Monday, October 28, 2013

Getting knocked out

Everyone is saying JavaScript is the future and i was wondering what all the hype was. Was never a big fan of JavaScript to begin with since i was involved mainly with writing back-end code. But i am always open for new things and being the enthusiast i am, thought to try out one of the plethora of JavaScript libraries out there.

Since MVC knowledge is inherent in anyone who would have written any serious web application, i wanted to try out something closer to my comfort zone. This is where i stumbled upon knockoutjs . It advocates more of a MVVM(Model-View-View-Model) architectural pattern which i believe is a very handy way to handle your front-end logic with a clear layer of separation.

I remember the code we wrote prior to these libraries where we used a separate JSON array for each page to hold all the elements in that page and do all validations based on the attributes we define on each JSON attribute.

With Knockout, this is all handled by the library. It handles the data bindings to the elements, changes to the elements as well dependency tracking. I did some reconnaissance of my own on the library and this article is just an introduction to what you can achieve with this library.

The code is simple, i enter my first name and last name and i have a separate field that displays the full name by concatenating the first name and the last name. Then i have a simple button to generate the JSON string which we could use to send the data back to the server.

Let us look at the code;

 
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
 <title>Personal Information</title>
</head>

<body>

 <div>
  <table>
   <tr>
    <td>First Name : </td>
    <td><input type="text" name="txtName" id="txtName" data-bind="value:firstname,valueUpdate:'afterkeydown'"></td>
   </tr>
   
   <tr>
    <td>Last Name : </td>
    <td><input type="text" name="txtLName" id="txtLName" data-bind="value:lastname,valueUpdate:'afterkeydown'"></td>
   </tr>
   
   <tr>
    <td colspan="2"><span id="lblFullName" data-bind="text:fullname"></span></td>
   </tr>
   
   <tr>
    <td colspan="2"><button data-bind="click: generateJSON">Generate JSON String</button></td>
   </tr>
  </table>
 </div>

<script src="http://code.jquery.com/jquery-1.10.1.min.js"></script>
<script type="text/javascript" src="http://cdnjs.cloudflare.com/ajax/libs/knockout/2.3.0/knockout-min.js"></script>

 <script type="text/javascript">
 
 $(function(){
   var person = {
    firstname:ko.observable(''),
    lastname:ko.observable(''),
    generateJSON:function(){
     alert(ko.toJSON(person));
    }
   };
   
   person.fullname = ko.computed(function(){
    
    return "The full name is : "+this.firstname()+" "+this.lastname();
   },person);
  
   ko.applyBindings(person);
  
  }
  );
 </script>
</body>
</html>

As you can see from the code this is just a normal HTML mark-up. The magic happens within the code between the script tags. For ease of use of this code i have included the knockout and jquery scripts directly from the CDN locations because otherwise you would have to manually download them and include the scripts.

In Knockout, everything revolves around your Model. This is the main point of interaction in terms of Knockout. Here i have defined by model as person. One thing to note is that i have defined the firstname and lastname properties as observable. In Knockout, observables are JavaScript functions since not all browsers support getters/setters.

Then i need to tell how to map each property in my model to the DOM elements. This is done through the data-bind attribute. As you can see, in each input element, i have defined the data-bind attribute and stated that the value property should be equal to the value of the property in my Model.

Knockout also has a concept called computed observables which was previously called Dependent-Observables. What this does is that it allows me to aggregate any fields within the model and show them as part of the result as well and any change in the properties will be reflected in the computed observable variable as well. You can see that if you keep typing values to the firstname and last name input boxes, the full name string also changes accordingly. I have specified another value in the data-bind attribute as valueUpdate:'afterkeydown' which allows the changes typed into the input boxes to be reflected as the user types in which otherwise would only have reflected if the user tabs into the fields.

To bind our Model to the Knockout library, we call ko.applyBindings passing in our Model. There are helper methods that allow us to generate a JSON string of our model which is what i have done in the click event function defined within my Model.

I hope this introduction would get you interested to try this library out if you have not already. I will follow up this post with more advanced topics such as templating, observable arrays, parent binding context etc..

As always your comments and suggestions are most welcome.

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