9. Controllers

# Controllers Controllers are a fairly familiar topic in other web development communities.Since most web developers rally around the mighty net/http interface, not manycontroller implementations have caught on strongly. However, there is greatbenefit in using a controller model. It allows for clean, well definedabstractions above and beyond what the net/http handler interface can aloneprovide. ## Handler Dependencies In this example we will experiment with building our own controllerimplementation using some standard features in Go. But first, lets start withthe problems we are trying to solve. Say we are using the `render` library thatwe talked about in previous chapters: ~~~ var Render = render.New(render.Options{}) ~~~ If we want our `http.Handler`s to be able access our `render.Render` instance,we have a couple options. **1. Use a global variable:** This isn't too bad for small programs, but whenthe program gets larger it quickly becomes a maintenance nightmare. **2. Pass the variable through a closure to the http.Handler: ** This is agreat idea, and we should be using it most of the time. The implementation endsup looking like this: ~~~ func MyHandler(r *render.Render) http.Handler { return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) { // now we can access r }) } ~~~ ## Case for Controllers When your program grows in size, you will start to notice that many of your`http.Handler`s will share the same dependencies and you will have a lot ofthese closurized `http.Handlers` with the same arguments. The way I like toclean this up is to write a little base controller implementation that affordsme a few wins: 1. Allows me to share the dependencies across `http.Handler`s that have similar goals or concepts. 1. Avoids global variables and functions for easy testing/mocking. 1. Gives me a more centralized and Go-like mechanism for handling errors. The great part about controllers is that it gives us all these things withoutimporting an external package! Most of this functionality comes from clever useof the Go feature set, namely Go structs and embedding. Let's take a look at theimplementation. ~~~ package main import "net/http" // Action defines a standard function signature for us to use when creating // controller actions. A controller action is basically just a method attached to // a controller. type Action func(rw http.ResponseWriter, r *http.Request) error // This is our Base Controller type AppController struct{} // The action function helps with error handling in a controller func (c *AppController) Action(a Action) http.Handler { return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) { if err := a(rw, r); err != nil { http.Error(rw, err.Error(), 500) } }) } ~~~ Thats it! That is all the implementation that we need to have the power ofcontrollers at our fingertips. All we have left to do is implement an examplecontroller: ~~~ package main import ( "net/http" "gopkg.in/unrolled/render.v1" ) type MyController struct { AppController *render.Render } func (c *MyController) Index(rw http.ResponseWriter, r *http.Request) error { c.JSON(rw, 200, map[string]string{"Hello": "JSON"}) return nil } func main() { c := &MyController{Render: render.New(render.Options{})} http.ListenAndServe(":8080", c.Action(c.Index)) } ~~~ ## Exercises 1. Extend `MyController` to have multiple actions for different routes in your application. 1. Play with more controller implementations, get creative. 1. Override the `Action` method on `MyController` to render a error HTML page.