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Exercise 3

In this part we'll learn how to send transactions to the blockchain using useDApp.

In the application we're building, go to the third tab. The tab has three small forms. The exercise is to wire up the UI so we're able to interact with the WETH10 via those forms. In this exercise you'll need to modify the Exercise3.tsx file.

Each form is extracted to a separate component.

👉 Let's start with DepositComponent.


We'll use the same pattern of creating an ethers contract instance as in the previous exercise. First we need to add few more imports to the top of our file.

import { useEthers } from '@usedapp/core';
import { WETH10ABI, WETH10 } from '@simple-dapp/contracts';
import { Contract, utils } from 'ethers';

And then in the DepositComponent component:

const { chainId } = useEthers();
const weth10Address = chainId && weth10Addresses[chainId];
const weth10Contract = new Contract(weth10Address, WETH10ABI.abi) as WETH10;

Next we need to implement logic for sending a transaction when the user clicks the Deposit button. We'll use the useContractFunction hook. Let's import it first:

- import { useEthers } from '@usedapp/core';
+ import { useContractFunction, useEthers } from '@usedapp/core';

And then in the DepositComponent component:

  const { send, state } = useContractFunction(weth10Contract, 'deposit');

The useContractFunction hook returns an object with two properties: send and state. The send function is used to send a transaction. The state object contains a variety of fields with information about the transaction, but we'll only use the status field in this exercise. The status field can have one of the following values: None, PendingSignature, Mining, Success, Fail, Exception, CollectingSignaturePool. The last one is used for sending transactions with wallets.

Let's now update the handleDeposit function to use the send function:

- // TODO: deposit `value` ETH to WETH10 contract
+ send({ value: utils.parseEther(value) });
+ setValue(''); // resetting the form values

Note how the deposit function in the WETH10 contract doesn't actually take any arguments. Instead we're passing an object of what is called overrides as the last parameter (the last parameter is the last one as well in this case). The overrides object is used to pass in additional information to the transaction. In this case we're passing in the value field which is the amount of ETH we want to deposit. The value field is a special field that is used to send ETH along with the transaction.

Let's now display the transaction status.

- value={''} // TODO: show transaction status
+ value={state.status}

Let's also ensure that the user doesn't send a second transaction before the first one is mined. We'll do that by disabling the Deposit button when the transaction is pending.

- disabled={false} // Disable button if transaction is in progress
+ disabled={state.status === 'Mining' || state.status === 'PendingSignature'}


The WithdrawComponent is very similar to the DepositComponent. The only difference is that we're calling the withdraw function instead of the deposit function. The withdraw function takes one argument - the amount of WETH10 we want to withdraw. We also don't need to pass in the overrides object this time because we're not sending any ether along the transaction (but note that your account we'll still get charged the gas fee).

In the WithdrawComponent component:

- // TODO: withdraw `value` WETH from WETH10 contract
+ send(utils.parseEther(value));
+ setValue('');

Update the rest of the TODO sections in the same way as in the DepositComponent.


The only difference between the TransferComponent and the WithdrawComponent is that we're calling the transfer function instead of the withdraw function. The transfer function takes two arguments - the address of the recipient and the amount of WETH10 we want to transfer.

In the TransferComponent component:

- // TODO: transfer `value` WETH to `address`
+ send(address, utils.parseEther(value));
+ setValue('');
+ setAddress('');

Update the rest of the TODO sections in the same way as in the DepositComponent and WithdrawComponent.

That's it!

Congratulations! Now you're able to send transactions to the blockchain using useDApp.

You can head on to the last page to summarize what we've learned.