Enumerate MIDIPacketList in Swift: Part 2

This is the conclusion of Enumerate MIDIPacketList in Swift.

In Part 1 we examined the structure of MIDIPacketList and built an Objective-C-influenced implementation of SequenceType. In this post we’ll look at Swift 2’s AnyGenerator type to achieve the same effect — but more concisely — thanks to Swift type inference and closures.

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AnyGenerator

Swift 2’s AnyGenerator type - and corresponding anyGenerator method - allow us to create inline generators. This removes the need to create a separate Generator class and — paired with type inference — allows us to remove a lot of boilerplate. Let’s re-examine our MIDIPacketList extension.

The essential compilable code for a SequenceType built with AnyGenerator looks like this:

extension MIDIPacketList: SequenceType {
  public func generate() -> AnyGenerator<MIDIPacket> {
    // TODO: Local state...

    return anyGenerator({
      // TODO: return nil if no additional elements

      // TODO: Iterator logic

      return nil // TODO: return the next sequence item
    })
  }
}

• Swift is able to infer the Generator type based on the return value of generate().

• A Generator returns nil to indicate that it has no more elements to generate.

The Generator in the code above will not generate anything. Let’s fill in the implementation.

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Step 1: Iterator state

First we define the generator’s iterator.

public func generate() -> AnyGenerator<MIDIPacket> {
  var iterator: MIDIPacket?
  var nextIndex: UInt32 = 0

• iterator stores the last packet we returned from the generator. Its initial nil value indicates that we haven’t enumerated a packet yet.

• nextIndex allows us to know once we’ve consumed all packets. This is required because MIDIPacketNext advances packet pointers by a fixed amount on each invocation, eventually pointing to memory not allocated for packets. nextPacket alone is not a reliable signal for having consumed all packets.

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Step 2: Iterator logic

Next we define the logic that will advance the iterator on each call of the generator.

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

• iterator’s nil state allows us to return self.packet initially.

• Each subsequent call advances iterator with MIDIPacketNext.

• MIDIPacketNext accepts and returns an UnsafePointer. withUnsafePointer is a helpful way to transform between MIDIPacket and UnsafePointer<MIDIPacket>, avoiding the need to initialize an UnsafeMutablePointer ourselves.

• The $0 is shorthand for the closure’s first argument.

• The enclosing () for the anyGenerator function has been removed because they’re not needed when the only argument is a closure.

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Step 3: Iterator termination

The final implementation advances and checks nextIndex against self.numPackets before iterating in order to avoid accessing invalid memory.

public func generate() -> AnyGenerator<MIDIPacket> {
  var iterator: MIDIPacket?
  var nextIndex: UInt32 = 0

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

  }
}

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Addendum

This series went through multiple iterations as I learned more Swift language features. Check out the revision history of this entry’s code.

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