# f_extending_golem

This Vignette discusses how you can extend `{golem}`.

## Project Hooks

### What it is

The `create_golem()` function comes with a `project_hook` parameter, a function run just after the `{golem}` project creation. It can be used to modify the project structure automatically just after its creation.

This allows you to define custom behavior when creating `{golem}` based app, that can be used for:

- Adding a different front-end template in `R/app_ui.R`

- Changing configuration options in `inst/golem-config.yml`

- Copying external files in `inst/app/www`

- Removing any file from the default template

- etc.

### How it works

The function is called after the default project has been created, and is executed in the directory of the created package. Here is a rough step by step of what happens when a project is created with `{golem}`:

1. The package name is generated
2. The directory that will receive the package is created
3. Default `{golem}` template is copied and pasted
4. R moves to the directory of the newly created project, and runs the `project_hook` function
5. R moves back to the previous directory, and removes comments if needed
6. Project is open

### Defining your own `project_hook`

The best way to extend `{golem}` project hook functionality is by defining this `project_hook` function in an external package. This will allow this function to be used inside the `{golem}` creation RStudio project creation widget:

> Note that inside this widget, the function should be explicitely namespaced (pkg::fun)

The `project_hook` function takes three mandatory parameters, which are passed from `create_golem`:

- `path`: the full path of the directory
- `package_name`: the name of the package
- `...` : further arguments that can be passed via `create_golem`

These parameters might not be used inside your own hook, but they need to be set in the hook function skeleton, for compatibility reasons.

### Example

Here is an example of a function that can be used to remove the `dev/` folder:

    no_dev <- function(path, package_name, ...) {
      fs::dir_delete("dev")
    }
    create_golem("ici", project_hook = no_dev)

This one will create a CSS:

    new_css <- function(path, package_name, ...) {
      css_path <- fs::path_abs("inst/app/www/custom.css")

      fs::file_create(css_path)

      write_there <- function(...) {
        write(..., file = css_path, append = TRUE)
      }

      write_there("body {")
      write_there("    background-color:red;")
      write_there("}")

      cli_cat_bullet("CSS generated")
    }

    create_golem("ici", project_hook = new_css)

## Module templates

### What it is

Module templates are a way to define your own content for creating the module script in `R/`. It allows to extend `{golem}` module template functionality by creating your own content inside the module file.

### How it works

The function is called after the file(s) creation. Here is a step by step of what happens when the `add_module` function is called:

1. Name is created, and so is the `R/` directory if needed.
2. The fct_ and utils_ files are created if necessary
3. The path to the module is generated
4. If the file already exists, it opens the file
5. If the file doesn’t exist, R creates it and the `module_template` function is called
6. File is generated, and potentially open

(Note that the `module_template` function is not called if the file already exists).

### Defining your own `module_template`

You can then define your own function inside your `{golem}` based application, but chances are you will be defining them into your own package.

Module template functions will receive, by default, the following parameters from `add_modules()`.

- `name`: the name of the module
- `path`: the path to the file in R/
- `export`: a TRUE/FALSE set by the `export` param of `add_module()`
- `...` further arguments

These parameters might not be used inside your own function, but they need to be set in the function skeleton, for compatibility reasons.

### Example

    my_tmpl <- function(name, path, export, ...) {
      # Define a template that only write the name of the
      # module in the file
      write(name, path)
    }
    golem::add_module(name = "custom", module_template = my_tmpl)

    my_other_tmpl <- function(name, path, ...) {
      # Copy and paste a file from somewhere else
      file.copy(..., path)
    }
    golem::add_module(name = "custom", module_template = my_other_tmpl)

## JS & CSS & SASS templates

### What it is

JavaScript, CSS and Sass template allow to use your own functions to add code when creating JavaScript files, JavaScript handlers and CSS or Sass files. These templates work inside `add_js_file()`, `add_js_handler()`, `add_css_file()` and `add_sass_file()`.

### How it works

The function is called after the file creation. Here is a step by step of what happens when these functions are called:

1. Name is created
2. The path is generated
3. If the file already exists, it opens the file
4. If the file doesn’t exist, R creates it and the `template` function is called
5. File is generated, and potentially open

(Note that the `template` function is not called if the file already exists).

### Defining your own `template`

You can then define your own function inside your `{golem}` based application, but chances are you will be defining them into your own package.

File template functions will receive, by default, the following parameters from the `add_*()` function.

- `path`: the path to the file
- `...` further arguments

These parameters might not be used inside your own function, but they need to be set in the function skeleton, for compatibility reasons.

### Example

    my_tmpl <- function(path, ...) {
      # Define a template that only write the name of the
      # module in the file
      write_there <- function(...) {
        write(..., file = path, append = TRUE)
      }

      write_there("body {")
      write_there("    background-color:red;")
      write_there("}")
    }
    golem::add_css_file(name = "custom", template = my_tmpl)

## Turn on the maintenance mode

### What it is

From time to time, you need your application to be unavailable: database update, API changes, etc. In order to keep your app running but make it unavailable, you can use a maintenance mode. When this maintenance mode is turned on, your application will be paused and a specific page will be displayed to your users.

`{golem}` comes with a default maintenance page, and you can replace it with you own page.

### How to set the maintenance mode

The maintenance mode will be turned on whenever the R process detects that the `GOLEM_MAINTENANCE_ACTIVE` environment variable is set to TRUE.

To visualize the maintenance page locally, you can run the following:

    withr::with_envvar(
      c("GOLEM_MAINTENANCE_ACTIVE" = TRUE),
      {
        golem::run_dev()
      }
    )

or

    Sys.setenv("GOLEM_MAINTENANCE_ACTIVE" = TRUE)
    golem::run_dev()

If you’re deploying on Posit Connect, you can set this variable in the setup panel.

If in command line, you can also do

    export GOLEM_MAINTENANCE_ACTIVE=TRUE && Rscript -e "mygolem::run_app()"

### The maintenance page

`{golem}` comes with a default maintenance page, but you can override it and use your own custom page.

In order to use your own page, you need to pass either an `html_document` or a `tagList` to the `with_golem_options` function in `run_app.R`:

    run_app <- function(
        onStart = NULL,
        options = list(),
        enableBookmarking = NULL,
        uiPattern = "/",
        ...) {
      with_golem_options(
        app = shinyApp(
          ui = app_ui,
          server = app_server,
          onStart = onStart,
          options = options,
          enableBookmarking = enableBookmarking,
          uiPattern = uiPattern
        ),
        golem_opts = list(...),
        maintenance_page = tagList(
          fluidRow(
            h1("Under maintenance"),
            span("Coming soon...")
          )
        )
      )
    }

or:

    run_app <- function(
        onStart = NULL,
        options = list(),
        enableBookmarking = NULL,
        uiPattern = "/",
        ...) {
      with_golem_options(
        app = shinyApp(
          ui = app_ui,
          server = app_server,
          onStart = onStart,
          options = options,
          enableBookmarking = enableBookmarking,
          uiPattern = uiPattern
        ),
        golem_opts = list(...),
        maintenance_page = shiny::htmlTemplate(
          filename = app_sys(
            "custom_maintenance_page.html"
          )
        )
      )
    }