swift - 如何在 Swift 中将 CGImage 保存到数据？

Question

此代码类型检查并编译但随后崩溃。如何将 CGImage 保存到 Data 以便稍后再次读取它。

let cgi: CGImage? = ...        
var mData = Data()
let imageDest = CGImageDestinationCreateWithData(mData as! CFMutableData, 
                                                 kUTTypePNG, 1, nil)!
CGImageDestinationAddImage(imageDest, cgi!, nil)
CGImageDestinationFinalize(imageDest)

最后一行崩溃了。控制台中的错误是:

2018-01-17 19:25:43.656664-0500 HelloPencil[2799:3101092] -[_NSZeroData 
  appendBytes:length:]: unrecognized selector sent to instance 0x1c80029c0
2018-01-17 19:25:43.658420-0500 HelloPencil[2799:3101092] *** Terminating app 
  due to uncaught exception 'NSInvalidArgumentException', reason: 
  '-[_NSZeroData appendBytes:length:]: unrecognized selector 
  sent to instance 0x1c80029c0'

从 Data 到 CFMutableData 的转换是 Xcode 推荐的，但它可能是错误的。

Answer 1

问题在于您创建可变数据的方式。 Data 不可转换为 NSMutableData。只需将强制转换从 Data 更改为 CFMutableData 到 CFDataCreateMutable(nil, 0)。另一种选择是使用 NSMutableData，它是免费桥接到 CFMutableData 的。像这样尝试:

if let cgi = cgi, 
    let mutableData = CFDataCreateMutable(nil, 0),
    let destination = CGImageDestinationCreateWithData(mutableData, "public.png" as CFString, 1, nil) {
    CGImageDestinationAddImage(destination, cgi, nil)
    if CGImageDestinationFinalize(destination) {
        let data = mutableData as Data
        if let image = UIImage(data: data) {
            print(image.size)
        }
    } else {
        print("Error writing Image")
    }
}

编辑/更新:Xcode 11 • Swift 5.1

extension CGImage {
    var png: Data? {
        guard let mutableData = CFDataCreateMutable(nil, 0),
            let destination = CGImageDestinationCreateWithData(mutableData, "public.png" as CFString, 1, nil) else { return nil }
        CGImageDestinationAddImage(destination, self, nil)
        guard CGImageDestinationFinalize(destination) else { return nil }
        return mutableData as Data
    }
}