First steps with Windows 8 development - Build Windows 8 Apps with HTML5 and JavaScript - page 107

The sixth principle can then be summarized by saying that applications may rely on public services
exposed by other applications in order to implement their functionality. Windows Store applications may
import functions from other applications via contracts. A contract is a formalized API for applications to
invoke functions from other applications. This saves you from rewriting the same functions over and over,
and at the same time, it brings users to the same action in mostly the same way.

For example, Windows Store applications can support the system-defined Search contract to
retrieve information from other applications and implement the Share contract to expose their
content publicly.

Above us only cloud
Finally, the seventh principle probably needs no further explanation. Local disks are no longer and
not necessarily the only place to save and read data. To ensure a continuous feel between the user
and the application, for years developers used to save personal data to cookies and local settings.
The cloud just adds another dimension by publishing personal data and making that information
available to others for social software interaction. Here the cloud indicates user-specific or even
application-specific storage that lives on some remote server that is publicly accessible.

Components for the presentation layer
The WinJS library is a library made of JavaScript objects expressly designed to provide easy access to
core Windows 8 features and subsequently simplify the development of Windows Store applications
with JavaScript.

WinJS consists of two main parts: a collection of behavioral objects to deal with core tasks such
as storage, networking, multimedia, and application lifecycle, and a set of widgets for user interface
arrangements. You'll use both parts of the WinJS library in the rest of the topic. Anyway, it is helpful
to have an overall vision of the visual widgets you can quickly incorporate in your applications as
building blocks.

Visual elements of WinJS
Table 5-1 provides a view of the visual elements you find in the WinJS library. You will compose
the user interface of your future Windows Store applications by integrating one or more of these
components in an HTML template.

TABLE 5-1 WinJS widgets

Widget Description

Displays a horizontal command bar that is usually placed at the bottom of the window. AppBar

Pops up a calendar and enables the user to pick up a date. DatePicker

Displays a collection of items and allows a user to flip through them, displaying one at
a time. A typical example can be displaying pictures and captions with the ability to flip
through them horizontally.

FlipView