Access Private UIKit Function Without Using Bridging Header - ios

Question

Consider the private C function _UICreateScreenUIImage, which returns a UIImage snapshot of the current device screen:
OBJC_EXTERN UIImage *_UICreateScreenUIImage(void) NS_RETURNS_RETAINED;
I can put this in a bridging header and access it in Swift like so:
MyApp-Bridging-Header.h
#import UIKit;
UIImage *_UICreateScreenUIImage(void) NS_RETURNS_RETAINED;
MyClass.swift
let image = _UICreateScreenUIImage()
print(image) // <UIImage: 0x7fc4ba6081c0>, {375, 667}
Is there a way I can access _UICreateScreenUIImage in pure Swift without using a bridging header?
An initial thought was to create an extension on UIImage, but the extension is expecting me to declare the body of the function in the extension:
extension UIImage {
public func _UICreateScreenUIImage(_: Void) -> UIImage // "Expected '{' in body of function declaration"
}
This implementation is flawed anyways, as _UICreateScreenUIImage isn't a method on UIImage.
Is exposing and accessing this method possible in pure Swift?
People seem to be confusing my question with "How do I take a screenshot?" That's not what I'm asking. I'm asking how do I access methods like UIImage *_UICreateScreenUIImage(void); in Swift. It could be any private method, such as +(UIImage *)_deviceSpecificImageNamed:(NSString *)name inBundle:(NSBundle *)bundle; or +(UIImage *)_pu_PhotosUIImageNamed:(NSString *)name;
.

Answer 1

It's a lot easier than you would expect:
#asmname("_UICreateScreenUIImage")
func _UICreateScreenUIImage() -> UIImage
// That's it – go ahead and call it:
_UICreateScreenUIImage()
As it happens, #asmname has actually just been changed in the 2.3 builds to #_silgen_name, so be ready to adjust accordingly:
#_silgen_name("_UICreateScreenUIImage")
func _UICreateScreenUIImage() -> UIImage
To my knowledge, #_silgen_name does not provide resolution of Objective-C methods. For this, there is the evenmore powerful Objective-C runtime API:
let invokeImageNamed: (String, NSTimeInterval) -> UIImage? = {
// The Objective-C selector for the method.
let selector: Selector = "animatedImageNamed:duration:"
guard case let method = class_getClassMethod(UIImage.self, selector)
where method != nil else { fatalError("Failed to look up \(selector)") }
// Recreation of the method's implementation function.
typealias Prototype = #convention(c) (AnyClass, Selector, NSString, NSTimeInterval) -> UIImage?
let opaqueIMP = method_getImplementation(method)
let function = unsafeBitCast(opaqueIMP, Prototype.self)
// Capture the implemenation data in a closure that can be invoked at any time.
return { name, interval in function(UIImage.self, selector, name, interval) }
}()
extension UIImage {
// Convenience method for calling the closure from the class.
class func imageNamed(name: String, interval: NSTimeInterval) -> UIImage? {
return invokeImageNamed(name, interval)
}
}
UIImage.imageNamed("test", interval: 0)
As far as handling NS_RETURNS_RETAINED, this won't be generated for you. Instead, you can use a return type of Unmanaged, and wrap that in a function to your convenience:
#_silgen_name("_UICreateScreenUIImage")
func _UICreateScreenUIImage() -> Unmanaged<UIImage>
func UICreateScreenUIImage() -> UIImage {
return _UICreateScreenUIImage().takeRetainedValue()
}