Messages sent to the embedder host may be one-way messages with no response
handler.  If the host calls FlutterEngineSendPlatformMessageResponse on a
one-way message, then just delete the message response handle.

Also update the documentation to indicate that
FlutterEngineSendPlatformMessageResponse must be called for all messages.
Previously the docs implied that some FlutterPlatformMessage objects may
have a null response_handle.  The embedder will now set a response_handle for
every message (even if the sender does not expect a response).

Corects a bnuch of typeos throughout teh engien codebsae. Also makes
a couple minor Commonwealth -> US spelling adjustments for consistency
with the rest of Flutter's codebase.

Made use of `misspell` tool:
https://github.com/client9/misspell

Rather than hard-coding the type of incoming events to mouse, and
synthesizing a primary button press for kDown/kUp, expose device kind
and buttons in the API.

For backwards compatibility, if the type is not set, the old behavior is
used. If an embedder sets the type to mouse explicitly, however, they
must also set correct button information.

For the touch type, the API abstracts away the framework's internal
expectation that a button is set for touch down/move for simplicity.

Fixes flutter/flutter#32854

PR #8498 made these constants extern, but forgot to export them so they
would be public symbols.

Currently, all Flutter threads are managed by the engine itself. This works for
all threads except the platform thread. On this thread, the engine cannot see
the underlying event multiplexing mechanism. Using the new task runner
interfaces, the engine can relinquish the task of setting up the event
multiplexing mechanism and instead have the embedder provide one for it during
setup.

This scheme is only wired up for the platform thread. But, the eventual goal
is to expose this message loop interoperability for all threads.

Add frame and target time metadata to vsync events and connect platform vsync events using flows. (#8172)

This will allow us to easily visualize the time the platform informed the engine of a vsync event, its arguments, and when the engine began its UI thread workload using this information.

Add an allocator specific check to ensure that strings passed to the timeline are not heap allocated. (#8168)

Verified that the tests fail on issues like https://github.com/flutter/engine/pull/8166. Unfortunately, there is no x-platform way to perform this check but this should gate incorrect traces being added to the engine.

Some embedders may have to wait on fences asynchronously before committing
contents. This allows them to post a task onto the engine managed thread used
for rendering.

Some clients (e.g., embedded devices) prefer generating persistent cache files for the specific device beforehand, and ship them as readonly files in OTA packages.

This updates the FlutterSemanticsAction enumerator identifiers for the
'move cursor forward/back one word' actions (added in
flutter/engine#8033) for consistency with the 'move cusor forward/back
on character' identifiers.

ABI compatibility is unaffected, but this with require the following
change in any embedder making use of these fields:

Rename:
    kFlutterSemanticsActionMoveCursorForwardByWordIndex
to:
    kFlutterSemanticsActionMoveCursorForwardByWord

Rename:
    kFlutterSemanticsActionMoveCursorBackwardByWordIndex
to:
    kFlutterSemanticsActionMoveCursorBackwardByWord

The current assumption is that the embedder will wait till the vsync event and
then fire the callback. However, some embedders have that information upfront.
Since the time point has already been specified by the embedder, there is no
reason to burden the embedder with having to setup a wait either.

Adds support for pointer signals, in a way that will support both discrete events (e.g., scroll wheels, flutter/flutter#22762) and continuous gestures (e.g., trackpad scroll, flutter/flutter#21953).

Also exposes these new event options to the embedder. Does not include code to send the
new events from the platform shells.

This brings the Dart and C++ semantics flag enums back in sync.

In #5902, the ability to move the cursor forward and backward one word
were added to dart:ui, and to the Android embedder, but not to the
SemanticsAction enum on the C++ side.

This brings the Dart and C++ semantics flag enums back in sync.

In #5941, implicit scrolling support was added to SemanticsFlag in
dart:ui, and to the Android embedder, but not to the SemanticsFlags enum
on the C++ side.

This also clarifies/corrects the documentation for this value in dart:ui
and in the embedder API.

occured -> occurred
preceeded -> preceded

Some embedders prefer to minimise the number of semantics node/custom
action updates sent back to the host platform -- for example due to
expensive serialisation mechanisms, etc.

This patch provides a 'batch end' signal that provides embedders with an
indication of when a self-consistent set of semantics node or custom action
updates have been sent.

We overload the node/action ID with information that conveys a batch end
by using an ID (-1) that is never allotted to semantics nodes by the
framework.

Correct struct members whose names were camelCase rather than
snake_case. This is a breaking API change, but does not break ABI.

Fixes https://github.com/flutter/flutter/issues/28240

Fixes https://github.com/flutter/flutter/issues/28283

Fixes https://github.com/flutter/flutter/issues/28278

Flutter's accessibility APIs consist of three main calls from the
embedder to the Dart application:

  1. FlutterEngineUpdateSemanticsEnabled: enables/disables semantics support.

  2. FlutterEngineUpdateAccessibilityFeatures: sets embedder-specific
     accessibility features.

  3. FlutterEngineDispatchSemanticsAction: dispatches an action (tap,
     long-press, scroll, etc.) to a semantics node.

and two main callbacks triggered by Dart code:

  1. FlutterUpdateSemanticsNodeCallback: notifies the embedder of
     updates to the properties of a given semantics node.

  2. FlutterUpdateSemanticsCustomActionCallback: notifies the embedder
     of updates to custom semantics actions registered in Dart code.

In the Flutter framework, when accessibility is first enabled, the
embedder will receive a stream of update callbacks notifying the
embedder of the full semantics tree. On further changes in the Dart
application, only updates will be sent.

Adds 'add', 'remove', and 'hover' to the set of pointer phases that are
available to embedders. This is necessary for them to send hover events
to the engine.

Allows embedders to specify a callback to be invoked in isolate scope
once root isolate has been created and marked runnable.

As an example of where this is useful, embedder unit test fixtures may
want to include Dart functions backed by a native implementation. On
isolate creation, this patch allows the unit test author to call
Dart_SetNativeResolver in root isolate scope.

Document make_resource_current on FlutterOpenGLRendererConfig and warn if the callback is not set. (#7648)

FlutterResult is also the name of a class in the Objective-C API
surface, which is problematic when building a framework that contains
both (such as a macOS implementation of the Flutter framework).

As of Dart 2, running from Dart source is no longer supported.  Dart
code should now be compiled to kernel form and will be loaded by from
kernel.blob in the assets directory. We retain the struct members for ABI
stability. package_path is also not required since kernel blobs are
self-contained.

This updates the embedder API but introduces no breaking ABI/API
changes.

* Allow embedder clients to provide a resource context. Closes flutter/flutter#14263.