## Microsoft Active Accessibility

Microsoft Active Accessibility (MSAA) is an older interface used for accessibility. It can interact with screen readers such as Windows Narrator or NVDA, and has a more limited set of available functionality compared to newer interfaces.
The base of MSAA is the `IAccessible` interface, which all a11y-providing classes extend. MSAA methods are prefixed with `acc_`. In the Flutter engine, `AXPlatformNodeWin` extends `IAccessible`. Any `IAccessible` objects used are COM interfaces, and should be initialized and reference counted accordingly.
When a program such a screen reader queries an application for an a11y object, it sends a `WM_GETOBJECT` window message, to which the window will respond with the result of passing the `IAccessible` of its root to the function `LresultFromObject`.

As switching from MSAA to UI Automation is a major task, MSAA has been extended with the `IAccessibleEx` interface which exposes the UI Automation new capabilities in MSAA-based implementations. In the Flutter engine, `AXPlatformNodeWin` implements `IAccessibleEx`, for example to get `AutomationId` accessibility property from Flutter `SemanticsNode::identifier`.

## UI Automation

UI Automation is one framework for Windows that is used for accessibility, for example, by screen readers such as Windows Narrator or NVDA. The interface class `IRawElementProviderFragment` must be implemented by any object representing a UIA fragment, i.e. an accessibility item that exists on the screen, such as a text label, a pane, a button, etc.. For the Flutter engine, the class `AXPlatformNodeWin` extends `IRawElementProviderFragment` in order to serve this purpose. Abstract methods in `IRawElementProviderFragment` provide functionality including retrieving the bounds of the fragment on screen, testing if an on-screen point is within the fragment, and retrieving properties of the fragment such as name and role. Each of the methods of `IRawElementProviderFragment` implemented by `AXPlatformNodeWin` are listed in `ax_platform_node_win.h`. Additionally, `IRawElementProviderFragment` contains a method that returns the fragment root of the fragment.
The fragment root represents the root of a fragment tree, and extends the class `IRawElementProviderFragmentRoot`. Generally, and in the case of Flutter, these correspond 1-1 to a window. We extend this class in `AXFragmentRootPlatformNodeWin` in ax_fragment_root_win.cc. This class contains methods to retrieve the fragment at a given point, and the current focus of the tree.
The Window class’s `OnGetObject` handles WM_GETOBJECT window messages, which a screen reader will dispatch to access the UIA fragment tree. When the `lparam` parameter of the message is equal to the constant `UiaRootObjectId`, the request is asking for the UIA root, and a reference is created out of the window’s fragment root with `UiaReturnRawElementProvider`, which is then returned to the message sender.
Patterns are optional interfaces which UIA elements can provide for additional functionality. For example, providing the `ITextProvider` allows a fragment to be treated as formatted text, and `IToggleProvider` indicates that a fragment can be toggled (e.g. a checkbox) and provides its current toggle state (i.e. on, off, mixed). The UIA method `GetPatternProvider` is used to return an instance of a requested interface type, or none if it is not provided. Provided patterns may or may not be the same as the fragment itself. For example, in `AXPlatformNodeWin`, when `IToggleProvider` is requested, a casted pointer to the node is returned, as `AXPlatformNodeWin` extends `IToggleProvider`. By contrast, when an `ITextProvider` is requested, a separate object is returned, rather than `AXPlatformNodeWin` being a subclass of `ITextProvider`. In this case, this is necessary, as other inherited methods of `AXPlatformNodeWin` share a name but different signature to those in `ITextProvider`, so it cannot extend both.

Whether or not the engine makes use of UI Automation is controlled in the `Window` class, in `Window::OnGetObject`. When a UIA message is received, a UIA response is only issued if the engine was built with the `FLUTTER_ENGINE_USE_UIA` macro defined.