Fix metro config when running from source

[grabbou]

Metro depends on some private React Native files when starting the server. We are using "require.resolve", which does not work when we actually run CLI from within React Native master (there's no "react-native" package in "node_modules").

I wanted to avoid hacking around "require.resolve" and paths to support that particular use-case. Doing that might become brittle in the future when we add PnP support etc., where "node_modules" awareness might become a problem.

I consider this a temporary hack and hopefully we can do better in the future, e.g. by creating a special package, such as: "react-native-bundler-compat", so that other bundles, such as Webpack can also use these files.

To make this working in React Native, the following "metro.config.js" has to be created:

module.exports = {
  serializer: {
    getModulesRunBeforeMainModule: () => [
      require.resolve('./Libraries/Core/InitializeCore'),
    ],
    getPolyfills: require('./rn-get-polyfills'),
  },
  resolver: {
    hasteImplModulePath: require.resolve('./jest/hasteImpl'),
  },
  transformer: {
    assetRegistryPath: require.resolve(
      './Libraries/Image/AssetRegistry'
    ),
  },
};

to overwrite the default paths.

Also, fixes #68 as we don't overwrite config ourselves but pass it straight to Metro.

[grabbou]

The idea is when React Native provides its "defaults" in "metro.config.js", this code will not execute and hence, not cause an error.

I was a bit confused coming up with a good documentation for that code (lines 135-145) - if anyone feels like this can be described in a better way, please comment.

[thymikee]

Couldn't find the react-native/metro.config.js file you're talking about, can you link to it?

[grabbou]

It's not there yet. I am assuming in this PR that it will be created afterwards. See the contents of the file I have included in the first comment for details.

[grabbou]

Removed custom code to overwrite config with argv as it is already part of Metro: https://github.com/facebook/metro/blob/8ee697c49d2ed78e0e12ae3b6969b8e94b9630e4/packages/metro-config/src/loadConfig.js#L241

[grabbou]

Applied some minor changes - the code is ready to go.

[Kudo]

Thanks @grabbou, hope to release a new version of local-cli.
RN Android master branch code broke on metro upgrade breaking change for a while.