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Microsoft Sharepoint 2013 : Understanding app patterns (part 2) - Building MVVM apps - Introducing knockout

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1/25/2015 2:57:04 AM

Introducing knockout

JavaScript does not natively support the kind of loose coupling that the MVVM pattern requires. Therefore, additional support must be introduced in the form of a third-party library. Although there are many third-party libraries available for JavaScript, few of them implement the MVVM pattern. One library that does implement it successfully is Knockout, which you can find at http://knockoutjs.com.

Knockout is a JavaScript library that you can add to your apps to implement the MVVM pattern. With Knockout, you can create ViewModel components in JavaScript that effectively bind the server-side SharePoint data sources to the webpages in your apps. The primary capabilities of Knockout that makes the MVVM pattern possible are declarative bindings and dependency tracking.

Declarative bindings make it possible for you to bind a ViewModel to HTML elements in a webpage. Instead of writing HTML elements in your JavaScript, the declarative bindings are defined within the app web page. This approach removes the knowledge of the webpage structure from the JavaScript code and creates the loose binding required by MVVM. Example 2 presents an example of declarative bindings in Knockout.

Example 2. Declarative bindings
<div id="resultsDiv" style="overflow: auto">
<table>
<thead>
<tr>
<th>Last Name</th>
<th>First Name</th>
<th>Phone</th>
</tr>
</thead>
<tbody id="resultsTable" data-bind="foreach: get_contacts()">
<tr>
<td data-bind="text: get_lname()"></td>
<td data-bind="text: get_fname()"></td>
<td data-bind="text: get_phone()"></td>
</tr>
</tbody>
</table>
</div>

In Example 2, the data-bind attribute binds a method from the ViewModel to HTML elements in the webpage. Note how the table body uses a foreach construct to iterate through a set of contacts returned from the get_contacts method and build a table row for each contact. The last name, first name, and phone number associated with each contact is then bound to a table cell within a row.

When you develop a ViewModel, you start by creating a library to hold the data for binding. Example 3 shows a JavaScript library that holds the data for an individual contact. Note how the properties of the contact are exposed through public methods, and these methods are the ones referenced in the associated HTML of Example 2.

Example 3. The contact data library
"use strict";

var Wingtip = window.Wingtip || {}
window.Wingtip.Contact = function (ln, fn, ph) {

//private members
var lname = 'undefined',
fname = 'undefined',
phone = 'undefined',
set_lname = function (v) { lname = v; },
get_lname = function () { return lname; },
set_fname = function (v) { fname = v; },
get_fname = function () { return fname; },
set_phone = function (v) { phone = v; },
get_phone = function () { return phone; };

//Constructor
lname = ln;
fname = fn;
phone = ph;

//public interface
return {
set_lname: set_lname,
get_lname: get_lname,
set_fname: set_fname,
get_fname: get_fname,
set_phone: set_phone,
get_phone: get_phone
};
}

The associated ViewModel performs the queries against the back-end data source and populates the data objects. The ViewModel also provides the public interface that is referenced in the data-bind attribute of the HTML for retrieving the collection of list items. Example 4 demonstrates a complete ViewModel that queries a contacts list and creates an array of contact information.

Example 4. The contacts ViewModel
"use strict";

var Wingtip = window.Wingtip || {}

Wingtip.ContactViewModel = function () {

//private members
var contacts = ko.observableArray(),
get_contacts = function () { return contacts; },

load = function () {
$.ajax(
{
url: _spPageContextInfo.webServerRelativeUrl +
"/_api/web/lists/getByTitle('Contacts')/items/" +
"?$select=Id,FirstName,Title,WorkPhone" +
"&$orderby=Title,FirstName",
type: "GET",
headers: {
"accept": "application/json;odata=verbose",
},

success: onSuccess,
error: onError
}
);
},

onSuccess = function (data) {
var results = data.d.results;

contacts.removeAll();

for (var i = 0; i < results.length; i++) {
contacts.push(
new Wingtip.Contact(
results[i].Title,
results[i].FirstName,
results[i].WorkPhone));
}

},

onError = function (err) {
alert(JSON.stringify(err));
};


//public interface
return {
load: load,
get_contacts: get_contacts
};

}();

The difference between Example 4 and Example 1 is significant. Example 1 relies on intimate knowledge of the user interface to create a table and push it onto the webpage for display. The ViewModel in Example 4 simply creates an array of contacts. You can then easily bind this array to various display forms such as a table or unordered list.

The most important aspect of the ViewModel is its use of the ko.observableArray type for handling the collection of contacts. The Knockout library provides the ko.observableArray type specifically to implement dependency tracking. With dependency tracking, automatic updating of the user interface occurs whenever data elements in the array change. When your app initializes, it should load the observable array and then bind the data. Knockout initializes the data binding when you call the applyBindings method. Example 5 illustrates how to initialize the data bindings with the ViewModel.

Example 5. Initializing data bindings
$(document).ready(function () {
Wingtip.ContactViewModel.load();
ko.applyBindings(Wingtip.ContactViewModel, $get("resultsTable"));
});
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