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Session / Flash Scopes

Revel provides Flash cookie based method to set temporary transient data. It also provides Session backed data to provide persistent user state data.


// A signed cookie, and thus limited to 4kb in size.
// Restriction: Keys may not have a colon in them.
type Session map[string]string

// Flash represents a cookie that gets overwritten on each request.
// It allows data to be stored across one page at a time.
// This is commonly used to implement success or error messages.
// e.g. the Post/Redirect/Get pattern: http://en.wikipedia.org/wiki/Post/Redirect/Get
type Flash struct {
	Data, Out map[string]string
}

NOTE: To set your own cookie, use Controller.SetCookie()

func (c MyController) MyMethod() revel.Result {
    new_cookie := &http.Cookie{Name: "foo", Value: "Bar"}
    c.SetCookie(new_cookie)
    return c.Render()
}

Session

revel.Session is a map[string]interface{}. By default you can still interact with string data as if it was a map[string]string. If you store objects in the session they must be able to convert to JSON and you must use the revel.Session.Get() function to extract the data. The Session.Get call will automatically inflate the object if it exists in the map. The inflated result will be a map[string]interface{}. You can also do a revel.Session.GetInto() passing a reference to the object you want inflated. and it will populate that object if it exists. If the session object does not exist, a SESSION_VALUE_NOT_FOUND error is returned.

The default session engine is the revel-cookie engine

func (c MyController) getUser(username string) models.User {
	user = &models.User{}
	_,  err := c.Session.GetInto("fulluser", user, false)
	if err==nil && user.Username == username {
		return user
	}
	// more
	
}

func (c MyController) MyMethod() revel.Result {

    c.Session["foo"] = "bar"
    c.Session["bar"] = 1 
    delete(c.Session, "abc") // Removed item from session
    return c.Render()
}

Flash

The Flash provides single-use string storage. It is useful for implementing the Post/Redirect/Get pattern, or for transient “Operation Successful!” or “Operation Failed!” messages.

Here’s an example of that pattern:

// Show the Settings form
func (c App) ShowSettings() revel.Result {
	return c.Render()
}

// Process a post
func (c App) SaveSettings(setting string) revel.Result {
    // Make sure `setting` is provided and not empty
    c.Validation.Required(setting)
    if c.Validation.HasErrors() {
        // Sets the flash parameter `error` which will be sent by a flash cookie
        c.Flash.Error("Settings invalid!")
        // Keep the validation error from above by setting a flash cookie
        c.Validation.Keep()
        // Copies all given parameters (URL, Form, Multipart) to the flash cookie
        c.FlashParams()
        return c.Redirect(App.ShowSettings)
    }
    saveSetting(setting)
    // Sets the flash cookie to contain a success string
    c.Flash.Success("Settings saved!")
    return c.Redirect(App.ShowSettings)
}

Walking through this example:

  1. User fetches the settings page.
  2. User posts a setting (POST)
  3. Application processes the request, saves an error or success message to the flash, and redirects the user to the settings page (REDIRECT)
  4. User fetches the settings page, whose template shows the flashed message. (GET)

It uses two convenience functions:

  1. Flash.Success(message string) is an abbreviation of Flash.Out["success"] = message
  2. Flash.Error(message string) is an abbreviation of Flash.Out["error"] = message

Flash messages may be referenced by key in templates. For example, to access the success and error messages set by the convenience functions, use these expressions:

{{.flash.success}}
{{.flash.error}}

Here is a second scenario where you want the flash variables returned without using a redirect

func (c Controller) Submit(Input UserName) revel.Result {
    Input.Validate(c.Validation);
    if c.Validation.HasErrors() {

        data := map[string]string{}
        for key, vals := range c.Params.Values {
            data[key] = strings.Join(vals, ",")
        }
        c.RenderArgs["flash"] =  data

        // Display input page
        return c.RenderTemplate("test.html")
    }
    ...
    }
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