Add the ability to change the packager commands

[pepicrft]

Summary:

At Shopify, we are using the React Native CLI in a monorepo setup. Since the CLI launches the packager taking the directory where React Native lives as the working directory, it causes the packager command to read the Metro configuration from the wrong directory.

This PR fixes it by giving the users the flexibility to customize the directory where the launch packager command and environment files are defined. On iOS, we changed the packager build phase to use our own launcher.

Please, let me know if you'd have taken a different approach. We only need to be able to specify where the Metro configuration lives.

Test Plan:

We tested it with our own monorepo and it works. We included that directory with the command and our own instructions on how to launch the packager.

cc @ElviraBurchik

[thymikee]

This seems very specific to a certain edge-case use case. Can't we make sure the custom Metro config is propagated to the server instance? E.g. by utilizing Metro's --config flag, which would be more generic and flexible

[pepicrft]

This seems very specific to a certain edge-case use case. Can't we make sure the custom Metro config is propagated to the server instance? E.g. by utilizing Metro's --config flag, which would be more generic and flexible

Sure! Do you think it's fine if I add a config argument to this list and then pass it to the launcher?

[thymikee]

I think for now we can go with --metro-config and add it for iOS as well, for parity. What do you think @grabbou @satya164?

[pepicrft]

Sounds good. Unless there are objections, I'll go ahead and use that flag instead.

[grabbou]

Thanks for the PR! I do agree with @thymikee that we should either utilise existing flags / Metro configuration instead of adding a new flag.

Right now, we obtain Metro configuration programatically here https://github.com/react-native-community/cli/blob/master/packages/cli/src/tools/loadMetroConfig.ts#L152 and we set cwd to root from Config. By design, Metro will look for a config there.

However, we also pass a second object, options, which - inside start and bundle commands, has historical config property ->

cli/packages/cli/src/commands/start/start.ts

Lines 87 to 89 in 5ec3fcf

	name: '--config [string]',
	description: 'Path to the CLI configuration file',
	parse: (val: string) => path.resolve(val),

You can use it to set a path to custom Metro configuration. The name probably needs to be updated, as historically, Metro/CLI config used to be a single file. While refactoring, we stopped relying on that property inside CLI, but kept it for Metro.

So, in general, I would say there are two ways going forward:

• either add --config to missing commands
• remove --config entirely and extend Config with metroConfig property. Users would have to create react-native.config.js file and define this property

While second option is more aligned with our long-term plan, I suspect it would break some tooling such as Expo. So as a short-term hack, I'd just stick to adding --config here and there.

One thing to consider on top of that would be to rename --config to --metro-config. While doing so, I would use Commander feature to define multiple names for an option and accept both occurrences. We don't want to break existing tooling, at the same time, we want to promote less confusing naming.