Graphics Reference
In-Depth Information
2
A picture is worth a thousand words. An interface is worth a thousand pictures.
Ben Shneiderman
Chapter 1, “The Layer Tree,” introduced the
CALayer
class and created a simple layer
with a blue background. Background colors are all very well, but layers would be rather
boring if all they could display was a flat color. A
CALayer
can actually contain a picture
of anything you like. This chapter explores the
backing image
of
CALayer
.
CALayer
has a property called
contents
. This property's type is defined as
id
,
implying that it can be any kind of object. This is true—in the sense that you can assign any
object you like to the
contents
property and your app will still compile—however, in
practice, if you supply anything other than a
CGImage
, then your layer will be blank.
This quirk of the
contents
property is due to Core Animation's Mac OS heritage. The
reason that
contents
is defined as an
id
is so that on Mac OS, you can assign either a
CGImage
or an
NSImage
to the property and it will work automatically. If you try to
assign a
UIImage
on iOS, however, you'll just get a blank layer. This is a common cause
of confusion for iOS developers who are new to Core Animation.
The headaches don't stop there, though. The type you actually need to supply is a
CGImageRef
, which is a pointer to a
CGImage
struct.
UIImage
has a
CGImage
property that returns the underlying
CGImageRef
. If you try to assign that to the
CALayer
contents
property directly, though, it won't compile because
CGImageRef
is not really a Cocoa object; it's a Core Foundation type.
