370 Working with Movie Clips
The following table contains brief descriptions of the new properties for movie clip instances:
These three properties are independent of each other, however, the
opaqueBackground and
scrollRect properties work best when an object is cached as a bitmap. You only see
performance benefits for the
opaqueBackground and scrollRect properties when you set
cacheAsBitmap to true.
To create a surface that’s also scrollable, you must set the
cacheAsBitmap and scrollRect
properties for the movie clip instance. Surfaces can nest within other surfaces. The surface
copies the bitmap onto its parent surface.
For information on alpha channel masking, which requires you to set the
cacheAsBitmap
property to
true, see “About alpha channel masking” on page 377.
You can find a sample source file that shows you how bitmap caching can be applied to an
instance. Find the file called cacheBitmap.fla, in the Samples folder on your hard disk.
■ In Windows, browse to boot drive\Program Files\Macromedia\Flash 8\Samples and
Tutorials\Samples\ActionScript\CacheBitmap.
Property Description
cacheAsBitmap Makes the movie clip instance cache a bitmap representation of itself.
Flash creates a surface object for the instance, which is a cached
bitmap instead of vector data. If you change the bounds of the movie
clip, the surface is recreated instead of resized. For more information
and an example, see “Caching a movie clip” on page 373.
opaqueBackground Lets you specify a background color for the opaque movie clip
instance. If you set this property to a numeric value, the movie clip
instance has an opaque (nontransparent) surface. An opaque bitmap
does not have an alpha channel (transparency), and renders faster. For
more information and an example, see “Setting the background of a
movie clip” on page 375.
scrollRect Lets you quickly scroll movie clip content and have a window for
viewing larger content. The movie clip’s contents are cropped, and the
instance scrolls with a specified width, height, and scroll offsets. This
lets the user quickly scroll movie clip content and have a window that
displays larger content than the Stage area. Text fields and complex
content that you display in the instance can scroll faster because Flash
does not regenerate the entire movie clip vector data. For more
information and an example, see
scrollRect (MovieClip.scrollRect
property)
.
NOTE
You cannot apply caching directly to text fields. You need to place text within a movie
clip to take advantage of this feature. For an example, see the sample file in Flash install
directory\Samples and Tutorials\Samples\ActionScript\FlashType.
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