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The Request Flow

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In the previous article we created a new Revel application called myapp. In this article we look at how Revel handles the HTTP request to http://localhost:9000/, resulting in the welcome message.

Routes

The first thing that Revel does is check the conf/routes file – the generated routes file contains a route:

GET     /                                       App.Index

This tells Revel to invoke the Index method of the App controller when it receives a GET request to /.

Actions

Let’s follow this call to the code, in app/controllers/app.go:

package controllers

import "github.com/robfig/revel"

type App struct {
	*revel.Controller
}

func (c App) Index() revel.Result {
	return c.Render()
}

All controllers must be structs that embed *revel.Controller in the first slot (directly or indirectly). Any method on a controller that is exported and returns a revel.Result may be treated as an Action.

The Revel controller provides many useful methods for generating Results. In this example, it calls Render(), which tells Revel to find and render a template as the response (200 OK).

Templates

All templates are kept in the app/views directory. When an explicit template name is not specified, Revel looks for a template matching the action. In this case, Revel finds the app/views/App/Index.html file, and renders it as a Go template.

{{set . "title" "Home"}}
{{template "header.html" .}}

<header class="hero-unit" style="background-color:#A9F16C">
  <div class="container">
    <div class="row">
      <div class="hero-text">
        <h1>It works!</h1>
        <p></p>
      </div>
    </div>
  </div>
</header>

<div class="container">
  <div class="row">
    <div class="span6">
      {{template "flash.html" .}}
    </div>
  </div>
</div>

{{template "footer.html" .}}

Beyond the functions provided by the Go templates, Revel adds a few helpful ones of its own.

This template is very simple. It:

  1. Adds a new title variable to the render context.
  2. Includes the header.html template (which uses the title).
  3. Displays a welcome message.
  4. Includes the flash.html template, which shows any flashed messages.
  5. Includes the footer.html.

If you look at header.html, you can see some more template tags in action:

<!DOCTYPE html>
<html>
  <head>
    <title>{{.title}}</title>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
    <link rel="stylesheet" type="text/css" media="screen" href="/public/stylesheets/main.css">
    <link rel="shortcut icon" type="image/png" href="/public/images/favicon.png">
    <script src="/public/javascripts/jquery-1.5.2.min.js" type="text/javascript" charset="utf-8"></script>
    {{range .moreStyles}}
      <link rel="stylesheet" type="text/css" href="/public/{{.}}">
    {{end}}
    {{range .moreScripts}}
      <script src="/public/{{.}}" type="text/javascript" charset="utf-8"></script>
    {{end}}
  </head>
  <body>

You can see the title being set, and you can also see that it accepts JS and CSS files included from calling templates in the moreStyles and moreScripts variables.

Hot-reload

Let’s change the welcome message. In Index.html, change

<h1>It works!</h1>

to

<h1>Hello World</h1>

Refresh your browser, and you should see the change immediately! Revel noticed that your template changed and reloaded it.

Revel watches:

  • All go code under app/
  • All templates under app/views/
  • Your routes file: conf/routes

Changes to any of those will cause Revel to update your running app with the newest code. Try it right now: open app/controllers/app.go and introduce an error.

Change

return c.Render()

to

return c.Renderx()

Refresh the page and Revel will display a helpful error message:

A helpful error message

Lastly, let’s pass some data into the template.

In app/controllers/app.go, change:

return c.Renderx()

to:

greeting := "Aloha World"
return c.Render(greeting)

In app/views/App/Index.html, change:

<h1>Hello World</h1>

to:

<h1>{{.greeting}}</h1>

Refresh the page and you should see your Hawaiian greeting.

A Hawaiian greeting

Next: Make a simple Hello World application.