LogoMelos

Getting Started

Melos requires a few one-off steps to be completed before you can start using it.

Installation#

Install Melos as a global package via pub.dev so it can be used from anywhere on your system:

dart pub global activate melos

Setup a workspace#

Melos is designed to work with a workspace. A workspace is a directory which contains all the packages that are going to be developed together. Its root directory must contain a melos.yaml and a pubspec.yaml file.

Recommended directory structure#

When using Melos you shouldn't have a project in the root of the workspace, since that is where the configuration for the workspace will live and those dependencies might clash with your project dependencies.

The following is the recommended workspace directory structure:

my_project
├── apps
│   ├── apps_1
│   └── apps_2
├── packages
│   ├── package_1
│   └── package_2
├── melos.yaml
├── pubspec.yaml
└── README.md

Install Melos in the workspace#

Different Melos workspaces might use different versions of Melos. To ensure everyone working in the workspace (as well as CI jobs) is using the same version of Melos, a dependency on the melos package has to be added to the pubspec.yaml file at the workspace root directory. The globally installed version of Melos will switch to the version specified in the pubspec.yaml file, if both versions are not the same.

If you don't have a pubspec.yaml file at the workspace root yet, create one now:

name: my_project

environment:
  sdk: '>=3.0.0 <4.0.0'

The corresponding pubspec.lock file should also be committed. Make sure to exclude it from the .gitignore file.

Add Melos as a development dependency by running the following command:

dart pub add melos --dev

Configure the workspace#

Next create a melos.yaml file at the repository root. Within the melos.yaml file, add the name and packages fields:

name: my_project

packages:
  - apps/**
  - packages/**

The packages list should contain paths to the individual packages within your project. Each path can be defined using the glob pattern expansion format.

Melos generates pubspec_overrides.yaml files to link local packages for development. Typically these files should be ignored by git. To ignore these files, add the following to your .gitignore file:

pubspec_overrides.yaml

Bootstrapping#

Once installed & setup, Melos needs to be bootstrapped. Bootstrapping has 2 primary roles:

  1. Installing all package dependencies (internally using pub get).
  2. Locally linking any packages together.

Bootstrap your project by running the following command:

melos bootstrap

Why do I need to bootstrap?#

In normal projects, packages can be linked by providing a path within the pubspec.yaml. This works for small projects however presents a problem at scale. Packages cannot be published with a locally defined path, meaning once you're ready to publish your packages you'll need to manually update all the packages pubspec.yaml files with the versions. If your packages are also tightly coupled (dependencies of each other), you'll also have to manually check which versions should be updated. Even with a few packages this can become a long and error-prone task.

Melos solves this problem by overriding local files which the Dart analyzer uses to read packages from. If a local package exists (defined in the melos.yaml file) and a different local package has it listed as a dependency, it will be linked regardless of whether a version has been specified.

Next steps#

Once successfully bootstrapped, you can develop your packages side-by-side with changes to a single package immediately reflecting across other dependent packages.

Melos also provides other helpful features such as running scripts across all packages. For example, to run dart analyzer in each package, add a new script item in your melos.yaml:

name: my_project

packages:
  - apps/**
  - packages/**

scripts:
  analyze:
    exec: dart analyze .

Then execute the command by running melos run analyze.

If you're looking for some inspiration as to what scripts can help with, check out the FlutterFire repository.

If you are using VS Code, there is an extension available, to integrate Melos with VS Code.