Environment: macOS 15.2 (24C101) with Xcode 16.2 (16C5032a) Goal: I am trying to build a simple IMKInputController-based input method. Problem: My .app bundle registers successfully and I can select it as an input source. When selected, it blocks keyboard input, but my handle method does not seem to execute or produce output. I have placed NSLog statements in my controller's init and handle methods. Code for the controller: import InputMethodKit // The IMKTextInput protocol is provided by the framework. // We don't need to define our own bridging protocol for this test. public class HelloWorldController: IMKInputController { public override init!(server: IMKServer!, delegate: Any!, client inputClient: Any!) { super.init(server: server, delegate: delegate, client: inputClient) NSLog("HelloWorldIME: Controller has been initialized.") } public override func handle(_ event: NSEvent!, client sender: Any!) -> Bool { NSLog("HelloWorldIME: handle() method was called.") // ================== FINAL FIX APPLIED HERE ================== // 1. First, we ensure the client is a fundamental Objective-C object. guard let clientObject = sender as? NSObject else { NSLog("HelloWorldIME: Error - client object is not an NSObject.") return false } NSLog("HelloWorldIME: Successfully cast client to NSObject.") // 2. Now that we have an NSObject, we can safely check if it responds to the selector. let selector = #selector(IMKTextInput.insertText(_:replacementRange:)) if !clientObject.responds(to: selector) { NSLog("HelloWorldIME: Error - client object does not respond to the insertText selector.") return false } NSLog("HelloWorldIME: Client responds to insertText. Preparing to insert text.") // 3. Since we've confirmed it responds, we can now safely treat it as an IMKTextInput // and call the method. let client = clientObject as! IMKTextInput let stringToInsert = "A" let replacementRange = NSRange(location: NSNotFound, length: 0) client.insertText(stringToInsert, replacementRange: replacementRange) NSLog("HelloWorldIME: Called insertText with string '\(stringToInsert)'. Action complete.") // ======================================================== return true } }