# Dynamic Web Apps with Rack (lab) ## Objectives 1. Translate a command line Ruby app to a dynamic web app 2. Use the `#write` method in a `Rack::Response` object to make a dynamic web app in Rack ## Creating a Dynamic Web App Making web apps that always give the same response are boring. Programming is fun because of its ability to create dynamic responses that change depending on the input. A dynamic web app in Rack is pretty straightforward. Let's say we wanted to create a simple slots game. First, let's set up our basic Rack app inside of `app/application.rb`: ``` class Application def call(env) resp = Rack::Response.new resp.write "Hello, World" resp.finish end end ``` Then run it with `rackup config.ru`. You should see something like ``` [2016-07-28 10:09:08] INFO WEBrick 1.3.1 [2016-07-28 10:09:08] INFO ruby 2.3.0 (2015-12-25) [x86_64-darwin15] [2016-07-28 10:09:08] INFO WEBrick::HTTPServer#start: pid=38967 port=9292 ``` Make note of `port=9292` — that shows which port we'll access the application on in the browser. But what's the host? If we're developing locally, we can just use `localhost` — so in this case we'd visit `http://localhost:9292`. If we're using the IDE, we should also see a line like ``` Starting server at 104.131.138.76:6868 ``` That is the full URL to use. (**Yours will most likely be different!**) So in this case, we'd visit [http://104.131.138.76:6868](http://104.131.138.76:6868/) in the browser. If we're using the IDE, **localhost will not work**. When we visit the appropriate URL in our browser, we should see "Hello, World". Let's liven things up a bit. The amazing part of Rack and everything (like Rails) that is built on top of Rack is that it's *just Ruby*. If you were writing a command line slots game generator, you would first need to generate three numbers between 1 and 20. You could do that like this: **NOTE**: Don't sweat the `Kernel` bit — [Kernel](http://ruby-doc.org/core-2.3.0/Kernel.html) is a module that holds many of Ruby's most useful bits. We're just using it here to generate some random numbers. ``` num_1 = Kernel.rand(1..20) num_2 = Kernel.rand(1..20) num_3 = Kernel.rand(1..20) ``` Then, to check to see if you won or not, we'd have an if statement like this: ``` num_1 = Kernel.rand(1..20) num_2 = Kernel.rand(1..20) num_3 = Kernel.rand(1..20) if num_1==num_2 && num_2==num_3 puts "You Win" else puts "You Lose" end ``` So how do we now make this application work on the web? Almost none of the code actually is specific to a command line interface. The only parts that require a command line are the two `puts` lines. All that needs to change to adapt this for the web is a different way than `puts` to express output to our user. Because this is the web, that means adding it to our response. Instead of `puts` now we'll use the `#write` method in our `Rack::Response` object. Remember that to modify our web server, we have to first exit out of the running server by typing CTRL-C. Then open up your Application and modify it to look like this: ``` class Application def call(env) resp = Rack::Response.new num_1 = Kernel.rand(1..20) num_2 = Kernel.rand(1..20) num_3 = Kernel.rand(1..20) if num_1==num_2 && num_2==num_3 resp.write "You Win" else resp.write "You Lose" end resp.finish end end ``` Notice that we only changed the `puts` statements into `resp.write` statements. That's it! Web servers are just big ruby apps that respond to the user in an HTTP response rather than via `puts` statements. Let's give the user a bit more information about what numbers they received by writing the numbers to the response as well. The `#write` method can be called many times to build up the full response. The response isn't sent back to the user until `#finish` is called. ``` class Application def call(env) resp = Rack::Response.new num_1 = Kernel.rand(1..20) num_2 = Kernel.rand(1..20) num_3 = Kernel.rand(1..20) resp.write "#{num_1}\n" resp.write "#{num_2}\n" resp.write "#{num_3}\n" if num_1==num_2 && num_2==num_3 resp.write "You Win" else resp.write "You Lose" end resp.finish end end ``` The `\n`s are just a special character which gets rendered by the browser as a new line. Kill your running server with CTRL-C and re-run it and refresh your browser. Feel free to cheat a bit and change the random numbers to just be between one and two. That way you can test that both work. Clone this repo here: --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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