Swift: Casting int tuples to custom type containing float vectors

问题

Both original answers to this questions are satisfactory but come to the solution in slightly different ways. I opted for the one that I found simpler to implement

I'm attempting to translate some ObjectiveC, from this apple metal doc/example, and metal code into swift but having some trouble with this bit:

here is the typedef I'm using, which is necessary so that the metal shaders can computer my vertex data (the float vectors from simd.h are significant):

#include <simd/simd.h>

typedef struct
{
  vector_float2 position;
  vector_float4 color;
} AAPLVertex;

In the objective C you can just do this to cast some data to this type:

static const AAPLVertex triangleVertices[] =
{
    // 2D positions,    RGBA colors
    { {  250,  -250 }, { 1, 1, 1, 1 } },
    { { -250,  -250 }, { 1, 1, 0, 1 } },
    { {    0,   250 }, { 0, 1, 1, 1 } },
};

But how do you do this in Swift? I've attempted this:

  private let triangleVertices = [
    ( (250,  -250), (1, 0, 1, 1) ),
    ( (-250,  -250), (1, 1, 0, 1) ),
    ( (0,  250), (0, 1, 1, 1) )
  ] as? [AAPLVertex]

but xcode tells me:

Cast from '[((Int, Int), (Int, Int, Int, Int))]' to unrelated type '[AAPLVertex]' always fails

and my app crashes on load.

回答1:

Try like this:

import simd

struct AAPLVertex {
    let position: float2
    let color: float4
}

let  triangleVertices: [AAPLVertex] = [
     // 2D positions,    RGBA colors
    .init(position: .init( 250, -250), color: .init(1, 0, 1, 1)),
    .init(position: .init(-250, -250), color: .init(1, 1, 0, 1)),
    .init(position: .init(   0, -250), color: .init(0, 1, 1, 1))]

回答2:

This is how I would implement it:

import simd

struct Vertex {
    var position: SIMD2<Float>
    var color: SIMD4<Float>
}

extension Vertex {
    init(x: Float, y: Float, r: Float, g: Float, b: Float, a: Float = 1.0) {
        self.init(position: SIMD2(x, y), color: SIMD4(r, g, b, a))
    }
}

let triangleVertices = [
    Vertex(x: +250, y: -250, r: 1, g: 0, b: 1),
    Vertex(x: -250, y: -250, r: 1, g: 1, b: 0),
    Vertex(x:    0, y: -250, r: 0, g: 1, b: 1),
]

However, I'm not sure to what extent the Swift native SIMD types are compatible with Metal, as compared to the Objective C ones, though I suspect they're interoperable.

来源：https://stackoverflow.com/questions/56781233/swift-casting-int-tuples-to-custom-type-containing-float-vectors

标签

swift

metal