Swift kCGImageAlphaPremultipliedLast unresolved

在使用CGBitmapContextCreate的時候，使用kCGImageAlpahPremultipliedLast，卻沒想到報錯，unresolved identifier error for ‘kCGImageAlphaPremultipliedLast’
也就是識別不了kCGImageAlphaPremultipliedLast，沒定義，該枚舉常量在OC裏面使用明明是好的啊。

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

Swift中該方法的定義最後一個參數類型是UInt32

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

在OC中，CGBitmapInfo的定義如下：

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

類似的還有CGImageAlphaInfo

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

所以我們可以在OC中直接使用kCGImageAlpahPremultipliedLast等枚舉常量。
但是在Swift中，CGBitmapInfo的定義卻有些不同，它是一個Struct。

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

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

而”alpha info” 被分開定義成了一個枚舉。

public enum CGImageAlphaInfo : UInt32 {

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

因此， 必須將enum轉換爲UInt32的值來創建CGBitmapInfo。

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

下邊一個”完整”的示例來說明兩種語言下的實現方式
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)

參考源