Misleading Debug Message Regarding Swift Closure

While Apple announced Swift 2.0, it’s also going to make it open source which is a really good news. However we’re still going to build products with the tools we currently have. In Swift 1.2 with Xcode 6.3.2, you may find debugging with closure is weird sometimes. For example, when I was trying to add MHVideoPhotoGallery into my project, I got some problem with the following code:

gallery.finishedCallback = {
	(index:Int, image:UIImage!, transition:MHTransitionDismissMHGallery!, mode:MHGalleryViewMode) -> Void in
	gallery.dismissViewControllerAnimated(true, dismissImageView:self.image_complaint.image, completion:nil)
}

The error message cannot assign a value of type '(Int, UIImage!, MHTransitionDismissMHGallery!, MHGalleryViewMode) -> Void' to a value of type '((Int, UIImage!, MHTransitionDismissMHGallery!, MHGalleryViewMode) -> Void)!' doesn’t help a lot and it appears in a misleading way, which makes it looks like it’s saying that there’s some problem with the closure parameters. But actually the real meaning is that there's some problem in the closure so that we cannot make it a proper non-nullable closure as it should be, and obviously, it doesn’t help at all. Instead the problem in the closure should be addressed. If you change the code a little bit like this:

let block = {
	(index:Int, image:UIImage!, transition:MHTransitionDismissMHGallery!, mode:MHGalleryViewMode) -> Void in
	gallery.dismissViewControllerAnimated(true, dismissImageView:self.image_complaint.image, completion:nil)
}
gallery.finishedCallback = block

Now we got the real problem: 'UIImage' is not convertible to 'UIImageView', and yes, I wrongly typed image_complaint.image instead of image_complaint. After I fixed the typo it looked like:

let block = {
	(index:Int, image:UIImage!, transition:MHTransitionDismissMHGallery!, mode:MHGalleryViewMode) -> Void in
	gallery.dismissViewControllerAnimated(true, dismissImageView:self.image_complaint, completion:nil)
}
gallery.finishedCallback = block

Which works in the same way as the following version, as it should do:

gallery.finishedCallback = {
	(index, image, transition, mode) -> Void in
	gallery.dismissViewControllerAnimated(true, dismissImageView:self.image_complaint, completion:nil)
}

I’m pretty sure Apple will address this issue in a future release, but for now if you’re debuggin with closure, you can use the work-around described above to find the problem more easily.

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