Particle system that's both calculated and rendered on the GPU using the Metal framework

This is the most highly optimised version of my Swift and Metal particles system; managing over 40 fps with four million particles and four gravity wells. It manages this by rendering to a CAMetalLayer rather than converting a texture to a UIImage and by passing in four particle definitions per step with a float4x4 rather than a particle struct.

You can read about these recent changes at my blog:

• CAMetalLayer work: http://flexmonkey.blogspot.co.uk/2015/03/swift-metal-four-million-particles-on.html
• Use of float4x4: http://flexmonkey.blogspot.co.uk/2015/03/mind-blowing-metal-four-million.html

This branch wraps up all the Metal code into one class so that it's easily implemented in other projects. To create a new particle system object, instantiate an instance of ParticleLab

let particleLab = ParticleLab()

...and when ready, add it as a sublayer to your view:

view.layer.addSublayer(particleLab)

The class has four gravity wells with propeties such as position, mass and spin. These are set with the setGravityWellProperties method:

particleLab.setGravityWellProperties(gravityWell: .One, normalisedPositionX: 0.3, normalisedPositionY: 0.3, mass: 11, spin: -4)
        
particleLab.setGravityWellProperties(gravityWell: .Two, normalisedPositionX: 0.7, normalisedPositionY: 0.3, mass: 7, spin: 3)
        
particleLab.setGravityWellProperties(gravityWell: .Three, normalisedPositionX: 0.3, normalisedPositionY: 0.7, mass: 7, spin: 3)
        
particleLab.setGravityWellProperties(gravityWell: .Four, normalisedPositionX: 0.7, normalisedPositionY: 0.7, mass: 11, spin: -4)

Gravity well positions can be displayed by setting the showGravityWellPositions property to true.

Classes can implement ParticleLabDelegate interface which includes particleLabDidUpdate. This method is invoked with each particle step and can be used, for example, for updating the position of gravity wells.