Chapter 6. Music Performance
This chapter describes the classes and functions that you use to create a musical performance with the MusicKit. The material presented here falls roughly into three sections: realizing music data, MusicKit scheduling, and guidelines for building an efficient application. To fully appreciate this chapter, you should be familiar with the material in the preceding two chapters, Chapter 4 and Chapter 5.
Design Philosophy
The primary task in a musical performance is to take a
representation of music and render it in an appropriate fashion.
Traditionally, this task is met by a performer wielding an instrument.
Similarly, the MusicKit has its
MKPerformer and
MKInstrument objects: A
MKPerformer obtains a
MKNote object, either by reading it from a
database such as a MKPart or scorefile or by
improvising the MKNote itself. The
MKPerformer then relays the
MKNote to an
MKInstrument that provides the machinery for
rendering, or realizing, the
MKNote.
Of MKInstrument and
MKPerformer, the former is the more crucial to
a MusicKit performance. Realization is the
whole point of a performance, thus
MKInstruments are ubiquitous.
MKPerformers provide a convenient means for
acquiring or generating MKNotes but they're not
essential to a performance application; in fact, some applications
can't use MKPerformers but, instead, must
manufacture MKNotes themselves and send them
directly to MKInstruments. In any case, the
means by which a MKNote is acquired is
separated from its means of realization and the mechanism for
connecting an agent of acquisition to that of realization is extremely
general. Specifically, any MKPerformer (or
your application) can be connected to any―and any number
of―MKInstruments. These connections can
be created and severed dynamically, allowing you to create a
kaleidoscopic network of MKNote sources and
destinations.
With MKPerformer and
MKInstrument, the
MKConductor class completes the triumvirate of
preeminent performance classes. A MKConductor
object acts as a time keeper as it determines the tempo of the
performance and starts and stops groups of
MKPerformers. Essentially, a
MKConductor is a timed-message sending object
that dispatches Objective-C messages at distinct times (in this
regard, it's similar to the timed entry mechanism of
NSTimer, but with a more sophisticated
interface). For example, it's through a
MKConductor that a
MKPerformer schedules the messages that relay
MKNotes to an
MKInstrument.
Because of the demands of music performance, the MusicKit provides hooks into the operating system that allow you to create applications that can run with heightened priority. Well-debugged and fine-tuned applications will find great benefit in these enhancements.
Figure 6-1 is a subset of the MusicKit's performance classes, showing their hierarchical relationship:
