Genny is a framework for writing modular generators, it however, doesn't actually generate anything. It just makes it easier for you to. :)

A github.com/gobuffalo/genny#Generator is used to build a blue print of what you want to generate.

A few of things that can be added to a Generator are:

• github.com/gobuffalo/genny#File
• os/exec#Cmd
• github.com/gobuffalo/packd#Box
• net/http#Request
• and more

A github.com/gobuffalo/genny#Generator does not actually generate anything; a github.com/gobuffalo/genny#Runner is needed to run the generator.

g := genny.New()

// add a file
g.File(genny.NewFileS("index.html", "Hello\n"))

// execute a command
g.Command(exec.Command("go", "env"))

// run a function at run time
g.RunFn(func(r *genny.Runner) error {
  // look for the `genny` executable
  if _, err := r.LookPath("genny"); err != nil {
    // it wasn't found, so install it
    c := gogen.Get("github.com/gobuffalo/genny/genny")
    if err := r.Exec(c); err != nil {
      return err
    }
  }
  // call the `genny` executable with the `-h` flag.
  return r.Exec(exec.Command("genny", "-h"))
})

When a github.com/gobuffalo/genny#Generator is run each item that was added to it will be run in FIFO order. In the above example this means the following will happen:

1. Create a new file r.Root/index.html
2. Run the command go env
3. Run a function that installs genny

Genny has two different components; the "generator" (or blueprint) and the "runner" which executes the generator. Often it is necessary to only run certain code when the generator is run, not built. For example, checking the existing of an executable and installing it if missing.

In these situations you will want to use a github.com/gobuffalo/genny#RunFn function.

In this example at runtime the RunFn will be called given the *Runner that is calling it. When called the function will ask the github.com/gobuffalo/genny#Runner.LookPath function to ask the location of the genny executable.

In github.com/gobuffalo/genny#DryRunner this will simply echo back the name of the executable that has been asked for, in this case return "genny", nil.

In github.com/gobuffalo/genny#WetRunner this will call the os/exec#LookPath and return its results.

If the genny binary is not found, it will attempt to install it. Should that succeed the method returns the execution of a call to genny -h.

g.RunFn(func(r *genny.Runner) error {
  // look for the `genny` executable
  if _, err := r.LookPath("genny"); err != nil {
    // it wasn't found, so install it
    c := gogen.Get("github.com/gobuffalo/genny/genny")
    if err := r.Exec(c); err != nil {
      return err
    }
  }
  // call the `genny` executable with the `-h` flag.
  return r.Exec(exec.Command("genny", "-h"))
})

The flexibility of the *Fn functions, combined with github.com/gobuffalo/genny#RunFn make for a powerful testing combination.

A github.com/gobuffalo/genny#Runner is used to run generators and control the environment in which those generators are run.

Genny ships with three implementations of Runner that cover most situations. They can also provide good starting points for customized implementations of Runner.

• github.com/gobuffalo/genny#DryRunner
• github.com/gobuffalo/genny#WetRunner
• github.com/gobuffalo/genny/gentest#NewRunner

To add a github.com/gobuffalo/genny#Generator to a github.com/gobuffalo/genny#Runner the github.com/gobuffalo/genny#Runner.With function can be used.

run := genny.DryRunner(context.Background())

// add a generator from the `simple` package
g := simple.New()
run.With(g)

// add a generator from the `notsimple` package
g := notsimple.New()
run.With(g)

Each github.com/gobuffalo/genny#Generator is run in FIFO order in which it was added to the github.com/gobuffalo/genny#Runner.

It is common to have a function that builds a new github.com/gobuffalo/genny#Generator or returns an error if there was a problem.

func New() (*genny.Generator, error) {
  g := simple.New()
  // do work which might error
  return g, nil
}

The github.com/gobuffalo/genny#Runner.WithNew function was designed to make adding a github.com/gobuffalo/genny#Generator with this return argument signature easier.

if err := run.WithNew(New()); err != nil {
  log.Fatal(err)
}

The idea of "dry" running means that no commands are executed, no files are written to disk, no HTTP requests are made, etc... Instead these steps are run "dry", which in the case of github.com/gobuffalo/genny#DryRunner is the case.

func main() {
  run := genny.DryRunner(context.Background())

  g := simple.New()
  run.With(g)

  if err := run.Run(); err != nil {
    log.Fatal(err)
  }
}

// output
DEBU[2018-12-06T15:13:47-05:00] Step: 4eac628c
DEBU[2018-12-06T15:13:47-05:00] Chdir: /go/src/github.com/gobuffalo/genny/internal/_examples/dry
DEBU[2018-12-06T15:13:47-05:00] File: /go/src/github.com/gobuffalo/genny/internal/_examples/dry/index.html
DEBU[2018-12-06T15:13:47-05:00] Exec: go env
DEBU[2018-12-06T15:13:47-05:00] LookPath: genny
DEBU[2018-12-06T15:13:47-05:00] Exec: genny -h

// file list
.
└── main.go

0 directories, 1 file

Using a "dry" runner can make testing easier when you don't have to worry about commands running, files being written, etc... It can also make it easy to provide a "dry-run" flag to your generators to let people see what will be generated when the generator is run for real.

While "dry" means to not execute commands or write files, "wet" running means the exact opposite; it will write files and execute commands.

Use the github.com/gobuffalo/genny#WetRunner when "wet" running is the desired outcome.

func main() {
  run := genny.WetRunner(context.Background())

  g := simple.New()
  run.With(g)

  if err := run.Run(); err != nil {
    log.Fatal(err)
  }
}

GOARCH="amd64"
GOBIN=""
// ...
A brief description of your application

Usage:
  genny [command]

Available Commands:
  help        Help about any command
  new         generates a new genny stub

Flags:
  -h, --help   help for genny

Use "genny [command] --help" for more information about a command.

// file list
.
├── index.html
└── main.go

0 directories, 2 files

$ cat index.html

Hello

The change the way github.com/gobuffalo/genny#DryRunner or github.com/gobuffalo/genny#WetRunner work, or to build your own github.com/gobuffalo/genny#Runner you need to implement the *Fn attributes on the github.com/gobuffalo/genny#Runner.

type Runner struct {
  // ...
  ExecFn     func(*exec.Cmd) error                                     // function to use when executing files
  FileFn     func(File) (File, error)                                  // function to use when writing files
  ChdirFn    func(string, func() error) error                          // function to use when changing directories
  DeleteFn   func(string) error                                        // function used to delete files/folders
  RequestFn  func(*http.Request, *http.Client) (*http.Response, error) // function used to make http requests
  LookPathFn func(string) (string, error)                              // function used to make exec.LookPath lookups
  // ...
}

These *Fn functions represent the FINAL end-point for the that is trying to be run.

Here are two implementations of the github.com/gobuffalo/genny#Runner.FileFn function.

The first will result in the file being printed to the screen. The second implementation writes the file to disk.

run.FileFn = func(f packd.SimpleFile) (packd.SimpleFile, error) {
  io.Copy(os.Stdout, f)
  return f, nil
}

run.FileFn = func(f genny.File) (genny.File, error) {
  if d, ok := f.(genny.Dir); ok {
    if err := os.MkdirAll(d.Name(), d.Perm); err != nil {
      return f, err
    }
    return d, nil
  }

  name := f.Name()
  if !filepath.IsAbs(name) {
    name = filepath.Join(run.Root, name)
  }
  dir := filepath.Dir(name)
  if err := os.MkdirAll(dir, 0755); err != nil {
    return f, err
  }
  ff, err := os.Create(name)
  if err != nil {
    return f, err
  }
  defer ff.Close()
  if _, err := io.Copy(ff, f); err != nil {
    return f, err
  }
  return f, nil
}

Working with files, both creating new ones as well as, existing ones, is a core component of writing a generator. Genny understands this and offers several ways of working with files that is flexible and helps to make writing and testing your generators easier.

The github.com/gobuffalo/genny#File interface is the heart of working with files in Genny.

Genny ships with several convenience method for creating a github.com/gobuffalo/genny#File.

• github.com/gobuffalo/genny#NewFile
• github.com/gobuffalo/genny#NewFileS
• github.com/gobuffalo/genny#NewFileB
• github.com/gobuffalo/genny#NewDir

To write a file you can add a github.com/gobuffalo/genny#File to your github.com/gobuffalo/genny#Generator.File and your file will then be handled by your *Runner when your generator is run.

g.File(genny.NewFile("index.html", strings.NewReader("Hello\n")))
g.File(genny.NewFileS("strings/string.html", "Hello\n"))
g.File(genny.NewFileB("bytes/byte.html", []byte("Hello\n")))

In the case of github.com/gobuffalo/genny#WetRunner will attemp to create any directories your files require.

When writing generators you may need to read an existing file, perhaps to modify it, or perhaps read it's contents. This presents a problem in generators.

The first problem is that anytime we have to read files from disk, we make testing more difficult.

The bigger problems, however, present themselves more with "dry" runners (for example testing), than they do with "wet" runners.

If generator A creates a new file and generator B wants to modify that file in testing and "dry" runners this is a problem as the file may not present on disk for generator B to access.

To work around this issue Genny has the concept of a github.com/gobuffalo/genny#Disk.

Now, instead of asking for the file directly from the file system, we can ask for it from the github.com/gobuffalo/genny#Runner.Disk instead.

g.RunFn(func(r *genny.Runner) error {
  // try to find main.go either in the virtual "disk"
  // or the physical one
  f, err := r.Disk.Find("main.go")
  if err != nil {
    return err
  }
  // print the contents of the file
  fmt.Println(f.String())
  return nil
})

When asking for files from github.com/gobuffalo/genny#Runner.Disk it will first check its internal cache for the file, returning it if found. If the file is not in the cache, then it try to read it from disk at filepath.Join(r.Root, name).

There are times that you may need to transform either certain files, or all files. This could be as simple as replacing a variable in a template's name to match some user input, or something more complex, such as running any templates with a given extension through a certain template engine.

The github.com/gobuffalo/genny#Transformer type can be used to implement these types of file transformations.

To create a new github.com/gobuffalo/genny#Transformer you can use the github.com/gobuffalo/genny#NewTransformer function.

The example below is taken from the github.com/gobuffalo/plushgen package.

// Transformer will plushify any file that has a ".plush" extension
func Transformer(ctx *plush.Context) genny.Transformer {
  t := genny.NewTransformer(".plush", func(f genny.File) (genny.File, error) {
    s, err := plush.RenderR(f, ctx)
    if err != nil {
      return f, errors.Wrap(err, f.Name())
    }
    return genny.NewFileS(f.Name(), s), nil
  })
  t.StripExt = true
  return t
}

The github.com/gobuffalo/genny#Transformer that is returned in the example will only be run on files that have a .plush extension in their name.

Should a file have a .plush extension, it will be sent to github.com/gobuffalo/plush to be rendered. The result of that rendering is returned as a new github.com/gobuffalo/genny#File. Finally, the extension .plush will be stripped from the file name.

g := genny.New()

// add a file
g.File(genny.NewFileS("index.html.plush", "Hello <%= name %>\n"))

// add the plush transformer
ctx := plush.NewContext()
ctx.Set("name", "World")
g.Transformer(plushgen.Transformer(ctx))

// output
DEBU[2018-12-07T10:35:56-05:00] Step: 09c9663e
DEBU[2018-12-07T10:35:56-05:00] Chdir: /go/src/github.com/gobuffalo/genny/internal/_examples/dry
DEBU[2018-12-07T10:35:56-05:00] File: /go/src/github.com/gobuffalo/genny/internal/_examples/dry/index.html
Hello World

Testing a generator can be difficult because creating, deleting, and modifying files can be painful to handle during testing. The same can be said of running functions and HTTP requests.

The *Fn attributes on github.com/gobuffalo/genny#Runner make it simplier to mock out different test cases.

Most of the time the out of the box defaults are "good enough" for testing. The github.com/gobuffalo/genny/gentest package offers several helpers to simplify testing further.

In this example we test the "happy" path of a github.com/gobuffalo/genny#Generator.

func Test_Happy(t *testing.T) {
  r := require.New(t)

  run := gentest.NewRunner()
  run.Disk.Add(genny.NewFileS("main.go", "my main.go file"))

  g := New()
  run.With(g)

  r.NoError(run.Run())
  res := run.Results()

  cmds := []string{"go env", "genny -h"}
  r.NoError(gentest.CompareCommands(cmds, res.Commands))

  files := []string{"index.html", "main.go"}
  r.NoError(gentest.CompareFiles(files, res.Files))
}

Notice how in the above example we had to add main.go to the github.com/gobuffalo/genny#Runner.Disk. That is because the file doesn't exist in our testing directory.

In the following example we test what happens when the genny executable can not be found when running the github.com/gobuffalo/genny#Generator.

We can simulate this experience by using the github.com/gobuffalo/genny#Runner.LookPathFn to return an error if it is asked about that particular executable.

func Test_Missing_Genny(t *testing.T) {
  r := require.New(t)

  run := gentest.NewRunner()
  run.Disk.Add(genny.NewFileS("main.go", "my main.go file"))

  g := New()
  run.With(g)

  // pretend we can't find genny
  run.LookPathFn = func(s string) (string, error) {
    if s == "genny" {
      return "", errors.New("can't find genny")
    }
    return s, nil
  }

  r.NoError(run.Run())
  res := run.Results()

  cmds := []string{"go env", "go get github.com/gobuffalo/genny/genny", "genny -h"}
  r.NoError(gentest.CompareCommands(cmds, res.Commands))

  files := []string{"index.html", "main.go"}
  r.NoError(gentest.CompareFiles(files, res.Files))
}

Genny ships with an executable that helps to generate new generators.

$ go get -u github.com/gobuffalo/genny/genny

$ genny -h

tools for working with genny

Usage:
  genny [command]

Available Commands:
  help        Help about any command
  new         generates a new genny stub

Flags:
  -h, --help   help for genny

Use "genny [command] --help" for more information about a command.

$ genny new coke -h

DEBU[2018-12-07T11:07:01-05:00] Step: a1d8eb2f
DEBU[2018-12-07T11:07:01-05:00] Chdir: /go/src/github.com/gobuffalo
DEBU[2018-12-07T11:07:01-05:00] File: /go/src/github.com/gobuffalo/coke/coke.go
DEBU[2018-12-07T11:07:01-05:00] File: /go/src/github.com/gobuffalo/coke/coke_test.go
DEBU[2018-12-07T11:07:01-05:00] File: /go/src/github.com/gobuffalo/coke/options.go
DEBU[2018-12-07T11:07:01-05:00] File: /go/src/github.com/gobuffalo/coke/options_test.go
DEBU[2018-12-07T11:07:01-05:00] File: /go/src/github.com/gobuffalo/coke/templates/example.txt