Web & iOS dev – Ruby, Rails, RubyMotion & React

Paul Sturgess

Setup Webpacker Webpack-dev-server With Docker-compose

The rails/webpacker gem is a fantastic way to configure webpack for your Rails application. However, it has gone through quite a few different configuration options to get it working with Docker.

Initially you could set the host by using command line arguments, but as of version 3.0.2 and this Pull Request the command line argument support was removed in favour of environment variables. By setting various environment variables you can override all dev server settings defined in config/webpacker.yml.

So now my docker-compose.yml config looks something like this:

    <<: *app
    command: /src/docker/web.sh
      - "3000:3000"
      - webpack

    <<: *app
    command: ./bin/webpack-dev-server
      - 3035:3035

To install yarn and nodejs I added the following to my Debian based dockerfile:

RUN apt-get update && apt-get install apt-transport-https

RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add - \
  && echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list \
  && curl -sL https://deb.nodesource.com/setup_8.x | bash -

RUN apt-get update && \
  apt-get install -y \

Debug Ruby Using Pry and Docker Compose

If you’re using Docker and docker-compose you probably found that by default you can’t debug your Ruby application with the binding.pry breakpoint.

However, if you comment out the command line for your web container. Then:

docker-compose up

Open a new tab and run:

docker-compose run --service-ports --rm web /the/command/you/commented/out

Then you will have interactive access to the command line with binding.pry!

Ruby Metaprogramming: Define_method and Instance_exec

This blog post runs through an example of iterating on some Ruby code. The bizarre fictional scenario develops to make the requirements ever more complicated. There’s certainly many different ways to solve the underlying problem, but I wanted to show an example where define_method and instance_exec can be combined with class methods to make for some very powerful and concise code.

For each change in requirements, I’ll post the code in full for how I’ve handled the new scenario.

I will say upfront that these two metaprogramming methods should be used with caution. Powerful and concise code does not necessarily make for easy to understand/debug/maintain code.

The Curry Restaurant

So, there’s a curry restaurant that sells Vindaloo and Tarka Daal and there’s a customer that likes to order curry from the restaurant… bear with me :). Upon receiving their meal, the customer will respond with a comment if the spiceyness isn’t to their taste. Unfortunately the restuarant isn’t consistent with the spices it uses when cooking.

With the above code we can do things like:

spicey_customer = SpiceyCustomer.new

If any of the meals are mild then the customer will respond with “That’s too mild!”.

A new greedy customer

Then along comes another customer, a greedy one. They also like to order curry and comment on the meal. However, rather than comment on the spiceyness, they like to comment on the size of the portion. Unfortunately the restuarant is also inconsistent with its porton sizes, but the greedy customer doesn’t care what type of curry it is, they just want it large!

Already we have a bit of duplication between the two customers but it’s not a massive problem at this point.

A new menu item - Jalfrezi

The restaurant is doing really well and introduces a new Jalfrezi curry to the menu. The greedy customer responds the same to it as for any curry, but the spicey customer responds to the Jalfrezi, the same as when recieving a Vindaloo - they want it hot!

We’ve added order_jalfrezi to both customers and added expect_hot_curry to SpiceyCustomer because they respond the same for jalfrezi and vindaloo. Now we really do have duplication between the two customers, so we’ll look at refactoring that next.

Refactor - Dining Out

We decide that the two customers have enough shared behaviour to introduce a new module called DiningOut. This means both customers can order all three currys and then respond in a way that is specific to them. With the custom response defined in each Customer in the respond_to_meal method.

You might be thinking it would be better to make Customer a class and have GreedyCustomer and SpiceyCustomer as subclasses that inherit from it, but for this example it doesn’t matter. This is a version of the Template method pattern.

We have our new DiningOut module and we’ve defined a respond_to_meal in both customers. This code is definitely cleaner but there is still a bit of a smell.

If the restaurant expands with lots of new items on the menu and these customers need to respond differently, then the respond_to_meal method is going to get pretty big and complicated pretty quickly. We’re also very reliant on the name of the meal not changing.

In addition the DiningOut module will bloat quickly with each new menu item. That isn’t a massive problem, but there’s already a fair bit of obvious duplication going on where the name of the order method closely matches the argument sent to order_meal.

Now you might be thinking, why can’t we just expose order_meal as a public method and call SpiceyCustomer.new.order_meal(:jalfrezi) instead of SpiceyCustomer.new.order_jalfrezi. However…

A new fussy customer

Along comes the Fussy customer who will only eat tarka daal and the newest item on the menu, chips. The fussy customer isn’t interested in responding to the meal and just eats in silence. In addition, the Greedy customer will order chips, but the Spicey customer would never order chips. Sounds complicated doesn’t it?

The simplest thing to do is to add an order_chips method to DiningOut and an empty respond_to_meal to the new FussyCustomer.

Now we have some complicated rules and they’re not properly enforced. There’s nothing stopping us calling FussyCustomer.new.order_vindaloo or SpiceyCustomer.new.order_chips even though neither of those customers would order those things.

We could start putting some logic into the order_* methods to check the type of customer before ordering the meal, but that would get ugly fast. Ideally we want a solution where we can tell by looking at the code which customer likes to eat which food and each customer wont even know how to order food they don’t want to order.

Introducing define_method and instance_exec

Before we get into the final solution, the end result is that we can now do things like the following:

spicey_customer = SpiceyCustomer.new
spicey_customer.order_chips # returns an undefined method error

greedy_customer = GreedyCustomer.new

fussy_customer = FussyCustomer.new
fussy_customer.order_tarka_daal # returns an undefined method error

Also when we look at each customer we can see very clearly what they like to eat and how they like to respond for each menu item.

The nice thing is that if the restaurant adds a new menu item, we don’t need to touch the DiningOut module at all. We just need to decide which customers would like to order the new menu item and how they might optionally respond.

It works by defining a new class method eats. This method takes the name of the menu item and a block to be called with the meal so the customer can inspect the meal to determine their response.

The eats class method in DiningOut uses define_method to dynamically create the order_* instance method on the customer. This means if the customer declares eats :chips then they will respond to the method order_chips (and conversely if they don’t define it then they will return undefined method error). This makes it very obvious what menu items the customer will eat and prevents mistakes.

The order method uses instance_exec to execute the response block passed in. This is required because we want to ensure the block is called on the customer instance and not in the context of the class method - if we just did block.call(meal) then an undefined method (for whatever method the response block called) would be raised.

By accepting a block and calling it with instance_exec gives the nice benefit of that block being able to call any method it likes on the instance. In this scenario each Customer can call a different response method, for each type of menu item, if they want. This is important because, for example, the Chips menu item does not respond to the method spiceyness - so if there was a single assumed respond_to_meal method each Customer implemented it would have to handle every type of meal (checking if it responds to methods such as spiceyness and all sorts of other logic).


Obviously this is a ridiculously contrived and long example to demonstrate two metaprogramming methods in Ruby but hopefully if you’ve read this far, you’ve got something out of it.

As I mentioned in the intro, these metaprogramming methods should be used with caution. Powerful and concise code does not necessarily make for easy to understand/debug/maintain code.

Using Enzyme to Test a React Component onClick With Event preventDefault

Enzyme is a React Testing utility written by the developers at Airbnb that is designed to make testing your components much simpler and easier.

Here’s a quick tip for testing onClick functions where your code calls event.preventDefault()

When triggering a click via the Enzyme api method simulate, you are not actually triggering a real event – the underlying implementation is simply calling the onClick prop of the node.

So if you’re calling event.preventDefault() in your code, you will need to pass a second argument to simulate, which is a mock object, that is passed through to the event handler.

For example:

const wrapper = shallow(<YourComponent />);
wrapper.find('a').simulate('click', { preventDefault() {} });

Easy when you know how, but not immediately obvious. Fortunately the Enzyme docs are brilliant and well worth checking out for further detail and examples.

Making and Testing Ajax Requests With Axios in a Redux App

This article is going to explain how to perform and test Ajax requests in a React application using Redux and Axios which is a promise based HTTP client.

I like Axios because of the way you can centrally configure the common behaviour for the requests your front-end needs to make and the responses it receives. To take one example, it has the concept of interceptors that allow you to write a single function that can the handle responses such as a 401 unauthorized or 500 exception.

When using Axios I like to create a utility ‘Service’ class to setup the centralised config and ensure all Ajax requests get handled consistently.

It also means calls to Axios are not sprinkled all over the codebase, so I could swap out Axios for an alternative library and I should only need to update this one wrapper class.

In a Redux application, actions are purely functional and thus ‘side-effect free’. So in order to perform Ajax requests you will need middleware such as ‘redux-thunk’. An alternative, that I’ve not used but heard good things of, is redux-loop.

Redux Thunk means you can return a function from your action to delay the dispatch of it, or only dispatch if a condition is met (like the response status is 200).

Once Redux Thunk is installed, you can return a function in your Action as shown by saveResource in the example below:

Then for any containers that you want to use the action, you need to pass through the dispatch to the function in the mapDispatchToProps. See saveResource below:

To test the action you can use standard jasmine spy functions:

Then finally to test the Service class:

There are a few bits here to go through here. Firstly I found that when running the specs I needed to add the babel-polyfill for Promises. I’m using Karma test runner so I needed to add 'node_modules/babel-polyfill/dist/polyfill.js' to the files array in the karma.config.js file.

Secondly at the top of the spec you’ll notice that jasmine-ajax is imported. This is the official plugin for faking Ajax requests in your Jasmine specs. What I like about this is that it has no knowledge of Axios. In the beforeEach it is turned on and then all Ajax requests triggered in the specs are captured and can be inspected.

Due to the asynchronous nature of the promise, I use setTimeout to grab the most recent request and check it was formed correctly. To test the callback I use the then function on the promise under test.

A small, but important, detail is the call to done() – this ensures the test does not exit early and waits until the asynchronous part of the test is complete before exiting.

And that’s it! There’s a lot of code there, but once the Service class is setup it should remain pretty much unchanged. Adding additional Ajax requests to your React application actions should be fairly straight forward.

Using Capybara and PhantomJS to Test an External Website

Capybara is a great way for performing integration testing of web applications. Typically it is used in development with the default Rack Test Driver. However, the Rack Test driver does not support JavaScript and is unable to access HTTP resources outside of your Rack application.

Enter PhantomJS – a headless implementation of Webkit that can execute JavaScript, external urls and works with Capybara. The easiest way to get the two to talk together is via the Poltergeist gem.

I created an example repo with all the required config and setup via Docker: https://github.com/paulsturgess/capybara_phantomjs

But essentially it boils down to:

require 'capybara'
require 'capybara/dsl'
require 'capybara/poltergeist'

$stdout.sync = true

class Demo
  include Capybara::DSL

  def self.run

  def initialize

  def run


  def setup_poltergeist
    Capybara.register_driver :poltergeist do |app|
      Capybara::Poltergeist::Driver.new(app, polyergeist_options)
    Capybara.default_driver = :poltergeist

  def polyergeist_options
    { js_errors: false }


Simple Method for Checking a File Mime Type in Ruby on Rails

When accepting file uploads from users of your application, you should always check the file to ensure malicious content isn’t permitted.

If you have a simple Rails file_field in a multipart form, you’ll get a ActionDispatch::Http::UploadedFile. This object wraps up the temporary uploaded file and has methods like content_type and original_filename.

At first glance it sounds sensible to check the extension (determined from original_filename) along with the content_type, against a whitelist of allowed values. However, if the pesky user has renamed the file extension before uploading the file, your whitelist will not help you as even content_type will return the incorrect value.

This is why checking the mime type of the file is important, but this is not something Ruby does natively. There are a few Ruby gems around which specifically handle this problem but the simplest solution I found, without requiring any third party dependencies, was to use the underlying Operating System and execute the file command via back ticks:

  `file --b --mime-type '/path/to/file.txt'`.strip

This works on both Linux and OS X and will return the mime type such as ‘text/plain’, which can then be used as part of your validation.

Functional Programming for a Rubyist

Functional programming has piqued my interest for a while – it appears to be reaching a tipping point with the rise of ReactJS, Redux, Elm and ImmutableJS bringing Functional Programming to a whole new set of developers through Javascript.

Elixir is also a functional programming language that’s attracting a lot of the Ruby community.

As an object-orientated Rubyist, I’ve been exposed to some functional programming ideas through Blocks, Procs and Lambdas but when programming in Ruby you’re typically mutating state all over the place.

Pure functional programming works with immutable state.


To learn more about functional programming, I’ve just successfully completed the Coursera course Functional Programming Principles in Scala taught by the creator of Scala, Martin Odersky.

Although I didn’t set out to specifically learn Scala, I decided to take the course because the reviews from fellow programmers highly recommended it for learning Functional Programming. I think they were right – it’s a very well put together course and it’s also really tough.

It completely challenges the way you think when you are used to writing imperative code. The course is definitely less about learning Scala and more focussed on the fundamentals of functional programming. Interestingly Scala actually allows both the worlds of functional and object-orientated to co-exist.

Each week you have an assignment to complete that you can upload your program directly from your IDE – there’s an automated test suite that will grade your submission.

Side effect free

So one of the reasons functional programming is gaining popularity is because one of the main cited benefits is that the code is easier to reason about due to being ‘side-effect free’.

It’s this immutability that is at the core of React & Redux, especially for performance.

When a component’s props or state change, React decides whether an actual DOM update is necessary. By using immutable state, it’s very easy to track if an object has changed. The React documentation has a really good detailed explanation on Optimizing Performance.


Functional programming languages are often talked about as being highly performant, but what does this mean?

The future trend for computing appears to be more cores and one the way to make best use of the hardware is to run code in parallel. This is where that ‘side-effect free’ code comes into play as makes it a lot easier to run in parallel.

The Future

Functional programming is certainly a completely different mindset for those of us used to object-oriented programs. It will be really interesting to see if it does become the mainstream approach to building web applications. I think I might start trying to apply some functional programming in Ruby and see how I get on.

Further Reading…

How to Write and Test a Redux Container Component for React

As a follow on from my last article about building and testing stateless presentational components, I’m going to explain Redux Container Components.

What is a container component?

A container component is concerned with how things work. This typically involves fetching data, watching the store for updates and updating the store itself by dispatching actions.

Essentially a container component will wrap a presentational component to add behaviour to it.

What does a container component look like?

The container component is created on Line 16 via the connect() function provided by Redux.

You can pass in additional functions to control how you want the state and the actions mapped to the props. These are mapStateToProps() and mapDispatchToProps().

In the example above, I’m using mapDispatchToProps() to setup the addTodo prop that will dispatch the addTodo action we’ve imported. How this is used is entirely down to the AddToForm presentational component and is outside the responsibility of this component.

Note the first argument to connect() is null because I’m not using mapStateToProps(), but the principle is the same as for mapDispatchToProps().

So how is a container component tested?

Container components should be fairly lightweight. There seems little point testing the connect() function, as this is not our code. But we do care that the props are going into our wrapped AddTodoForm component correctly.

So to test this component, all I’m interested in is that the addTodo prop has been setup to dispatch the required action to the store.

The mock store and the Provider are required to render the connected component.

After that I’m just reaching into the component to grab the addTodo prop and calling the function. I’m spying on the dispatch method in the store, that means I can check it’s called with the action that’s formed correctly.

My test for the presentational component AddTodoForm will take care of checking that the prop is actually triggered when it’s supposed to.

How to Write and Test a Stateless React Component

Recently I’ve been learning Redux state container for React apps and it actively encourages ‘stateless’ components. This blog post will describe how these can be written and tested.

Note that you can write stateless components whether you’re using Redux or not!

What is a stateless component?

Redux introduces the concept of ‘presentational’ and ‘container’ components and the major difference between the two is that your presentational components should not be aware of Redux.

Without getting into the details of exactly how Redux works, the consequence of this separation of responsibilities means your presentational components can be fairly simple.

Certainly if they don’t make use of any React lifecycle methods (like componentDidUpdate) they can start life as a simple function that just returns the required output.

What does a stateless component look like?

Below is an example taken from a Todo app, what it does isn’t important. The key thing to notice is that it’s just a function that returns a list item.

Three props are defined onClick, completed and text. There’s no state manipulation.

So how is stateless component tested?

When using ReactTestUtils.renderIntoDocument it will not work with a stateless component – you’ll get an error ‘instance must be a composite component’.

The way around this is to use a component created by extending React.Component to wrap the stateless component. You’ll see in the test below a Wrapper component is created.

Once you’ve done that you can reach into the component to grab out specific DOM elements or other components.