MICROSOFT_PRESS_EBOOK_PROGRAMMING_WINDOWS_8_APPS_WITH_HTML_CSS_AND_JAVASCRIPT_PDF

Chapter 16
WinRT Components: An
Introduction
At the very beginning of this book, in the first two pages of Chapter 1, “The Life Story of a Windows
Store App,” we learned that apps can be written in a variety of languages and presentation
technologies. For the better part of this book we’ve spoken of this as a somewhat singular choice: you
choose the model that best suits your experience and the needs of your app and go from there.
At the same time, we’ve occasionally encountered situations where some sort of mixed language
approach is possible, even necessary. In Chapter 1, in “Sidebar: Mixed Language Apps,” I introduced the
idea that an app can actually be written in multiple languages. In Chapter 8, “State, Settings, Files, and
Documents,” I mentioned that gaining access to database APIs beyond IndexedDB could be
accomplished with a WinRT component. In Chapter 10, “Media,” we saw that JavaScript might not
necessarily the best language in which to implement a pixel crunching routine. And in Chapter 13, “Tiles,
Notifications, the Lock Screen, and Background Tasks” we encountered the Notifications Extensions
Library, a very useful piece of code written in C# that made the job of constructing an XML payload
much easier from JavaScript. We also saw that background tasks can be written in languages that differ
from that of the main app.
With the primary restriction that an app that uses HTML and CSS for its presentation layer cannot
share a drawing surface with WinRT components written in other languages, the design of WinRT really
makes the mixed language approach possible. As discussed in Chapter 1, WinRT components written in
any language are made available to other languages through a projection layer that translates the
component’s interface into what’s natural in the target language. All of WinRT is written this way, and
custom WinRT components take advantage of the same mechanism. (We’ll see the core characteristics
of the JavaScript projection later in this chapter.)
What this means for you—writing an app with HTML, CSS, and JavaScript—is that you can
implement various parts of your app in a language that’s best suited to the task or technically necessary.
As a dynamic language, JavaScript shows its strength in easily gluing together functionality provided by
other components that do the heavy lifting for certain kinds of tasks (like camera capture, background
transfers, etc.). Those heavy-lifting components are often best written in other language such as C++,
where the compiled code runs straight on the CPU instead of going through runtime layers like
JavaScript.
716