mix_templates v0.1.6 MixTemplates

NOTE: This documentation is intended for folks who want to write their own templates. If you just want to use a template, then have a look at the README, or try mix help template and mix help gen.

This is the engine that supports templated directory trees.

A template is a trivial mix project that acts as the specification for the projects you want your users to be able to generate. It contains a single source file in lib that contains metadata and option parsing. It also contains a top-level directory called template. The directories and files underneath template/ copied to the destination location.

The copying function takes a map containing key-value pairs. This is passed to EEx, which is used to expand each individual file. Thus a template file for mix.exs may contain:

defmodule <%= @project_name_camel_case %>.Mixfile do
  use Mix.Project

  @name    :<%= @project_name %>
  @version "0.1.0"

  . . .

The <%= ... %> constructs are expanded using the passed-in map.

In addition, the template looks for the string $PROJECT_NAME\$ in the names of files and directories. It replaces each occurrence with the name of the project, taken from assigns.project_name.

Thus the directory structure for a standard Elixir project might be:

template
├── $PROJECT_NAME$
│   ├── README.md
│   ├── config
│   │   └── config.exs
│   ├── lib
│   │   └── $PROJECT_NAME$.ex
│   ├── mix.exs
│   └── test
│       ├── $PROJECT_NAME$_test.exs
│       └── test_helper.exs
└── templates_project.ex

Write a Template

Make sure you have the underlying tools installed:

$ mix archive.install hex mix_templates
$ mix archive.install hex mix_generator

Then install the template for templates (yup :).

$ mix template.install hex gen_template_template

Now create your template project:

$ mix gen template my_template

Wander into the directory that is created:

$ cd my_template/
$ tree
.
├── README.md
├── lib
│   └── my_template.ex
├── mix.exs
└── template
    └── $PROJECT_NAME$
        └── your_project_tree_goes_here

Add a Description

Your first job is to update the metadata in lib/«whatever».ex:

defmodule MyTemplate do

  @moduledoc File.read!(Path.join([__DIR__, "../README.md"]))

  use MixTemplates,
    name:       :my_template,
    short_desc: "Template for ....",
    source_dir: "../template"

end

The only change you’re likely to make to the metadata is to update the short description. This is used to display information about the template when you list the templates you have installed, so you probably want to keep it under 70 characters.

Add the Files

The job of your template is to contain a directory tree that mirrors the tree you want your users to produce locally when they run mix gen.

  • The easiest way to start is with an existing project that uses the same layout. Copy it into your template under template/$PROJECT_NAME$.

  • Remove any files that aren’t part of every project.

  • Look for files and directories whose names include the name of the project. Rename these, replacing the project name with the string $PROJECT_NAME$. For example, if you’re following the normal convention for test files, you’ll have a file called

    test/myapp_test.exs

    Rename this file to

    test/$PROJECT_NAME$.exs

  • Now you need to look through the files for content that should be customized to each new project that’s generated. Replace this content using EEx substitutions:

    For example, the top-level application might be an Elixir file:

    defmodule MyApp do # . . . end

    Replace this with

    defmodule <%= project_name_camel_case %> do # . . . end

    There’s a list of the available values in the next section.

Test Your Template

You can use mix gen to test your template while you’re developing it. Simply give it the path to the directory containing the generator (the top level, with mix.exs in it). This path must start with a dot (“.”) or slash (“/“).

    $ mix gen ../work/my_generator test_project

Publish Your Template

Wander back to the mix.exs file at the top of your project, and update the @description, @maintainers, and @github attributes. Then publish to hex:

    $ mix hex.publish

and wait for the praise.

Standard Substitutions

The following values are available inside EEx substitutions in templates. (Remember that the inside of a <%= ...%> is just Elixir code, so you aren’t limited to this list. The next section describes how you can extend this set even further in your own templates.)

Project Information

Assuming the template was invoked with a project name of my_app:

@project_name               my_app
@project_name_camel_case    MyApp

Date and Time

These examples are from my computer in US Central Daylight Time (GMT-5)

@now.utc.date               "2017-04-11"
@now.utc.time               "00:49:37.505034"
@now.utc.datetime           "2017-04-11T00:49:37.505034Z"

@now.local.date             "2017-04-10"
@now.local.time             "19:49:37"
@now.local.datetime         "2017-04-10 19:49:37"

The Environment

@host_os                    "os-name" or "os-name (variant)" eg: "unix (darwin)"
@original_args              the original args passed to mix

@elixir_version             eg: "1.5.3"
@erlang_version             eg: "8.2"
@otp_release                eg: "19"

@in_umbrella?               true if we're in the apps_path directory of an 
                            umbrella project

Stuff About the Template

@template_module            the module containing your template metadata
@template_name              the name of the template (from the metadata)

@target_dir                 the project directory is created in this
@target_subdir              the project directory is called this

Handling Command Line Parameters

You may need to configure the output of your template depending on the options specified on the command line. For example, the standard project template lets you generate basic and supervised apps. To indicate you want the latter, you add a command line flag:

    $ mix gen project my_app --supervised

This option is not handled by the gen task. Instead, it passes it to your template module (the file in your top-level lib/). You can receive the parameters by defining a callback

defmodule MyTemplate do

  @moduledoc File.read!(Path.join([__DIR__, "../README.md"]))

  use MixTemplates,
    name:       :my_template,
    short_desc: "Template for ....",
    source_dir: "../template"

  def populate_assigns(assigns, options) do
    # ...
  end
end

The populate_assigns function is called immediately after the standard set of assigns have been created, and before any templating is done. It receives the current assigns (a map) and the options passed to mix gen (another map). It must return a (potentially updated) assigns map.

For example, if the user invoked your template with

    $ mix gen a_template my_app --pool 10 --logging

The options passed to populate_assigns would be

%{into: ".", logging: true, pool: "10"}

(The :into entry is used by the generator—it is basically the target directory)

You can add these options to your assigns, and then subsequently use them in your templates.

def populate_assigns(assigns, options) do
  assigns = add_defaults_to(assigns)
  options |> Enum.reduce(assigns, &handle_option/2)
end

defp add_defaults_to(assigns) do
  assigns
  |> Map.merge(%{ is_supervisor: false })
end

defp handle_option({ :app, val }, assigns) do
  %{ assigns | project_name: val }
end

defp handle_option({ :application, val }, assigns) do
  handle_option({ :app, val }, assigns)
end

defp handle_option({ :supervisor, val }, assigns) do
  %{ assigns | supervisor: val }
end

# ...

defp handle_option({ :into, _ }, assigns), do: assigns

defp handle_option({ opt, val }, _) do
  Mix.shell.error([ :red,    "\nError: ",
                    :reset,  "unknown option ",
                    :yellow, "--#{opt} #{inspect val}\n"])
  Process.exit(self(), :normal)
end

Dealing with optional files and directories

Sometimes you need to include a file or directory only if some condition is true. Use these helpers:

  • MixTemplates.ignore_file_and_directory_unless(«condition»)

    Include this in a template, and the template and it’s immediate directory will not be generated in the output unless the condition is true.

    For example, in a new mix project, we only generate lib/«name»/application.ex if we’re creating a supervised app. The application.ex template includes the following:

    <%
    #   ------------------------------------------------------------
        MixTemplates.ignore_file_and_directory_unless @is_supervisor?
    #   ------------------------------------------------------------
    %>
    defmodule <%= @project_name_camel_case %>.Application do
       # ...
    end

Summary

Functions

find(name)
generate(template, assigns)
ignore_file_and_directory_unless(flag)