Specialized Layers - iOS Core Animation - page 110

Figure 6.4 A rich text label implemented using CATextLayer
Leading and Kerning
It's worth mentioning that the leading (line spacing) and kerning (spacing between letters)
for text rendered using CATextLayer is not completely identical to that of the string
rendering used by UILabel due to the different drawing implementations (Core Text and
WebKit, respectively).
The extent of the discrepancy varies (depending on the specific font and characters used)
and is generally fairly minor, but you should keep this mind if you are trying to exactly
match appearance between regular labels and a CATextLayer .
A UILabel Replacement
We've established that CATextLayer has performance benefits over UILabel , as well
as some additional layout options and support for rich text on iOS 5. But it's fairly
cumbersome to use by comparison to a regular label. If we want to make a truly usable
replacement for UILabel , we should be able to create our labels in Interface Builder, and
they should behave as much as possible like regular views.
We could subclass UILabel and override its methods to display the text in a
CATextLayer that we've added as a sublayer, but we'd still have the redundant empty
backing image created by the presence of UILabel -drawRect: method. And because
CALayer doesn't support autoresizing or autolayout, a sublayer wouldn't track the size of
the view bounds automatically, so we would need to manually update the sublayer bounds
every time the view is resized.