@ngdoc overview
@name Tutorial: 4 - Two-way Data Binding
@description

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In this step, you will add a feature to let your users control the order of the items in the phone
list. The dynamic ordering is implemented by creating a new model property, wiring it together with
the repeater, and letting the data binding magic do the rest of the work.


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You should see that in addition to the search box, the app displays a drop down menu that allows
users to control the order in which the phones are listed.

The most important differences between Steps 3 and 4 are listed below. You can see the full diff on
{@link https://github.com/angular/angular-phonecat/compare/step-3...step-4 GitHub}:


## Template

__`app/index.html`:__
<pre>
  Search: <input ng-model="query">
  Sort by:
  <select ng-model="orderProp">
    <option value="name">Alphabetical</option>
    <option value="age">Newest</option>
  </select>


  <ul class="phones">
    <li ng-repeat="phone in phones | filter:query | orderBy:orderProp">
      {{phone.name}}
      <p>{{phone.snippet}}</p>
    </li>
  </ul>
</pre>

We made the following changes to the `index.html` template:

* First, we added a `<select>` html element named `orderProp`, so that our users can pick from the
two provided sorting options.

      <img  class="diagram" src="img/tutorial/tutorial_04.png">

* We then chained the `filter` filter with {@link api/ng.filter:orderBy `orderBy`}
filter to further process the input into the repeater. `orderBy` is a filter that takes an input
array, copies it and reorders the copy which is then returned.

Angular creates a two way data-binding between the select element and the `orderProp` model.
`orderProp` is then used as the input for the `orderBy` filter.

As we discussed in the section about data-binding and the repeater in step 3, whenever the model
changes (for example because a user changes the order with the select drop down menu), Angular's
data-binding will cause the view to automatically update. No bloated DOM manipulation code is
necessary!



## Controller

__`app/js/controllers.js`:__
<pre>
function PhoneListCtrl($scope) {
  $scope.phones = [
    {"name": "Nexus S",
     "snippet": "Fast just got faster with Nexus S.",
     "age": 0},
    {"name": "Motorola XOOM™ with Wi-Fi",
     "snippet": "The Next, Next Generation tablet.",
     "age": 1},
    {"name": "MOTOROLA XOOM™",
     "snippet": "The Next, Next Generation tablet.",
     "age": 2}
  ];

  $scope.orderProp = 'age';
}
</pre>

* We modified the `phones` model - the array of phones - and added an `age` property to each phone
record. This property is used to order phones by age.

* We added a line to the controller that sets the default value of `orderProp` to `age`. If we had
not set the default value here, the model would stay uninitialized until our user would pick an
option from the drop down menu.

  This is a good time to talk about two-way data-binding. Notice that when the app is loaded in the
browser, "Newest" is selected in the drop down menu. This is because we set `orderProp` to `'age'`
in the controller. So the binding works in the direction from our model to the UI. Now if you
select "Alphabetically" in the drop down menu, the model will be updated as well and the phones
will be reordered. That is the data-binding doing its job in the opposite direction — from the UI
to the model.



## Test

The changes we made should be verified with both a unit test and an end-to-end test. Let's look at
the unit test first.

__`test/unit/controllersSpec.js`:__
<pre>
describe('PhoneCat controllers', function() {

  describe('PhoneListCtrl', function(){
    var scope, ctrl;

    beforeEach(function() {
      scope = {},
      ctrl = new PhoneListCtrl(scope);
    });


    it('should create "phones" model with 3 phones', function() {
      expect(scope.phones.length).toBe(3);
    });


    it('should set the default value of orderProp model', function() {
      expect(scope.orderProp).toBe('age');
    });
  });
});
</pre>


The unit test now verifies that the default ordering property is set.

We used Jasmine's API to extract the controller construction into a `beforeEach` block, which is
shared by all tests in the parent `describe` block.

You should now see the following output in the Karma tab:

        Chrome 22.0: Executed 2 of 2 SUCCESS (0.021 secs / 0.001 secs)


Let's turn our attention to the end-to-end test.

__`test/e2e/scenarios.js`:__
<pre>
...
    it('should be possible to control phone order via the drop down select box',
        function() {
      //let's narrow the dataset to make the test assertions shorter
      input('query').enter('tablet');

      expect(repeater('.phones li', 'Phone List').column('phone.name')).
          toEqual(["Motorola XOOM\u2122 with Wi-Fi",
                   "MOTOROLA XOOM\u2122"]);

      select('orderProp').option('Alphabetical');

      expect(repeater('.phones li', 'Phone List').column('phone.name')).
          toEqual(["MOTOROLA XOOM\u2122",
                   "Motorola XOOM\u2122 with Wi-Fi"]);
    });
...
</pre>

The end-to-end test verifies that the ordering mechanism of the select box is working correctly.

You can now rerun `./scripts/e2e-test.sh` or refresh the browser tab with the end-to-end test
`runner.html` to see the tests run, or you can see them running on {@link
http://angular.github.com/angular-phonecat/step-4/test/e2e/runner.html
Angular's server}.

# Experiments

* In the `PhoneListCtrl` controller, remove the statement that sets the `orderProp` value and
you'll see that Angular will temporarily add a new "unknown" option to the drop-down list and the
ordering will default to unordered/natural order.

* Add an `{{orderProp}}` binding into the `index.html` template to display its current value as
text.

* Reverse the sort order by adding a `-` symbol before the sorting value: `<option value="-age">Oldest</option>`

# Summary

Now that you have added list sorting and tested the app, go to {@link step_05 step 5} to learn
about Angular services and how Angular uses dependency injection.


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