Generator - entities in multiple files

[vaind]

Verify (= have a test case) the generator supports generating a single database model for entities spread around multiple files.

[vaind]

OK, there are a few issues. I'll try to put some of them into words.

1. build_runner allows us to either target all .dart files or one of the predefined directories, like lib, web: Extend the list of supported synthetic inputs with $assets$ or similar dart-lang/build#1132
2. When a build is run for a file, there is no way of knowing about other files in the same package. Therefore, we can't create just a single declaration of a model that could then be loaded when initializing objectbox. Generating it for all files, on the other hand, results in duplicate code which then can't be used.
3. We currently create objectbox-model.json directly, in the directory where the build was executed from. Also, this way build_runner doesn't know this file is created/updated by our builder so it doesn't allow other builders to depend on it.
   Another issue with this is that we don't differentiate between entities defined in lib and those in test (or other potential directories). While this works fine for our test code (that's actually what we want), it will pose problems for some library users.

I've tried creating another builder that would take objectbox-model.json and create an objectbox-model.dart file for it. Couldn't make this work yet, probably because the json file is in the root directory so I can't make build_runner to consider it. Probably because it's in the root dir: https://github.com/dart-lang/build/blob/master/docs/faq.md#how-can-i-include-additional-sources-in-my-build

  model:
    import: "package:objectbox_generator/objectbox_generator.dart"
    builder_factories: ["modelLoaderBuilder"]
    build_extensions: {"$lib$": ["objectbox-model.g.dart"]}
    auto_apply: dependents
    build_to: cache
    applies_builders: ["source_gen|library_builder"]
    defaults:
      generate_for:
        include: ["objectbox-model.json"]

I guess I'll just have to write the file each time any entity is encountered. The same way the json file is written. It will be up to the librery user to then include this file and use the objectBoxModel it defines to initialize Store().

In the meantime, I realized we shouldn't generate just one objectbox-model.json but rather have it scoped (lib folder, test folder, etc). That should probably be a separate issue, though.

[vaind]

Another problem: while generating a single objectbox_model.dart is possible with the information from model json, it needs to somehow reference the other generated entities. Namely, the flatbuffers marshalling code.

Therefore, having all the code in this single file seems like the easiest solution to all the problems. I'm going to trie and convert the builder to work on the whole lib directory. Hopefully it would be possible to override this dir to something else by the user.

[vaind]

A way to go: https://github.com/dart-lang/build/blob/master/docs/writing_an_aggregate_builder.md