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Contracts and common tasks
Microsoft Windows 8 is an operating system that imposes several constraints on applications. In
particular, applications are isolated from each other. How can applications communicate with one
another under these conditions? Contracts are the answer.
A contract defines a system-wide protocol for applications developed by different companies to
communicate and exchange data. A contract refers to a particular common task, such as sharing data,
searching for data, picking files, defining settings, and more. Applications that need that particular
behavior can rely on existing implementations of that contract instead of coding their own. At the
same time, applications may expose themselves to a given contract so that other applications can
consume it.
Aspects of Windows 8 contracts
There are three aspects of a contract that deserve a bit more attention: how to discover available
services, how to consume services, and how to expose services.
The Charms bar
In Windows 8, the Charms bar is a system's toolbar that users can access at any time, regardless of the
application that is currently active. The Charms bar slides in from the right edge of the screen as soon
as you point the mouse to the top/right or bottom/right corner of the screen. You can also display the
bar by pressing Windows+C on your keyboard. Finally, on touch-enabled devices, you invoke the bar
by swiping from the side. The Charms bar is shown in Figure 9-1.
FIGURE 9-1 The Charms bar in action.
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