MICROSOFT_PRESS_EBOOK_PROGRAMMING_WINDOWS_8_APPS_WITH_HTML_CSS_AND_JAVASCRIPT_PDF

}
ImageProperties.savePropertiesAsync(properties).done(function () {
// ...
}, function (error) {
// Some error handling as some properties may not be supported by all image formats.
});
A few noteworthy features of this code include the following:
• It separates keywords in the UI control and separately appends each to the keywords property
vector.
• It creates a new collection of properties of type Windows.Foundation.Collections.PropertySet
and uses its insert method to add properties to the list. This property set is what’s expected by
the savePropertiesAsync method.
• The Helpers.convertStartsToSystemRating method (see js/default.js) converts between 1–5
stars, as used in the WinJS.UI.Rating control, to the System.Rating value that uses a 1–99 range.
The documentation for System.Rating specifically indicates this mapping.
In general, all the detailed information you want for any particular Windows property can be found
on the reference page for that property. Again start at the Windows Properties and drill down from
there.
Image Manipulation and Encoding
To do something more with an image than just loading and displaying it (where again you can apply
various CSS transforms for effect), you need to get to the actual pixels by means of a decoder. This
already happens under the covers when you assign a URI to an img.src., but to have direct access to
pixels means decoding manually. On the flip side, saving pixels back out to an image file means using an
encoder.
WinRT provides APIs for both in the Windows.Graphics.Imaging namespace, namely in the
BitmapDecoder, BitmapTransform, and BitmapEncoder classes. Loading, manipulating, and saving an
image file often involves these three classes in turn, though the BitmapTransform object is focused on
rotation and scaling so you won’t use it if you’re doing other manipulations.
One demonstration of this API can be found in Scenario 2 of the Simple imaging sample. I’ll leave it
to you to look at the code directly, however, because it gets fairly involved—up to 11 chained promises
to save a file! It also does all decoding, manipulation, and encoding within a single function such as
saveHandler (js/scenario2.js). Here’s the process it follows:
• Open a file with StorageFile.openAsync, which provides a stream.
• Pass that stream to the static method BitmapDecoder.createAsync which provides a specific
instance of BitmapDecoder for the stream.
• Pass that decoder to the static method BitmapEncoder.createForTranscodingAsync, which
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