Swift kCGImageAlphaPremultipliedLast unresolved

在使用CGBitmapContextCreate的时候，使用kCGImageAlpahPremultipliedLast，却没想到报错，unresolved identifier error for ‘kCGImageAlphaPremultipliedLast’
也就是识别不了kCGImageAlphaPremultipliedLast，没定义，该枚举常量在OC里面使用明明是好的啊。

CGBitmapContextCreate(&pixelData, width: width, height: height, bitsPerComponent: 8, bytesPerRow: width*4, imageCS, bitmapInfo: kCGImageAlphaPremultipliedLast)

Swift中该方法的定义最后一个参数类型是UInt32

CGBitmapContextCreate(data: UnsafeMutablePointer<Void>, _ width: Int, _ height: Int, _ bitsPerComponent: Int, _ bytesPerRow: Int, _ space: CGColorSpace?, _ bitmapInfo: UInt32)

在OC中，CGBitmapInfo的定义如下：

typedef CF_OPTIONS(uint32_t, CGBitmapInfo) {
  kCGBitmapAlphaInfoMask = 0x1F,
  kCGBitmapFloatComponents = (1 << 8),
  ...
} CF_ENUM_AVAILABLE(10_4, 2_0);

类似的还有CGImageAlphaInfo

typedef CF_ENUM(uint32_t, CGImageAlphaInfo) {
  kCGImageAlphaNone,               /* For ex, RGB. */
  kCGImageAlphaPremultipliedLast,  /* For ex, premultiplied RGBA */
  kCGImageAlphaPremultipliedFirst, /* For ex, premultiplied ARGB */
  ...
};

所以我们可以在OC中直接使用kCGImageAlpahPremultipliedLast等枚举常量。
但是在Swift中，CGBitmapInfo的定义却有些不同，它是一个Struct。

public struct CGBitmapInfo : OptionSetType {
    public init(rawValue: UInt32)

    public static var AlphaInfoMask: CGBitmapInfo { get }
    public static var FloatComponents: CGBitmapInfo { get }
    ...
}

而”alpha info” 被分开定义成了一个枚举。

public enum CGImageAlphaInfo : UInt32 {

    case None /* For example, RGB. */
    case PremultipliedLast /* For ex, premultiplied RGBA */
    case PremultipliedFirst /* For ex, premultiplied ARGB */
    ...
}

因此， 必须将enum转换为UInt32的值来创建CGBitmapInfo。

let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.PremultipliedLast.rawValue)
let context = CGBitmapContextCreate(..., bitmapInfo)

下边一个”完整”的示例来说明两种语言下的实现方式
OC:

CGContextRef newContext = CGBitmapContextCreate(baseAddress,                                                  width, height, 8, bytesPerRow, colorSpace,                                                  kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst);

Swift:

let bmpInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.PremultipliedFirst.rawValue|CGBitmapInfo.ByteOrder32Little.rawValue)
let newContext = CGBitmapContextCreate(baseAddress, width, height, 8, bytesPerRow, colorSpace, bmpInfo.rawValue)

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