[Question]: is it possible to run a task during the build time?

Do you want to request a feature or report a bug?

It's a question, I need a help :)

What is the current behavior?

I want to call a node function during the build time. What it does - parsing a json and stores the result into the application cache folder. The result of this function may be used then during the run time. This function also has a check inside it to determine whether it should perform parsing to avoid cyclic reloading of the application (storing/modifying a new file causes it).

If the current behavior is a bug, please provide the steps to reproduce and a minimal repository on GitHub that we can yarn install and yarn test.

What is the expected behavior?

My question is: am I right about using reporter to achieve this?
It works for me, but only on the first launch. Then I guess Metro uses a dependency graph to resolve whether a particular file was changed and should be included into re-compiled bundle. So the old file is used and I need to completely restart a bundler.

How can I handle it?

Please provide your exact Metro configuration and mention your Metro, node, yarn/npm version and operating system.

const MetroConfig = require('metro-config/src/defaults');
const defaultMetroConfig = MetroConfig.getDefaultValues();

module.exports = {
  reporter: {
    update: (event) => {
      defaultMetroConfig.reporter.update(event);

      if (event.type === 'bundle_build_started') {
        myParseJsonFn();
      }
    },
  },
}; 

Yarn 1.19.2
Metro from the master branch (I use it experimentally)
MacOS Catalina

Start with the Metro configuration's reporter.update callback and the bundle_build_started event shown here. Investigate how Metro's dependency graph handles the generated cache file on later builds and whether a supported build-time hook exists. Done should mean a documented way to run the task repeatedly without requiring a full bundler restart.