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Create a custom Snap to extend the functionality of MetaMask.

Create a Snap to calculate gas fee percentages

This tutorial walks you through creating a Snap that calculates the percentage of gas fees they would pay for their transaction.

It gets the current gas price by calling the eth_gasPrice RPC method using the global Ethereum provider made available to Snaps, and displays this as a percentage of gas fees in a tab in MetaMask's transaction confirmation window.

Prerequisites​

Steps​

1. Set up the project​

Create a new Snap project using the @metamask/create-snap starter kit by running:

yarn create @metamask/snap transaction-insights-snap

Next, cd into the transaction-insights-snap project directory and run:

yarn install

This initializes your development environment with the required dependencies.

Did you get a warning?

You may get a warning such as:

@lavamoat/allow-scripts has detected dependencies without configuration. explicit configuration required.
run "allow-scripts auto" to automatically populate the configuration.

You can resolve the issue by running:

yarn run allow-scripts auto

2. (Optional) Customize your Snap's UX​

This Snap is generated from a TypeScript template Snap. We recommend customizing your Snap to improve its UX, but this is optional for testing. If you don't wish to customize your Snap, skip to Step 3.

2.1. Provide an icon​

Optimize your metadata and display an icon for your Snap in MetaMask.

Create a new folder images in the Snap package packages/snap/:

mkdir packages/snap/images

Download this gas.svg icon file into that images folder.

Icon attribution

This is a free icon, "Gas" by Mello from the Noun Project.

Your file structure should look like this:

transaction-insights-snap/
├─ packages/
│  ├─ site/
|  |  |- src/
|  |  |  |- App.tsx
|  |  ├─ package.json
|  |  |- ...(React app content)
|  |
│  ├─ snap/
|  |  ├─ images/
|  |  |  |- gas.svg
|  |  ├─ src/
|  |  |  |- index.test.ts
|  |  |  |- index.ts
|  |  ├─ snap.manifest.json
|  |  ├─ package.json
|  |  |- ... (Snap content)
├─ package.json
├─ ... (other stuff)

Open packages/snap/snap.manifest.json in a text editor. This file contains the main configuration details for your Snap. Edit the npm object, within the location object, and add iconPath with the value "images/gas.svg" to point to your new icon:

snap.manifest.json
"location": {
  "npm": {
    "filePath": "dist/bundle.js",
    "iconPath": "images/gas.svg",
    "packageName": "snap",
    "registry": "https://registry.npmjs.org/"
  }
}

Open packages/snap/package.json in a text editor. Edit the files array and reference the images/ folder:

package.json
"files": [
  "dist/",
  "images/",
  "snap.manifest.json"
],

2.2. Update your Snap's name​

Optimize your metadata and update your Snap's name in MetaMask. MetaMask uses the proposedName of the Snap, currently "TypeScript Example" in the template.

Open packages/snap/snap.manifest.json in a text editor. Edit the "proposedName" property within the metadata to provide a functional name such as "Gas Percentage Calculator":

snap.manifest.json
{
  "version": "0.1.0",
  "description": "An example Snap written in TypeScript.",
  "proposedName": "Gas Percentage Calculator",
  ...
}

2.3. Disable the non-functional button​

Open packages/site/src/pages/index.tsx in a text editor. The template comes with a button, SendHelloButton, that is non-functional for this example. To prevent a user triggering errors, disable SendHelloButton by updating disabled={!installedSnap} as follows:

index.tsx
button: (
  <SendHelloButton
    onClick={handleSendHelloClick}
    disabled={true}
  />
),

These three updates are the minimum required to ensure that each user interaction with your Snap is well-informed. However, your Snap will function without these tweaks.

3. Enable transaction insights and the Ethereum provider​

To enable your Snap to provide transaction insights and use the global Ethereum provider, request the endowment:transaction-insight and endowment:ethereum-provider permissions in packages/snap/snap.manifest.json:

snap.manifest.json
"initialPermissions": {
  "endowment:transaction-insight": {},
  "endowment:ethereum-provider": {}
},
tip

In this tutorial, you can replace what was previously in initialPermissions. You do not need any permissions other than endowment:transaction-insight and endowment:ethereum-provider.

4. Calculate and display the percentage of gas fees​

To calculate and display the gas fees a user would pay as a percentage of their outgoing transaction, replace the code in packages/snap/src/index.ts with the following:

index.tsx
import type { OnTransactionHandler } from "@metamask/snaps-sdk";
import { Box, Heading, Text, Bold } from "@metamask/snaps-sdk/jsx";

// Handle outgoing transactions.
export const onTransaction: OnTransactionHandler = async ({ transaction }) => {

  // Use the Ethereum provider to fetch the gas price.
  const currentGasPrice = await ethereum.request({
    method: "eth_gasPrice",
  }) as string;

  // Get fields from the transaction object.
  const transactionGas = parseInt(transaction.gas as string, 16);
  const currentGasPriceInWei = parseInt(currentGasPrice ?? "", 16);
  const maxFeePerGasInWei = parseInt(transaction.maxFeePerGas as string, 16);
  const maxPriorityFeePerGasInWei = parseInt(
    transaction.maxPriorityFeePerGas as string,
    16,
  );

  // Calculate gas fees the user would pay.
  const gasFees = Math.min(
    maxFeePerGasInWei * transactionGas,
    (currentGasPriceInWei + maxPriorityFeePerGasInWei) * transactionGas,
  );

  // Calculate gas fees as percentage of transaction.
  const transactionValueInWei = parseInt(transaction.value as string, 16);
  const gasFeesPercentage = (gasFees / (gasFees + transactionValueInWei)) * 100;

  // Display percentage of gas fees in the transaction insights UI.
  return {
    content: (
      <Box>
        <Heading>Transaction insights Snap</Heading>
        <Text>
          As set up, you are paying <Bold>{gasFeesPercentage.toFixed(2)}%</Bold>
          in gas fees for this transaction.
        </Text>
      </Box>
    ),
  };
};
Notes

  • Learn more about the parameters of a submitted transaction.

  • If you have previously developed a dapp, you're likely familiar with accessing the Ethereum provider using window.ethereum. In a Snap, the window object is not available. Instead, when you request the endowment:ethereum-provider permission, your Snap is granted access to the ethereum global object.

5. Build and test your Snap​

Complete the following steps to build and test your Snap:

5.1. Build your Snap​

From the command line, run yarn start. The following displays:

You can now view site in the browser.

 http://localhost:8000/

Open localhost:8000 in your browser (with MetaMask Flask installed). A template test dapp displays, for installing and testing your Snap.

5.2. Test your Snap​

Select Connect and accept the permission request.

After connecting, you're prompted to install the Snap with the following permissions:

  • Access the Ethereum provider
  • Fetch and display transaction insights

Next, select Confirm > OK.

From MetaMask Flask, create a new testnet ETH transfer.

tip

You can set up multiple accounts to transfer between your accounts.

Next, from the transaction confirmation window in MetaMask, switch to the tab named TYPESCRIPT EXAMPLE SNAP (or Gas Percentage Calculator if you followed Step 2). Switching to the tab activates the onTransaction entry point of your Snap and displays the percentage of gas fees in the transaction insights UI:

Transaction insights UI

warning

The template Send message button is inoperable. The default template Snap used in Create a gas estimation Snap is configured to expose a JSON-RPC API with a simple hello command, which brings up a dialog box. In contrast, the Snap you're creating in this tutorial doesn't expose an API. Instead, it provides transaction insights directly in a tab in MetaMask's transaction confirmation window.

Next steps​

Consider improving your Snap's functionality further.

Display a different UI for contract interactions​

Your Snap displays a gas fee percentage for ETH transfers initiated by the user. For contract interactions, add the following code to the beginning of the onTransaction entry point:

index.tsx
if (typeof transaction.data === "string" && transaction.data !== "0x") {
  return {
    content: (
      <Box>
        <Heading>Gas Percentage Calculator</Heading>
        <Text>
          This Snap only provides transaction insights for simple ETH transfers.
        </Text>
      </Box>
    ),
  };
}

Customize your Snap​

You can improve your Snap's UX by completing Step 2. Consider updating packages/site/src/pages/index.tsx to remove the non-functional Send message button.

Before publishing a Snap, it's also important to customize the metadata and properties of your Snap, for example:

  • Update the location in snap.manifest.json to your Snap's published location.
  • Update the description in snap.manifest.json to a description of your Snap.
  • Update the name, version, description, and repository fields of /packages/snap/package.json (even if you do not plan to publish your Snap to npm).
note

When editing source, the shasum is set automatically when you build from the command line.

important

The version and repository fields in snap.manifest.json inherit the values from package.json and overwrite them in snap.manifest.json. We recommend updating version and repository in package.json first, then building the Snap project.

After you've made all necessary changes, you can publish your Snap to npm.

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