Swift convenience initializer extension for SKPhysicsBody

Question

extension SKPhysicsBody {

    /// anchorPoint version of init(rectangleOfSize:center:)
    convenience init(rectangleOfSize s: CGSize, withAnchorPoint anchorPoint: CGPoint) {
        var center = CGPoint()
        center.x = (s.width / 2) - ( s.width * anchorPoint.x)
        center.y = (s.height / 2 ) - ( s.height * anchorPoint.y)
        self.init(rectangleOfSize: s, center: center)
    }

}

I got this error on runtime

-[PKPhysicsBody initWithRectangleOfSize:withAnchorPoint:]: unrecognized selector sent to instance 0x7f9b03c4fff0

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[PKPhysicsBody initWithRectangleOfSize:withAnchorPoint:]: unrecognized selector sent to instance 0x7f9b03c4fff0'

This is how I call in code

// redBox is a typical SKSpriteNode()
redBox.physicsBody = SKPhysicsBody(rectangleOfSize: redBox.frame.size, withAnchorPoint: redBox.anchorPoint)

I basically want to extend the SKPhysicsBody class to provide a convenience initializer to its factory method

`redBox.physicsBody = SKPhysicsBody(rectangleOfSize: redBox.frame.size, withAnchorPoint: redBox.anchorPoint)` — Kent Liau, Jan 20 '16 at 08:25
my current workaround is using factory method `SKPhysicsBody.bodyWithRectangleOfSize(:withAnchorPoint:)` to return a physics body. This totally feel not swift-ty, as all obj-c factory is map nicely to respective init method. — Kent Liau, Jan 20 '16 at 10:06
Seems that SKPhysicsBody is a cluster class and it's `alloc` method returns a private `PKPhysicsBody` instance that doesn't have the `init` just added. — Cristik, Jan 20 '16 at 10:13
I read some other SpriteKit project with SKPhysicsBody extension to conveniently create physic body, they mostly use a class method create a body and return it, is this the only neat way? — Kent Liau, Jan 20 '16 at 10:28

rickster · Accepted Answer · 2017-11-17T01:43:12.110

As @Cristik guessed in the comments, this is the same root problem as this question and this question: the public SKPhysicsBody class is a convenience interface for the private PKPhysicsBody class that provides its implementation.

In the past, this approach relied heavily on the "duck typing" behavior of Objective-C — as long as ClassA responds to all the same selectors as ClassB, you can call any of those selectors on a pointer whose static type (the type declared to the compiler in source code) is ClassB, even if the actual object at run time is really an instance of ClassA.

Swift is stricter about runtime type correctness than ObjC, so "duck typing" alone is not enough. Since iOS 9 / OS X 10.11, SpriteKit has some workarounds that allow PKPhysicsBody instances to better pretend to be SKPhysicsBody instances.

But those don't cover all cases — in particular, it doesn't catch (ObjC) [SKPhysicsBody alloc] returning a PKPhysicsBody instance, which means any attempt to add initializers to SKPhysicsBody in Swift will fail. (Because the ObjC alloc/init process is reduced to one initializer call in Swift.)

I'd consider this a bug and recommend filing it with Apple.

Edit/update: And until that bug gets fixed (it's been a year and some now), the workaround is to make your convenience "initializer" into a class method instead. (Or a global function if you must, but... ewww.)

Swift convenience initializer extension for SKPhysicsBody

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