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A bare-bones implementation for iOS focus engine support, to enable basic full keyboard access (FKA) (except for scrolling which will be implemented in a different patch). Partially f1xes https://github.com/flutter/flutter/issues/76497 https://github.com/user-attachments/assets/427db87e-cc15-483a-85a1-56bf1c02c285 On iOS 15 and above, FKA, if enabled, always consumes relevant key events, so the Flutter framework can't see those key events as they won't be delivered via the `UIResponder` chain (https://developer.apple.com/documentation/uikit/uikeycommand/3780513-wantspriorityoversystembehavior). This patch provides the basic focus-related information to the iOS focus engine, derived from the information already available in the accessibility tree, so the iOS focus engine can navigate the UI hierarchy and invoke `accessibilityActivate` on the current focus when the user presses the space key. This at the moment seems to be the best option: - There doesn't seem to be a way to reliably prevent FKA from consuming the key events and that seems to be by design. - The user can remap the FKA keys in iOS system settings, but that key mapping isn't available to apps, so even if the framework can get the key events it won't be able to honor custom key maps. - When FKA is on, `-[FlutterView isAccessibilityElement]` is called without user interaction (presumably it's called when the view appears), so when the user interacts with the app using FKA, it's likely that the accessibility is already enabled, we don't have to worry detecting whether FKA is on (at least for now). Scrolling using FKA currently does not work despite `FlutterScrollableSemanticsObject` conforms to `UIFocusItemScrollableContainer`. `setContentOffset:` must be implemented using a new API that informs the framework of the new contentOffset in the scroll view. `accessibilityScroll` does not work because it scrolls too much in most cases, and it tells the framework "how much to scroll" instead of "where to scroll to" so it tends to be jumpy. ## What happens on iOS versions earlier than 15 I couldn't find iOS 14 runtime for simulators in xcode 16 so I couldn't test it. But since the key events will be delivered to the framework first regardless of whether FKA is enabled, the framework is supposed to handle keyboard focus navigation even when FKA is on. [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style