[SR-2388] Forming a CFDictionary from a Swift Dictionary is now really difficult

[mattneub]

Previous ID	SR-2388
Radar	rdar://problem/32003073
Original Reporter	@mattneub
Type	Bug
Status	Resolved
Resolution	Done

Attachment: Download

Environment

Xcode Version 8.0 beta 6 (8S201h)

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	@belkadan
Priority	Medium

md5: 0803c238279e54d409d7b137b9e8c7d0

Issue Description:

Before seed 6, I used to be able to say this:

        let d : [NSObject:AnyObject] = [
            kCGImageSourceShouldAllowFloat : true,
            kCGImageSourceCreateThumbnailWithTransform : true,
            kCGImageSourceCreateThumbnailFromImageAlways : true,
            kCGImageSourceThumbnailMaxPixelSize : w
        ]
        let imref = CGImageSourceCreateThumbnailAtIndex(src, 0, d)!

Now I have to take everything across the bridge by hand. All the CFString keys have to be cast as String or NSString. We don't get any automatic bridge-crossing, so we have to tell Swift explicitly what the value types are, and then we have to cast the resulting dictionary to CFDictionary as well. Even if we get past the compiler, it is still possible to do this wrong and thus end up with an invalid CFDictionary so that at runtime CGImageSourceCreateThumbnailAtIndex yields nil. The only way I've found to get things right is like this:

        let d = [
            kCGImageSourceShouldAllowFloat as String : true as NSNumber,
            kCGImageSourceCreateThumbnailWithTransform as String : true as NSNumber,
            kCGImageSourceCreateThumbnailFromImageAlways as String : true as NSNumber,
            kCGImageSourceThumbnailMaxPixelSize as String : w as NSNumber
        ]
        let imref = CGImageSourceCreateThumbnailAtIndex(src, 0, d as CFDictionary)!

This is absolutely horrible.

The situation is compounded by the fact that no one seems to have told NSDictionary that its Swift equivalent is now [AnyHashable : Any]. We see that in the APIs and it works, but when you try to cast a CFDictionary to it, the compiler complains:

        let result = CGImageSourceCopyPropertiesAtIndex(src, 0, nil)! as [AnyHashable:Any]
        // compile error

In my opinion, Swift now goes too far with its refusal to help things get across the bridges, the Swift to Objective-C bridge and the CFTypeRef toll-free bridge. (In fact, there now seems to be rather a heavy toll on that bridge. 🙂)

[jckarter]

To work around the second problm, you can do nsDict as [NSObject: AnyObject] as [AnyHashable: Any], which is silly but should satisfy the compiler for now.

[belkadan]

I think cfDict as NSDictionary as [AnyHashable: Any] also works.

[jckarter]

Not yet, unfortunately, we still only allow explicit as conversions from NS classes to the "old" value type mappings [AnyObject]/[NSObject: AnyObject]/etc.

[jckarter]

You can bridge `CFString` to `NSString` using `as NSString`. If you must make `CFString` `Hashable`, you can add this extension as a workaround:

extension CFString: Hashable {
  public static func ==(a: CFString, b: CFString) -> Bool {
    return CFEqual(a, b)
  }
}

[mattneub]

@jckarter That compiles (thank you!) but it doesn't actually work. Without the explicit as Number we fail to get a valid CFDictionary and we crash when we force-unwrap.

The reason is that merely saying true or w does not actually get us across the bridge to AnyObject — and typing the dictionary as [CFString:AnyObject] doesn't help, because we don't get automatic bridge-crossing that way any more either.

[jckarter]

@mattneub What type is w? Bool, Int, and UInt should bridge to NSNumber when you convert to NSDictionary or CFDictionary, but if it's some other number type it may be getting boxed.

[jckarter]

For example, this prints `true` for me:

let w = 1024

let d1 : [CFString: Any] = [
  kCGImageSourceShouldAllowFloat : true,
  kCGImageSourceCreateThumbnailWithTransform : true,
  kCGImageSourceCreateThumbnailFromImageAlways : true,
  kCGImageSourceThumbnailMaxPixelSize : w
]
let d2 : [CFString: AnyObject] = [
  kCGImageSourceShouldAllowFloat : true as NSNumber,
  kCGImageSourceCreateThumbnailWithTransform : true as NSNumber,
  kCGImageSourceCreateThumbnailFromImageAlways : true as NSNumber,
  kCGImageSourceThumbnailMaxPixelSize : w as NSNumber
]
print((d1 as NSDictionary).isEqual(to: d2 as NSDictionary))

[mattneub]

@jckarter It comes out as an NSDictionary but, as I said, it doesn't make it as a valid CFDictionary that ImageIO can understand. That is the bridge we are not crossing.

Here's the full code (along with your CFString extension):

        let url = Bundle.main.url(forResource:"colson", withExtension: "jpg")!
        let src = CGImageSourceCreateWithURL(url as CFURL, nil)!
        let scale = UIScreen.main.scale
        let w = self.iv.bounds.width * scale
        // have to cross over to Objective-C manually
        let d : [CFString:Any] = [
            kCGImageSourceShouldAllowFloat : true as NSNumber,
            kCGImageSourceCreateThumbnailWithTransform : true as NSNumber,
            kCGImageSourceCreateThumbnailFromImageAlways : true as NSNumber,
            kCGImageSourceThumbnailMaxPixelSize : w as NSNumber
        ]
        let imref = CGImageSourceCreateThumbnailAtIndex(src, 0, d as CFDictionary)!

I've attached my test project; launch, then tap the second button to view the image in the interface. I crash at that moment if all four as NSNumber casts have been removed. bk2ch23p829imageIO.zip

[jckarter]

@belkadan, is there a chance the crash in CF is related to #4366 ? Does CFNumber not toll-free-bridge with NSNumber subclasses?

[belkadan]

It does, but CFBoolean is a distinct type from CFNumber at the CF layer, so it very well could be related.

[jckarter]

Ah, that could definitely be it then.

[jckarter]

@mattneub If you do true as Bool as NSNumber, do you see the same crash as if you do no coercions at all? Do you crash if you pass w without as NSNumber?

[swift-ci]

Comment by Wil Shipley (JIRA)

When I used this form (this is what the auto-updating stuff in Xcode ended up spitting out) it compiled but I do NOT get an actual image — the load fails.

            let thumbnailSideLength = 512
            let optionsDictionary = [
                kCGImageSourceCreateThumbnailFromImageIfAbsent as NSString: true,
                kCGImageSourceThumbnailMaxPixelSize as NSString: thumbnailSideLength,
            ] as CFDictionary
            guard let thumbnail = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, optionsDictionary) else {
                    print("\(#function) failed to load thumbnail for \(name)/\(maker)/\(productLine)")
                    throw PlatonicPieceOfFurnitureError.cannotCreateImageFromThumbnailImageSource
            }

When I change my dictionary to this it succeeds:

            let optionsDictionary = [
                kCGImageSourceCreateThumbnailFromImageIfAbsent as NSString: true as NSNumber,
                kCGImageSourceThumbnailMaxPixelSize as NSString: thumbnailSideLength as NSNumber,
            ] as CFDictionary

[belkadan]

Confirmed that with #4366 this succeeds without the as NSNumber. I'm using this bug to track making all CF types Hashable by default.

[belkadan]

#4394