Accessing Objective-C Pointers in Swift

Question

I have this Objective-C Code fragment, which I want to express in Swift

CFArrayRef windowList;
AXUIElementCopyAttributeValue(appRef, kAXWindowsAttribute, (CFTypeRef *)&windowList);

if ((!windowList) || CFArrayGetCount(windowList)<1)
        continue;

AXUIElementRef windowRef = (AXUIElementRef) CFArrayGetValueAtIndex( windowList, 0);
CFTypeRef role;
AXUIElementCopyAttributeValue(windowRef, kAXRoleAttribute, (CFTypeRef *)&role);

The first thing I´m not sure about: Who allocates the memory behind the windowListPointer. I tried with this fragment:

var windowListPointer : UnsafeMutablePointer<Optional<AnyObject>>
AXUIElementCopyAttributeValue(appRef, kAXWindowsAttribute as CFString, windowListPointer );

But that does not even compile: It complains, the windowListPointer is not initialised. What Object I could create, to let the WindowListPointer point to?

@MartinR TNT, As in the example var windowList : CFArray? AXUIElementCopyAttributeValue(appRef, kAXWindowsAttribute as CFString, &windowList ) does not compile too: Cannot pass immutable value of type 'CFTypeRef?' (aka 'Optional<AnyObject>') as inout — mica
– mica, Commented Nov 25, 2017 at 15:10

Martin R · Accepted Answer · 2017-11-25 15:55:59Z

3

If you pass an UnsafeMutablePointer<Optional<AnyObject>> as the last argument to AXUIElementCopyAttributeValue() then you must initialize it by allocating (and ultimately releasing) memory:

var resultPtr: UnsafeMutablePointer<Optional<AnyObject>> = UnsafeMutablePointer.allocate(capacity: 1)
resultPtr.initialize(to: nil)

let result = AXUIElementCopyAttributeValue(appRef, kAXWindowsAttribute as CFString, resultPtr)
// ...

resultPtr.deinitialize()
resultPtr.deallocate(capacity: 1)

It is easier to pass the address of an Optional<AnyObject> variable with &. Then conditionally cast the received object to the expected type, in this case an array of AXUIElement:

var value: AnyObject?
let result = AXUIElementCopyAttributeValue(appRef, kAXWindowsAttribute as CFString, &value)
if result == .success, let windowList = value as? [AXUIElement] {
    // use `windowList`
}

and similarly:

if let window = windowList.first {
    var value: AnyObject?
    let result = AXUIElementCopyAttributeValue(window, kAXRoleAttribute as CFString, &value)
    if result == .success, let role = value as? String {
        // use `role` ...
    }
}

One could define a generic utility function which encapsulates all the casting:

func axUICopyAttributeValue<T>(of element: AXUIElement, attribute: String, as type: T.Type) -> T? {
    var value: AnyObject?
    let result = AXUIElementCopyAttributeValue(element, attribute as CFString, &value)
    if result == .success, let typedValue = value as? T {
        return typedValue
    }
    return nil
}

Example usage:

if let windowList = axUICopyAttributeValue(of: appRef, attribute: kAXWindowsAttribute, as:[AXUIElement].self) {

    for window in windowList {
        if let role = axUICopyAttributeValue(of: window, attribute: kAXRoleAttribute, as: String.self) {

            // ...
        }
    }
}

edited Nov 25, 2017 at 15:55

answered Nov 25, 2017 at 15:23

Martin R

541k98 gold badges1.3k silver badges1.4k bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

mica Over a year ago

Thanks! Two Questions, 1) where can I find, that I have to cast to [AXUIElement] 2) var value: AnyObject? Does that declaration allocate any memory for an Object or just a reference to AnyObject (or nil)

Martin R Over a year ago

1) developer.apple.com/documentation/applicationservices/… states that the values is "an array of accessibility objects". – 2) Optional variables are implicitly initialized to nil. No memory is allocated there.

mica Over a year ago

Compiles fine now, but returns AXError . Got the other parameters as in this post: stackoverflow.com/questions/47480873/…

mica Over a year ago

The row value of the AXError is kAXErrorCannotComplete = -25204, Doku says: A fundamental error has occurred, such as a failure to allocate memory during processing. Could it be possible, that I have to allocate anything for var value: AnyObject??

Martin R Over a year ago

Sorry, I have no idea. I tested it with a reference to the current application let appRef = AXUIElementCreateApplication(getpid()), and that worked.

|

Josh Homann · Accepted Answer · 2017-11-25 15:01:00Z

1

CFArray is the Foundation C version of NSArray (since C doesn't understand Objective C NSObjects). Swift papers over both NSArray and CFArray for you so you don't need to use a pointer; you should just be able to cast it to a Swift array of the appropriate type with as?

answered Nov 25, 2017 at 15:01

Josh Homann

16.4k3 gold badges33 silver badges34 bronze badges

1 Comment

mica Over a year ago

What Swift Type would you use for windowList in `AXUIElementCopyAttributeValue(appRef, kAXWindowsAttribute as CFString, windowList )'

Collectives™ on Stack Overflow

Accessing Objective-C Pointers in Swift

2 Answers 2

6 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related