foodsoft/doc/design_patterns/publish_subscribe.md

# Publish/subscribe pattern
## Handling DOM updates after AJAX database manipulation

As an example, let us consider the manipulation (create, update...) of `StockArticles`. This can be done in different views, e.g., `stock_articles/index`, `stock_articles/show` and `deliveries/_form` through modals using AJAX requests. As an advantage of the AJAX technique, the user does not need to reload the entire page. However, (after the update of the `StockArticle` in the database) it is generally required to update the DOM in the current view such that the page properly reacts to the asynchronous actions.

The process can be divided in two steps: **1.** AJAX database manipulation and **2.** DOM updates for the particular view. The crucial point is the coupling of the two steps since the controller for the first step offers the same functionality to all views and does not need to know anything about the current view.

### 1. AJAX database manipulation
**(i)** Example: current view `deliveries/_form` offers a link for the AJAX action `StockArticle#new`. This opens a modal filled with `stock_articles/_form`.

**(ii)** AJAX form post addresses the `StockArticle#create` action which handles the database manipulation.

**(iii)** The database manipulation is finished by the rendering of, e.g., `stock_articles/create.js.erb`. The key task there is to **publish** the database changes by calling `trigger`, i.e.,

    $(document).trigger({
      type: 'StockArticle#create',
      stock_article_id: <%= @stock_article.id %>
    });

### 2. DOM updates for the particular view
**(i)** Each view has the opportunity to **subscribe** to particular events of the previous step. A very simple example is the update of the `stock_articles/index` view after `StockArticle#destroy`:

    $(document).on('StockArticle#destroy', function(e) {
      $('#stockArticle-' + e.stock_article_id).remove();
    });

However, in most of the situations you will like to use the full power of the MVC framework in order to read new data from the database and render some partial. Let us consider this slightly more advanced case in the following.

The view `stock_articles/index` could listen (amongst others) to `StockArticle#create` like this:

    $(document).on('StockArticle#create', function(e) {
      $.ajax({
        url: '#{index_on_stock_article_create_stock_articles_path}',
        type: 'get',
        data: {id: e.stock_article_id},
        contentType: 'application/json; charset=UTF-8'
      });
    });

**(ii)** The action `StockArticles#index_on_stock_article_create` is a special helper action to handle DOM updates of the `stock_articles/index` view after the creation of a new `StockArticle` with the given `id`.