CGContext.init() -- 不再允许 NULL 颜色空间

TL;DR：在旧版 Obj-C 代码中，颜色空间参数值为NULL。这在 Swift 等价物中是不允许的。使用什么价值？

我继承的代码如下：

unsigned char pixel[1] = {0};
CGContextRef context = CGBitmapContextCreate(
    pixel,1, 1, 8, 1, NULL, (CGBitmapInfo)kCGImageAlphaOnly
);

Swift 4 的移植CGContext很简单，除了NULL色彩空间值。使用合理的值，我得到nil从...回来CGContext.init?()。我的翻译是：

var pixelValue = UInt8(0)
var pixel = Data(buffer: UnsafeBufferPointer(start:&pixelValue, count:1))
let context = CGContext(
    data            : &pixel,
    width           : 1,
    height          : 1,
    bitsPerComponent: 8,
    bytesPerRow     : 1,
    space           : CGColorSpace(name:CGColorSpace.genericRGBLinear)!,
    bitmapInfo      : CGImageAlphaInfo.alphaOnly.rawValue
)! // Returns nil; unwrapping crashes

Q: 合适的值是多少space？ （我提供的值没有返回nil;这是CGContext()调用自己。

设置环境变量CGBITMAP_CONTEXT_LOG_ERRORS产生如下错误日志：

Assertion failed: (0), function get_color_model_name, 
file /BuildRoot/Library/Caches/com.apple.xbs/Sources/Quartz2D_Sim/
Quartz2D-1129.2.1/CoreGraphics/API/CGBitmapContextInfo.c, line 210.

对于更多背景故事，上下文用于查找 a 中单个像素的 alpha 值UIImage通过以下方式：

unsigned char pixel[1] = {0};
CGContextRef context = CGBitmapContextCreate(pixel,1, 1, 8, 1, NULL, (CGBitmapInfo)kCGImageAlphaOnly);
UIGraphicsPushContext(context);
[image drawAtPoint:CGPointMake(-point.x, -point.y)];
UIGraphicsPopContext();
CGContextRelease(context);
CGFloat alpha = pixel[0]/255.0;

（我确实有可能的替代方案来寻找 alpha，但为了不影响遗留代码，我想保持这种方式。）

我最近正在研究类似的主题，也许这个代码示例会对某人有所帮助：

let image = UIImage(named: "2.png")
guard let cgImage = image?.cgImage else {
    fatalError()
}

let width = cgImage.width
let height = cgImage.height
//CGColorSpaceCreateDeviceGray - 1 component, 8 bits
//i.e. 1px = 1byte
let bytesPerRow = width
let bitmapByteCount = width * height

let bitmapData: UnsafeMutablePointer<UInt8> = .allocate(capacity: bitmapByteCount)
defer {
    bitmapData.deallocate()
}
bitmapData.initialize(repeating: 0, count: bitmapByteCount)

guard let context = CGContext(data: bitmapData, width: width, height: height,
                              bitsPerComponent: 8, bytesPerRow: bytesPerRow,
                              space: CGColorSpaceCreateDeviceGray(), bitmapInfo: CGImageAlphaInfo.alphaOnly.rawValue) else {
                                fatalError()
}
//draw image to context
var rect = CGRect(x: 0, y: 0, width: width, height: height)
context.draw(cgImage, in: rect)

// Enumerate through all pixels
for row in 0..<height {
    for col in 0..<width {
        let alphaValue = bitmapData[row * width + col]
        if alphaValue != 0 {
            //visible pixel
        }
    }
}