ios - 什么是 UnsafeMutablePointer？如何修改底层内存？

Question

我正在尝试使用 SpriteKit 的 SKMutableTexture 类，但我不知道如何使用 UnsafeMutablePointer< Void > .我有一个模糊的想法，它是指向内存中一连串字节数据的指针。但是我该如何更新呢？这在代码中实际上是什么样的？

编辑

这是一个可以使用的基本代码示例。我如何让它做一些像在屏幕上创建一个红色方 block 这样简单的事情？

    let tex = SKMutableTexture(size: CGSize(width: 10, height: 10))
    tex.modifyPixelDataWithBlock { (ptr:UnsafeMutablePointer<Void>, n:UInt) -> Void in

        /* ??? */

    }

Answer 1

来自 SKMutableTexture.modifyPixelDataWithBlock 的文档:

The texture bytes are assumed to be stored as tightly packed 32 bpp, 8bpc (unsigned integer) RGBA pixel data. The color components you provide should have already been multiplied by the alpha value.

因此，当您获得 void* 时，底层数据采用 4x8 位流的形式。

您可以像这样操纵这样的结构:

// struct of 4 bytes
struct RGBA {
    var r: UInt8
    var g: UInt8
    var b: UInt8
    var a: UInt8
}

let tex = SKMutableTexture(size: CGSize(width: 10, height: 10))
tex.modifyPixelDataWithBlock { voidptr, len in
    // convert the void pointer into a pointer to your struct
    let rgbaptr = UnsafeMutablePointer<RGBA>(voidptr)

    // next, create a collection-like structure from that pointer
    // (this second part isn’t necessary but can be nicer to work with)
    // note the length you supply to create the buffer is the number of 
    // RGBA structs, so you need to convert the supplied length accordingly...
    let pixels = UnsafeMutableBufferPointer(start: rgbaptr, count: Int(len / sizeof(RGBA))

    // now, you can manipulate the pixels buffer like any other mutable collection type
    for i in indices(pixels) {
        pixels[i].r = 0x00
        pixels[i].g = 0xff
        pixels[i].b = 0x00
        pixels[i].a = 0x20
    }
}