swift - Trouble using callbacks with CGPattern in Swift3

Question

I'm attempting to create a colored pattern using CGPattern in Swift. Apple provides a nice Objective-C example in the Quartz 2D Programming Guide in their section on Painting Colored Patterns. But getting all of that syntax converted from Objective-C is not straight forward. Plus I'd like to make use of the info parameter in the drawing callback and there is no example of doing that.

Here's my first attempt:

class SomeShape {
    func createPattern() -> CGPattern? {
        let bounds = CGRect(x: 0, y: 0, width: someWidth, height: someHeight)
        let matrix = CGAffineTransform.identity
        var callbacks = CGPatternCallbacks(version: 0, drawPattern: nil, releaseInfo: nil)

        let res = CGPattern(info: nil, bounds: bounds, matrix: matrix, xStep: bounds.width, yStep: bounds.height, tiling: .noDistortion, isColored: true, callbacks: &callbacks)

        return res
    }
}

Clearly this needs a proper value for the drawPattern parameter to CGPatternCallbacks and I need to pass self as the info parameter to the CGPattern initializer.

What is the proper syntax to complete this?

Answer 1

As you say in your answer, CGPatternDrawPatternCallback is defined as:

typealias CGPatternDrawPatternCallback =
                               @convention(c) (UnsafeMutableRawPointer?, CGContext) -> Void

The @convention(c) attribute (which only appears to show up in the generated header) means that the function value used must be compatible with C, and therefore cannot capture any context (because C function values are nothing more than raw pointers to a function, and don't store an additional context object).

So if you want to have context available in the function, you need to pass your own UnsafeMutableRawPointer? to the info: parameter of CGPattern's initialiser. This will then be passed as the first parameter of the given draw pattern function upon being called.

In order to pass self to this parameter, you can use Unmanaged. This allows you to convert between references and opaque pointers and, unlike unsafeBitCast, also lets you control the memory management of the reference when doing so.

Given that we have no guarantee that the caller of createPattern() will keep self retained, we cannot just pass it off to the info: parameter without retaining it ourselves. If it were passed without retaining (for example with unsafeBitCast), and was then deallocated before drawing the pattern – you would get undefined behaviour when attempting to use a dangling pointer in the drawing callback.

With Unmanaged:

• You can pass a reference as a +1 retained opaque pointer with passRetained(_:).toOpaque()

• You can get back a reference from this pointer with fromOpaque(_:).takeUnretainedValue() (and the instance will stay retained)

• You can then consume the +1 retain with fromOpaque(_:).release(). You'll want to do this when the CGPattern is freed.

For example:

class SomeShape {
    // the bounds of the shape to draw
    let bounds = CGRect(x: 0, y: 0, width: 40, height: 40)

    func createPattern() -> CGPattern? {

        var callbacks = CGPatternCallbacks(version: 0, drawPattern: { info, ctx in

            // cast the opaque pointer back to a SomeShape reference.
            let shape = Unmanaged<SomeShape>.fromOpaque(info!).takeUnretainedValue()

            // The code to draw a single tile of the pattern into "ctx"...
            // (in this case, two vertical strips)
            ctx.saveGState()
            ctx.setFillColor(UIColor.red.cgColor)
            ctx.fill(CGRect(x: 0, y: 0,
                            width: shape.bounds.width / 2, height: shape.bounds.height))

            ctx.setFillColor(UIColor.blue.cgColor)
            ctx.fill(CGRect(x: 20, y: 0,
                            width: shape.bounds.width / 2, height: shape.bounds.height))
            ctx.restoreGState()

        }, releaseInfo: { info in
            // when the CGPattern is freed, release the info reference,
            // consuming the +1 retain when we originally passed it to the CGPattern.
            Unmanaged<SomeShape>.fromOpaque(info!).release()
        })

        // retain self before passing it off to the info: parameter as an opaque pointer.
        let unsafeSelf = Unmanaged.passRetained(self).toOpaque()

        return CGPattern(info: unsafeSelf, bounds: bounds, matrix: .identity,
                         xStep: bounds.width, yStep: bounds.height,
                         tiling: .noDistortion, isColored: true, callbacks: &callbacks)
    }
}

Alternatively, a better solution if you want value semantics for SomeShape, you could make it a struct. Then when creating the pattern, you can just wrap it up in a Context heap-allocated box before passing it off to the info: parameter:

struct SomeShape {

    // the bounds of the shape to draw
    let bounds = CGRect(x: 0, y: 0, width: 40, height: 40)

    func createPattern() -> CGPattern? {

        final class Context {
            let shape: SomeShape
            init(_ shape: SomeShape) { self.shape = shape }
        }

        var callbacks = CGPatternCallbacks(version: 0, drawPattern: { info, ctx in

            // cast the opaque pointer back to a Context reference,
            // and get the wrapped shape instance.
            let shape = Unmanaged<Context>.fromOpaque(info!).takeUnretainedValue().shape

            // ...

        }, releaseInfo: { info in
            // when the CGPattern is freed, release the info reference,
            // consuming the +1 retain when we originally passed it to the CGPattern.
            Unmanaged<Context>.fromOpaque(info!).release()
        })

        // wrap self in our Context box before passing it off to the info: parameter as a
        // +1 retained opaque pointer.
        let unsafeSelf = Unmanaged.passRetained(Context(self)).toOpaque()

        return CGPattern(info: unsafeSelf, bounds: bounds, matrix: .identity,
                         xStep: bounds.width, yStep: bounds.height,
                         tiling: .noDistortion, isColored: true, callbacks: &callbacks)
    }
}

This now also takes care of any retain cycle concerns.