber.toFixed(dp)\n}\n\n/\n * Turn an EVM big number into normal human understandable percision.\n * @param {BigNumber} aBigNumber An EVM big number.\n * @param {Number} decimals Percisions that EVM number has. Default is 18.\n */\nconst evmToHuman = function (aBigNumber, decimals = 18) {\n return aBigNumber.dividedBy(DECIMALS(decimals))\n}\n\n/\n * Turn a human understandable number to an EVM Big number.\n * @param {String} aNumber A normal float/int from user input.\n * @param {Number} decimals Percisions that EVM number has. Default is 18.\n * @returns {String} String represented number.\n */\nconst humanToEVM = function (aNumber, decimals = 18) {\n const a = makeBN(aNumber)\n return a.multipliedBy(DECIMALS(decimals)).toString(10)\n}\n\n/**\n * Shortcut turing EVM big number string into human readable short format.\n * @param {String} aString String representing the EVM big number.\n * @param {Number} decimals Percisions that EVM number has. Default is 18.\n * @param {BigNumber} dp decimal points that result shall keep.\n */\nconst evmToPrintable = function (aString, decimals = 18, dp = 2) {\n const evmNumber = makeBN(aString)\n const humanNumber = evmToHuman(evmNumber, decimals)\n return printBN(humanNumber, dp)\n}\n\nexport {\n makeBN,\n printBN,\n evmToHuman,\n humanToEVM,\n evmToPrintable\n}”, “language”: “javascript” } ] } [/block] Great! All is done, now we put things together and make our app alive!
Put Things Together¶
Time to go back to App.vue to fill up the Javascript section.
Firstly, let’s call the VTHO contract and display our VTHO balance, here is what I did in code:
- Fix in the VTHO smart contract address.
- Fix in my account address. (Generated from the previous episode of tutorials)
- Initialization the VTHO balance query before the component is mounted. [block:code] { “codes”: [ { “code”: “\n”, “language”: “javascript” } ] } [/block]
Note: you can always use ticker() instead of setTimeout() to recursively query the balance of your account. 😉
Secondly, we add the function of sending VTHO token transactions to the button, so we can get alert whenever it succeeds or fails.
[block:code]
{
“codes”: [
{
“code”: “”,
“language”: “javascript”
}
]
}
[/block]
Great! Now update our template to bind to those data:
[block:code]
{
“codes”: [
{
“code”: “\n <div id=”app”>\n {{myamount}} <span class=”text-primary”>VTHO
GitHub repo of all the source code: [GitHub Link]
