An Approach to View-Models in Backbone

Wednesday, January 7th, 2015

One of the difficult parts I am experiencing when working with Backbone is separating out display logic properties from models. Let’s look at a simple example where we display a user model in a view and allow the user’s email address to be edited.

The Problem

var User = Backbone.Model.extend({
  url: '/users'
});

var UserView = Backbone.View.extend({
  template: Handlebars.compile($('script[data-template-name="user"]').html()),
  events: {
    'click .edit-email': 'editEmail',
  },

  editEmail: function(e) {
    e.preventDefault();
    this.model.set('editEmail', true);
  }
});

var user = new User({
  id: 1,
  first: 'David',
  last: 'Tang',
  email: 'dtang85@gmail.com'
});

var userView = new UserView({
  model: user
});

userView.render();

And the corresponding UserView template using Handlebars.


{{#if editEmail}}
  <input type="text" value="{{email}}">
{{else}}
  {{email}}
  <a href="#" class="edit-email">Edit</a>
{{/if}}

Whenever the edit button is clicked, the template switches from displaying the email and edit button to a text input where the email can be edited.

So imagine I click on that edit button. If I call user.toJSON(), the object will be:

{
  "id": 1,
  "first": "David",
  "last": "Tang",
  "email": "dtang85@gmail.com",
  "editEmail": true
}

Now let’s say the API I am working with only allows certain properties to be sent across. In the case of this example, the API expects only properties id, first, last, and email. If unexpected properties are sent across, like editEmail, the API returns an error response.

Essentially the User model has beome polluted with display specific properties. One solution you might consider would be to write a custom AJAX call and override the save() method. This could work, but do you really want to do that for all your models? Probably not.

One of the things I really liked about Ember is how it defines controllers as a way to decorate models with display logic properties. There is a clear separation between the data models and the UI specific properties. Similarly in Angular, you can bind data to $scope or this if you are using the controller-as syntax, both which act as view-models and can be decorated with display specific properties.

A Solution

A solution I came up with was to introduce a view-model into Backbone called ViewModel that extends Backbone.Model. Here are the requirements:

1. The view-model should accept any number of properties containing Backbone models

var vm = new ViewModel({
  user: user, // a User model instance
  address: address // an Address model instance
});

This could be improved to allow for infinite nested models, but this should suffice.

2. The view-model should call toJSON() on the data models attached to it

var vm = new ViewModel({
  user: user
});

vm.toJSON();
// { user: { id: 1, first: 'David', last: 'Tang', email: 'dtang85@gmail.com' } }

vm.set('editEmail', true);
// { user: { id: 1, first: 'David', last: 'Tang', email: 'dtang85@gmail.com' }, editEmail: true }

3. The view-model should emit “change” and “change:{property}” events when its models change.

var vm = new ViewModel({
  user: user
});

vm.on('change', function() {
  console.log('change event emitted');
});

vm.on('change:user', function() {
  console.log('change:user event emitted');
});

vm.get('user').set('email', 'davidtang@someemail.com');

// "change" event emitted
// "change:user" event emitted

The Implementation

var ViewModel = Backbone.Model.extend({
  constructor: function(options) {
    Object.keys(options).forEach(function(key) {
      var vm = this;

      if (options[key] instanceof Backbone.Model) {
        options[key].on('change', function() {
          vm.trigger('change');
          vm.trigger('change:' + key);
        });
      }
    }, this);

    return Backbone.Model.apply(this, arguments);
  },

  toJSON: function() {
    var json = Backbone.Model.prototype.toJSON.apply(this, arguments);

    Object.keys(json).forEach(function(key) {
      var value = this.get(key);

      if (value instanceof Backbone.Model) {
        json[key] = value.toJSON();
      }
    }, this);

    return json;
  }
});

Conclusion

This is just one implementation I am experimenting with. I haven’t tried it in the application I am building at work, but I am considering it. Please let me know what you think in the comments and what solutions you have tried/considered.

Disclaimer: Any viewpoints and opinions expressed in this article are those of David Tang and do not reflect those of my employer or any of my colleagues.

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