Enumerating a MIDIPacketList in Swift 2.

MIDIPacketList+SequenceType.swift

// Blogged at http://design.featherless.software/enumerate-midipacketlist-in-swift-part-1/
//        and http://design.featherless.software/enumerate-midipacketlist-in-swift-part-2/
extension MIDIPacketList: SequenceType {
  public func generate() -> AnyGenerator<MIDIPacket> {
    var iterator: MIDIPacket?
    var nextIndex: UInt32 = 0

    return anyGenerator {
      if nextIndex++ >= self.numPackets { return nil }
      if iterator != nil {
        iterator = withUnsafePointer(&iterator!) { MIDIPacketNext($0).memory }
      } else {
        iterator = self.packet;
      }
      return iterator
    }
  }
}