Thread by @rohantaneja_

What is _$? #Dart #Flutter

Back in 2020, I had been working with Flutter/Dart for a while, but I had zero clue about the concept of generated code.

Here’s what followed ⬇️

I saw this strange _$ syntax everywhere.

I knew that a prefixed underscore is Dart’s way of saying that a certain thing is private but I wasn’t sure if the _ in _$ also meant the same thing.

It felt alien to see a $ sign in class/method names.

One day, I saw some code on @FlutterComm’s Slack, that used _$.

It took some courage to DM the author of that code and ask what that syntax was because it seemed that I was the only one who didn’t understand it, which is why I shied away from asking on the public channel 🙊

That person was kind enough to clearly explain that adding a $ sign to class/method name is intended to denote that its implementation comes from auto-generated code.

The _ in front of the $ sign still represents “private”, just like anywhere else in Dart.

$, as also explained on SO:

"...is just another letter to the language, but it's reserved for generated code. That allows code generators to (largely) avoid worrying about naming conflicts as long as they put a `$` somewhere in the names they create."

stackoverflow.com/a/62179466/506…

It finally made sense.

The next part was to figure out how do I benefit from this generated code and how was it actually being generated?

So I searched for “generate code dart”. The top results mentioned “build_runner” and “source_gen”.

I had read about `build_runner` before but never really understood it because not understanding the `_$` part put me off.

But now with `_$` out of the way, I felt confident that I could understand this concept.

Long story short:

`build_runner` is a package that generates Dart code.

`source_gen` is a package that helps define the logic for generating Dart code.

“Give it to me in English”?

- You write 1️⃣ line of code.

- You run `build_runner` and get 1️⃣0️⃣0️⃣ lines of code related to that 1️⃣ line magically.

- `source_gen` is the magician 🪄. It helps define the logic for getting the 1️⃣0️⃣0️⃣ lines from that 1️⃣ line.

For most of us who are just building Flutter apps for end-users, we may only need `build_runner`.

We can still automate writing some repetitive code by using `source_gen` if needed but for most cases, if we only want to consume some packages, `build_runner` is enough:

I then followed a tutorial to try these^ commands and it started to make more sense.

Then gradually I got acquainted with packages like `json_serializable` and `freezed` that heavily use code generation and make our lives as devs easier.

I now use these 2 in my apps everyday.

“Where do I go from here?”

I’d suggest starting with this codelab from the Flutter docs that walks you through using `json_serializable` using `build_runner`:

docs.flutter.dev/development/da…

It’s easy to follow and you’ll clearly see the benefits of using generated code by its end.

That's it! ⭐️

So just like Rohan from 2020, if you found the concept of generated code overwhelming, I hope this thread made it slightly easier for you to understand it 🙌