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This is just a toy experiment as an attempt to use David Nolen's Om along side Cognitect's Pedestal to implement a TodoMVC example.


Each todo will have the following attributes:

  • id (string) - a globally-unique ID for the todo
  • title (string) - the title of the todo
  • body (string) - details of the todo
  • ord (integer) - the ordinal of a todo with respect to all other todos
  • completed? (boolean) - true if the todo has been completed
  • created (integer) - Unix Timestamp when todo was created
  • updated (integer) - Unix Timestamp when todo was last updated

This is represented by a record using Prismatic's Schema, see the pedestal-om-todo.models namespace for the straightforward implementation.


A user of the app should be able to:

  • Add a todo item

  • Delete a todo item - click the red 'Delete' button next to the item

  • Delete all completed todo items - this button should only appear at the bottom left when there are. Clicking

  • Filter todo items by their status - only show me one of "all todos", "completed todos", "active todos"

  • Check a todo item as being completed or active - click the tick next to the item to toggle the status of a todo

  • Check all todo items as being completed or active - only visible when there are todo items. Clicking it should toggle all todos to be active or completed. If this is set to completed, and a particular todo is made active again, this should also become active.

  • View and edit details of a todo item - clicking a todo should bring you to a detailed item view where you can edit the title and body.

The following routes (URLs) are available:

  • / - shows all todos as a list
  • /:filter - shows all todos as list that have the status :filter. :filter is one of all, active, or completed
  • /item/:id - shows the detailed view of the todo item with id being :id

Weird Stuff

Modifications had to be made in order to use Om in the application. Below are some gotchas that I ran into while building what is currently on master. Many of these are opposed to Pedestal's spirit of having pure transformation on data, but oh well .....


There are 2 foci in the app -- a List view, and an Item view.

The List view shows a list of todos, filtered to show only those that satisfy the supplied filter. See the pedestial-om-todo.todos.list namespace for implementation.

The Item view shows the details of a todo item, and allows the user to edit the title and body. See the pedestial-om-todo.todos.item namespace for implementation.

Each of those views is an Om app. As such we don't swap templates in and out like shown in the tutorial.

Instead, what happens is that a root HTML template is initiated on startup, and then the appropriate Om apps are mounted and unmounted as focus changes. See the pedestal-om-todo.rendering namespace for the intended render config used to implement this behaviour.

A side effect of this is that the design control panel of standard Pedestal is basically useless, since everything is populated "dynamically" by javascript. The upside of using Om. Recording a series of deltas and then replaying them could probably be an adequate substitute, though this probably doesn't scale that well with larger apps, so I'll have to look into better methods of unit testing.


Routing is implementated in the pedestal-om-todo.history and pedestal-om-todo.routes namespaces.

Start by looking at pedestal-om-todo.routes, whereby the routes are defined using secretary, and the appropriate set-focus messages are dispatched.


There is some hairy and un-clojurery uses of state.

Firstly, the pedestal-om-todo.state namespace is intended to keep globally available state. For now, this is only used to keep a reference to the Pedestal App's input queue, which hopefully shouldn't change through the lifetime of the app.

Secondly, the use of Om requires that access to the Om App's state be available during the lifetime of that app. You will see that the List and Item views keep a reference to an app-state atom, whereby transformations are applied as needed.

The Problem of Rendering Refresh when switching Focus

Say you are viewing the todo list with 2 items:

  1. Take out the trash
  2. Feed the cat

You click todo (1) and view its details.

Then you hit your browser back button to go back to the list without making any edits.

In this case, the todos have not changed, and hence no rendering deltas are produced. But you still need to refresh the todo list, or else no todos will be displayed ...

The way around this is for the app model to actually change, and force pedestal to emit the needed deltas.

Using the above example, we actually store todos under the path [:todos :modify todo-id]. We also store a particular value [:todos :modify :req-id], which is a globally unique request id.

"Refreshing the list" is then a matter of changing the value at [:todos :modify :req-id], which will cause the entire [:todos :modify] tree to be emitted and then used in rendering.

The same idea applies to switching filters and viewing items.


Documentation can be generated using Marginalia using the command:

lein marg app

Stuff to Add (TODOs)

  • actually figure out how to unit test the damn thing

  • Allow re-ordering of todos (click and drag as per David's sortable example)

  • Add tags to todos, with typeahead search for easy entry (use David's typeahead example)

  • Smoother animation between transitions

    • navigation between List View and Item View
    • when checking/unchecking a todo
  • undo support as per David Nolen's example

    This should only support specific operations (eg: undo delete of a todo, undo creation of a todo).

    A related feature would be todo snapshotting, so you can reset todos to a particular state

  • persistence API

    This should just be an interface that can be pluggable into any adapter (localStorage, some database server, etc ...)

    This should play nicely with undo support (which would require changes to the data model).

  • externs file for React

    At this point in time, clojurescript advanced compilation is not supported, and therefore the production aspect does not yet work. (see this section of the pedestal tutorial for an overview of aspects)


Code is in the public domain.


TodoMVC using Pedestal and Om






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