metamask-extension/ui/app/hooks/useTokenTracker.js

import { useState, useEffect, useRef, useCallback } from 'react';
import TokenTracker from '@metamask/eth-token-tracker';
import { useSelector } from 'react-redux';
import { getCurrentNetwork, getSelectedAddress } from '../selectors';
import { useEqualityCheck } from './useEqualityCheck';

export function useTokenTracker(tokens, includeFailedTokens = false) {
  const network = useSelector(getCurrentNetwork);
  const userAddress = useSelector(getSelectedAddress);

  const [loading, setLoading] = useState(() => tokens?.length >= 0);
  const [tokensWithBalances, setTokensWithBalances] = useState([]);
  const [error, setError] = useState(null);
  const tokenTracker = useRef(null);
  const memoizedTokens = useEqualityCheck(tokens);

  const updateBalances = useCallback((tokenWithBalances) => {
    setTokensWithBalances(tokenWithBalances);
    setLoading(false);
    setError(null);
  }, []);

  const showError = useCallback((err) => {
    setError(err);
    setLoading(false);
  }, []);

  const teardownTracker = useCallback(() => {
    if (tokenTracker.current) {
      tokenTracker.current.stop();
      tokenTracker.current.removeAllListeners('update');
      tokenTracker.current.removeAllListeners('error');
      tokenTracker.current = null;
    }
  }, []);

  const buildTracker = useCallback(
    (address, tokenList) => {
      // clear out previous tracker, if it exists.
      teardownTracker();
      tokenTracker.current = new TokenTracker({
        userAddress: address,
        provider: global.ethereumProvider,
        tokens: tokenList,
        includeFailedTokens,
        pollingInterval: 8000,
      });

      tokenTracker.current.on('update', updateBalances);
      tokenTracker.current.on('error', showError);
      tokenTracker.current.updateBalances();
    },
    [updateBalances, includeFailedTokens, showError, teardownTracker],
  );

  // Effect to remove the tracker when the component is removed from DOM
  // Do not overload this effect with additional dependencies. teardownTracker
  // is the only dependency here, which itself has no dependencies and will
  // never update. The lack of dependencies that change is what confirms
  // that this effect only runs on mount/unmount
  useEffect(() => {
    return teardownTracker;
  }, [teardownTracker]);

  // Effect to set loading state and initialize tracker when values change
  useEffect(() => {
    // This effect will only run initially and when:
    // 1. network is updated,
    // 2. userAddress is changed,
    // 3. token list is updated and not equal to previous list
    // in any of these scenarios, we should indicate to the user that their token
    // values are in the process of updating by setting loading state.
    setLoading(true);

    if (!userAddress || network === 'loading' || !global.ethereumProvider) {
      // If we do not have enough information to build a TokenTracker, we exit early
      // When the values above change, the effect will be restarted. We also teardown
      // tracker because inevitably this effect will run again momentarily.
      teardownTracker();
      return;
    }

    if (memoizedTokens.length === 0) {
      // sets loading state to false and token list to empty
      updateBalances([]);
    }

    buildTracker(userAddress, memoizedTokens);
  }, [
    userAddress,
    teardownTracker,
    network,
    memoizedTokens,
    updateBalances,
    buildTracker,
  ]);

  return { loading, tokensWithBalances, error };
}