Efficiently copying Swift Array to memory buffer for iOS Metal

Question

I am writing an iOS application using Apple\'s new Metal framework. I have an array of Matrix4 objects (see Ray Wenderlich\'s tutorial) that I need to pass in to a shader via

Answer 1

I've done this with an array of particles that I pass to a compute shader.

In a nutshell, I define some constants and declare a handful of mutable pointers and a mutable buffer pointer:

let particleCount: Int = 1048576
var particlesMemory:UnsafeMutablePointer = nil
let alignment:UInt = 0x4000
let particlesMemoryByteSize:UInt = UInt(1048576) * UInt(sizeof(Particle))
var particlesVoidPtr: COpaquePointer!
var particlesParticlePtr: UnsafeMutablePointer!

var particlesParticleBufferPtr: UnsafeMutableBufferPointer!

When I set up the particles, I populate the pointers and use posix_memalign() to allocate the memory:

    posix_memalign(&particlesMemory, alignment, particlesMemoryByteSize)

    particlesVoidPtr = COpaquePointer(particlesMemory)
    particlesParticlePtr = UnsafeMutablePointer(particlesVoidPtr)

    particlesParticleBufferPtr = UnsafeMutableBufferPointer(start: particlesParticlePtr, count: particleCount)

The loop to populate the particles is slightly different - I now loop over the buffer pointer:

    for index in particlesParticleBufferPtr.startIndex ..< particlesParticleBufferPtr.endIndex
    {
        [...]

        let particle = Particle(positionX: positionX, positionY: positionY, velocityX: velocityX, velocityY: velocityY)

        particlesParticleBufferPtr[index] = particle
    }

Inside the applyShader() function, I create a copy of the memory which is used as both the input and output buffer:

    let particlesBufferNoCopy = device.newBufferWithBytesNoCopy(particlesMemory, length: Int(particlesMemoryByteSize),
        options: nil, deallocator: nil)

    commandEncoder.setBuffer(particlesBufferNoCopy, offset: 0, atIndex: 0)

    commandEncoder.setBuffer(particlesBufferNoCopy, offset: 0, atIndex: 1)

...and after the shader has run, I put the shared memory (particlesMemory) back into the buffer pointer:

    particlesVoidPtr = COpaquePointer(particlesMemory)
    particlesParticlePtr = UnsafeMutablePointer(particlesVoidPtr)

    particlesParticleBufferPtr = UnsafeMutableBufferPointer(start: particlesParticlePtr, count: particleCount)

There's an up to date Swift 2.0 version of this at my GitHub repo here