如何在 Swift 中将 CGImage 保存到数据？

Question

This code type checks and compiles but then crashes. How do I save a CGImage to Data so that I can read it in again later.

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

The last line crashes. Error in console is:

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'

That cast from Data to CFMutableData was recommended by Xcode, but maybe it's wrong.

Answer 1

The problem is the way you are creating your mutable data. Just change your forced casting from Data to CFMutableData to CFDataCreateMutable(nil, 0)

Try like this:

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")
    }
}

edit/update: 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
    }
}