Javascript for info-style navigation - Week 2 & 3

by Mathieu Lirzin — ven. 16 juin 2017

This is an update on the work I am doing this summer for my Google Summer of Code. Please see the first article of this serie of reports for a general introduction on what this project is about.

Implementing backward and forward navigation keys

I have implemented the navigation with the [ and ] keys which respectively correspond to backward and forward. Those keys allow the user to traverse the manual linearly, compared to the p and n keys which traverse the nodes of the manual considering only considering the nodes of the same level and not their sub-nodes.

A manual is actually a tree of nodes with the beginning of the manual representing the root of the tree, and the sections it is composed of representing the children nodes. With that model in mind, the linear traversal of the manual correspond to a tree traversal using a depth first search algorithm.

Contrary to the previously implemented navigation links the backward and forward links are not present in the HTML pages. As a consequence they need to be computed from the table of content, which contains all the information needed. Indeed the table of content contains an <ul> element with <li> children elements that corresponds to tree structure of the manual. Using the depth first search traversal of those elements allows us to achieve the computation we want with the create_link_dict function which takes the top <ul> node as an argument and for each of its children apply the add_link internal function. add_link populates links the appropriate key value pairs by using prev_id which store the previously seen href attribute. Since we use a depth first search traversal the previously seen element correspond to the backward link and reciprocally the current href corresponds to the forward link of the previously seen href.

function
create_link_dict (nav)
{
  let prev_id = "*TOP*";
  let links = {};

  function
  add_link (elem)
  {
    if (elem.matches ("a") && elem.hasAttribute ("href"))
      {
        let id = elem.getAttribute ("href");
        links[prev_id] = Object.assign ({}, links[prev_id], { forward: id });
        links[id] = Object.assign ({}, links[id], { backward: prev_id });
        prev_id = id;
      }
  }

  depth_first_walk (nav, add_link, Node.ELEMENT_NODE);
  return links;
}

let links = create_link_dict (document.querySelector ("ul"));

The actual implementation of depth_first_walk is simple since it consists of applying func to node and recursively applying func to all its children in order. The particularity of this implementation is that we have an additional node_type argument which allows to filter the nodes we want to consider with func. This argument is a Node type constant which is compared to the actual nodeType property of the node argument.

function
depth_first_walk (node, func, node_type)
{
  if (!node_type || (node.nodeType === node_type))
    func (node);

  for (let child = node.firstChild; child; child = child.nextSibling)
    depth_first_walk (child, func, node_type);
}

The dictionary returned by create_link_dict is used by the global key event handler to retrieve the iframe identifier we need to make visible when going forward or backward.

Improving the architecture

The original prototype I have started working with, had no particular architecture attached to it, meaning there was no separate model, in other words the model was the DOM itself. This approach which is nice when prototyping doesn't scale well when combining features. The problem is that the asynchronicity of events combined with constant mutation of the DOM is hard to reason about and make it easy to introduce bugs. The solution to that issue is to separate the view from the model. The model contains the logic and the data structures of the application. The view(s) contain(s) a particular representation of the model. In our case the representation is the DOM. The fuzzy part is to make the model and the view interact nicely. The traditional solution consists of using an Model View Controller architecture (MVC) which brings a Controller to manage the interactions between the view and the model. This means handling click events and updating the view when the model changes. While this architecture is traditional, it brings with it a dose of complexity because of the two-way dataflow between the model and the view. A modern alternative to that architecture is to use a Unidirectional dataflow like what is implemented by the Redux Javascript framework. The idea is basically to implement the model using an unique state object and reducers which are pure functions taking an action and a state and returning a new state. The view is then only a function of the state to void that renders the state inside the DOM. Actions are sent to a global store via event handlers. The store is the object responsible of the state of the application and applying the reducers to it. This short explanation without any diagram is obviously not sufficient to get whole picture, so I encourage everyone to look around the internet for more information.

For this project, I have tried to use this Unidirectional dataflow by reimplementating something similar to what Redux does. For now this works fine, and allows me to reason about the program more easily.

Next step

The basic key navigation implementation is now done. The next step of this project will be to implement the menu navigation which consists of allowing the users based on a text input field to access a particular sub-node of the manual. This would serve as a basis for further features based on text inputs such as the index search and regexp search.

Follow the developpement

The development of this project is done in public. You can checkout the Git repository to see what is the current state of the project.