Prepare() in Beego Web Framework

November 04, 2014 · 3 min. read

Prepare() is a method exposed by beego.Controller{} and it is executed prior to the method corresponding to your route and HTTP request (the action). It is not a filter and thus, it can easily be extended (much like the Get() and Post() method) for a variety of functions such as:

Session control: ACL-based authentication and authorization
Globals: Setting [ REUSABLE ] defaults for your template (stylesheets, JavaScipts, variables...)

Core / base controller (globals)

We may for example create a core / base controller that would set layout.tpl as its default layout and assign a template variable to a string array of stylesheets or JavaScripts to import.

controllers/base.go:

type BaseController struct {
    // Embedding: "Inherit" beego.Controller
    beego.Controller
}

func (this *BaseController) Prepare() {
    // Overwrite beego.Controller.Layout (string)
    this.Layout = "layout.tpl"

    // beego.Controller.Data comprises of a map of template variables
    this.Data["HeadStyles"] = []string{
        "//cdn.jsdelivr.net/pure/0.5.0/pure-min.css",
        "//yui.yahooapis.com/pure/0.5.0/grids-responsive-min.css",
        "//fonts.googleapis.com/css?family=Roboto:400,500,300",
        "/static/css/base.css",
    }

    this.Data["HeadScripts"] = []string{
        "//cdn.jsdelivr.net/modernizr/2.8.3/modernizr.min.js",
    }
}

Any new struct embedding BaseController{} will now inherit its methods and variables (only Prepare() in this case) as well as those in the anonymous member beego.Controller{} as it is embedded in BaseController{} (refer to the first few lines). Unless the new struct sets its own Prepare() method, Beego will fallback to using the next defined Prepare() which in this case, belongs to BaseController{}. Our new struct will now have a Layout variable that is set to layout.tpl (overwriting beego.Controller{} default) and an array of styles and scripts in HeadStyles and HeadScripts template variable (set via Data) respectively, by default.

Fun fact: beego.Controller.Prepare() does not contain any logic.

This is useful for reducing redundancy in our controllers. Indeed, we do not have to define the same Layout in every controller, and we may, on a per controller method basis, add or remove styles and scripts from our template just by altering the HeadStyles and HeadScripts array. For example, assuming we have a layout...

views/layout.tpl:

<!DOCTYPE html>
<html class="no-js" lang="en">
    <head>
        <meta charset="utf-8" />
        <meta http-equiv="X-UA-Compatible" content="IE=edge" />

        <title>Beego App: {{.Title}}</title>
        <meta name="description" content="" />
        <meta name="viewport" content="width=device-width, initial-scale=1" />

        {{range .HeadStyles}}
        <link rel="stylesheet" href="{{.}}" />
        {{end}} {{range .HeadScripts}}
        <script src="{{.}}"></script>
        {{end}}
    </head>

    <body>
        {{.LayoutContent}}
    </body>
</html>

And a new struct controller named HelloWorldController implementing BaseController{}.

controllers/helloworld.go:

type HelloWorldController struct {
    // Embedding: "Inherit" BaseController
    BaseController
}

func (this *HelloWorldController) Get() {
    // Set {{.Title}}
    this.Data["Title"] = "Hello World!"

    // TplNames is parsed into {{.LayoutContent}}
    this.TplNames = "hello_world.tpl"
}

Beego will automatically render (if the AutoRender config is enabled) hello_world.tpl AND layout.tpl. The contents of hello_world.tpl will be imported into the LayoutContent template variable [ in the curly braces ]. Additionally, {{range .HeadStyles}} and {{range .HeadScripts}} will loop through our string array of stylesheets and JavaScripts so that we can import them into our HTML page. Our HTML document title will be "Beego App: Hello World!" once successfully rendered.

Notice that in our HelloWorldController.Get() method we did not have to define what Layout is as it was already defined by HelloWorldController{} parent, the BaseController{}. Thus, when the Render() method is fired, it will be in the context of HelloWorldController{}. We can even include specific stylesheets and scripts specific for the HelloWorldController.Get() method by using Go's append function on this.Data["HeadScripts"].

If we were to rewrite HelloWorldController{} to implement beego.Controller{} instead, Layout will not be defined as well as the other template variables we are using for our JavaScript and stylesheet imports. The resulting HTML output will simply be a parsed hello_world.tpl without layout.tpl.

You may visualise the inheritance of our controllers in the following manner:

beego.Controller
beego.Controller -> BaseController
beego.Controller -> BaseController -> HelloWorldController

By extending the original beego.Controller{} we may overwrite its methods and variables, and implement our own logic that will better cater to our needs. Best of all, we can abide by DRY (Do Not Repeat Yourself) principles.

Execute parent Prepare()

If we were to create a new Prepare() method in HelloWorldController{}, the Prepare() method in BaseController{} will no longer be fired (for reasons we have highlighted above - it is overwritten). We may still however, execute the "parent" method by calling the struct name directly (for anonymous members) or the field name. For example:

controllers/helloworld_with_parent.go:

func (this *HelloWorldController) Prepare() {
    this.BaseController.Prepare()

    // Do other stuff
}

Both HelloWorldController.Prepare() and BaseController.Prepare() will now be fired in that order.

Through this additional extension, we may not only maintain the logic and variables established in our parent controller, but extend them further to encompass new logic or variables for all methods in a controller. We may for a specific controller, validate a user session to see if they are logged in and redirect them away from it if they are not, and at the same time, we can still keep our parent variables which can be used to render the template to those who are logged in and validated.

As a final note, it is probably worth exploring Beego Godocs, Beego source code and WeTalk, a project made in Beego by... Beego to learn more about the ins and outs of the framework (not as well documented on the official website, probably due to the lack of English contributors). You can see Prepare() in action in Anna & Daphne's admin control panel.