What is an Opaque type?

• a type that wraps lower-level types
• used when
  • the underlying implementation is complex
  • the user does not need to know about the inner workings.
• The individual fields of an object based on an opaque type are hidden from clients, but the type’s functions offer access to most values of these fields

A. Opaque types in Core Foundation

Core foundation has many “opaque types’.

What is Core Foundation?

Core Foundation is a library with a set of programming interfaces conceptually derived from the Objective-C-based Foundation framework but implemented in the C language.

1. Makes it possible for the different frameworks and libraries on OS X to share code and data.

Applications, libraries, and frameworks can define C routines that incorporate Core Foundation types in their external interfaces; they can thus communicate data—as Core Foundation objects—to each other through these interfaces.

2. Makes some degree of operating-system independence possible

Some Core Foundation types and functions are abstractions of things that have specific implementations on different operating systems.

3. Supports internationalization with Unicode strings

Uses CFString, instances of which represent an array of 16-bit Unicode characters. Flexible enough to hold megabytes worth of characters and yet simple and low-level enough for use in all programming interfaces communicating character data

Opaque type examples in Core Foundation

• CFString: an opaque type that “represents” and operates on Unicode character arrays.
• CFArray: an opaque type for indexed-based collection functionality.

B. Opaque types in Quartz2D

• CGPathRef: vector graphics
• CGImageRef: bitmap images
• CGLayerRef: drawling layer that can be used for repeated drawing and offscreen drawing
• CGPatternRef: repeated drawing using patterns
• CGShadingRef: for gradients
• CGGradientRef: for gradients
• CGColorRef: used for colors in Quartz2D
• CGImageSourceRef: move data into and out of Quartz
• CGFontRef: to draw text