feat(bob): support custom target definitions (#732)

### Summary

This adds the `custom` target to bob. Users are able to pass arbitrary
scripts via this target and bob will call those scripts.

### Test plan

1. Configure bob in a new project
2. Define the `custom` target
3. Define a script that generates some files
4. Call bob build and make sure the script was called with the right
package manager

---------

Co-authored-by: Satyajit Sahoo <[email protected]>

docs/pages/build.md

@@ -188,6 +188,18 @@ Generates the [React Native Codegen](https://reactnative.dev/docs/the-new-archit
 
 You can ensure your Codegen generated scaffold code is stable through different React Native versions by shipping it with your library. You can find more in the [React Native Official Docs](https://reactnative.dev/docs/the-new-architecture/codegen-cli#including-generated-code-into-libraries).
 
+#### `custom`
+
+Define a custom build target. This is useful to call code generators during the build process.
+
+##### `script`
+
+Accepts a script name. `bob` will call the matching script defined under `package.json`'s `scripts` property. The build process **will throw and exit** if the target is defined without this option.
+
+##### `clean`
+
+You can pass a path to this option and `bob` will delete all the files on that path. The path is resolved relatively to where `build` was called from.
+
 #### `commonjs`
 
 Enable compiling source files with Babel and use CommonJS module system.

packages/react-native-builder-bob/src/index.ts

@@ -11,6 +11,7 @@ import buildCommonJS from './targets/commonjs';
 import buildModule from './targets/module';
 import buildTypescript from './targets/typescript';
 import buildCodegen from './targets/codegen';
+import customTarget from './targets/custom';
 import type { Options, Report, Target } from './types';
 
 type ArgName = 'target';
@@ -584,6 +585,14 @@ async function buildTarget(
         report,
       });
       break;
+    case 'custom':
+      await customTarget({
+        options: targetOptions,
+        source: path.resolve(root, source),
+        report,
+        root,
+      });
+      break;
     default:
       logger.exit(`Invalid target ${kleur.blue(targetName)}.`);
   }

packages/react-native-builder-bob/src/targets/custom.ts

@@ -0,0 +1,72 @@
+import kleur from 'kleur';
+import path from 'path';
+import fs from 'fs-extra';
+import type { Input } from '../types';
+import { spawn } from '../utils/spawn';
+import dedent from 'dedent';
+import del from 'del';
+
+type Options = Omit<Input, 'output'> & {
+  options?: {
+    script?: string;
+    clean?: string;
+  };
+};
+
+export default async function customTarget({ options, root, report }: Options) {
+  if (options?.script == null) {
+    report.error(
+      dedent(
+        `No script was provided with the custom target.
+         Example: ${kleur.green('{["custom", { "script": "generateTypes" }}')}`
+      )
+    );
+    process.exit(1);
+  }
+
+  const pathToClean = options.clean
+    ? path.relative(root, options.clean)
+    : undefined;
+
+  if (pathToClean) {
+    report.info(`Cleaning up ${kleur.blue(pathToClean)}`);
+
+    await del([path.resolve(root, pathToClean)]);
+  }
+
+  const packageManagerExecutable = process.env.npm_execpath ?? 'npm';
+  const packageManagerArgs = ['run', options.script];
+
+  // usr/bin/yarn -> yarn
+  const packageManagerName = path.basename(packageManagerExecutable);
+  report.info(
+    `Running ${kleur.blue(packageManagerName)} ${kleur.blue(
+      packageManagerArgs.join(' ')
+    )}`
+  );
+
+  try {
+    await spawn(packageManagerExecutable, packageManagerArgs, {
+      stdio: ['ignore', 'ignore', 'inherit'],
+    });
+  } catch (e) {
+    report.error(
+      `An error occurred when running ${kleur.blue(options.script)}`
+    );
+    process.exit(1);
+  }
+
+  report.success(`Ran the ${kleur.blue(options.script)} script succesfully`);
+
+  if (options.clean && pathToClean && !(await fs.pathExists(pathToClean))) {
+    report.warn(
+      `Custom target with the ${kleur.blue(
+        options.script
+      )} script has ${kleur.blue(options.clean)} as the ${kleur.bold(
+        'clean'
+      )} option but this path wasn't created after running the script. Are you sure you've defined the ${kleur.bold(
+        'clean'
+      )} path correctly?`
+    );
+  }
+}

packages/react-native-builder-bob/src/types.ts

@@ -13,7 +13,12 @@ export type Input = {
   report: Report;
 };
 
-export type Target = 'commonjs' | 'module' | 'typescript' | 'codegen';
+export type Target =
+  | 'commonjs'
+  | 'module'
+  | 'typescript'
+  | 'codegen'
+  | 'custom';
 
 export type Options = {
   source?: string;