[Android] Perform eager gesture recognition on MotionEvents

content/browser/renderer_host/input/gesture_event_queue.cc

+// Copyright 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "content/browser/renderer_host/input/gesture_event_queue.h"
+
+#include "base/auto_reset.h"
+#include "base/logging.h"
+#include "content/common/input/web_input_event_traits.h"
+
+using blink::WebGestureEvent;
+using blink::WebInputEvent;
+using blink::WebTouchEvent;
+using blink::WebTouchPoint;
+
+namespace content {
+namespace {
+
+InputEventAckState TransitionToMostRestrictiveState(

[tdresser, 2014/01/16 14:31:45]

TransitionToMostRestrictiveState implies that this mutates something. No
"transition" is taking place.
You could actually mutate ack_state_ in here, in which case the name would make
sense.

Otherwise I think naming it GetMostRestrictiveState or MostRestrictiveState
would be reasonable.

[jdduke (slow), 2014/01/16 17:17:53]

Yeah, except the order matters (i.e., we never go from a restrictive state to a
less restrictive state).  I agree this isn't the best wording, but I'd like the
name to indicate that arg order is important.

+    InputEventAckState ack_old,
+    InputEventAckState ack_new) {
+  if (ack_new == INPUT_EVENT_ACK_STATE_UNKNOWN) {
+    NOTREACHED() << "New acks should never be undefined.";
+    return ack_old;
+  }
+
+  if (ack_old == INPUT_EVENT_ACK_STATE_UNKNOWN ||
+      ack_old == INPUT_EVENT_ACK_STATE_NOT_CONSUMED)
+    return ack_new;
+
+  // If a partial touch sequence has been consumed, or has no consumer,
+  // subsequent touches have no bearing on the overall gesture disposition.
+  return ack_old;
+}
+
+WebGestureEvent CreateGesture(WebInputEvent::Type type) {
+  DCHECK(WebInputEvent::isGestureEventType(type));
+  WebGestureEvent event;
+  event.type = type;
+  event.sourceDevice = WebGestureEvent::Touchscreen;
+  return event;
+}
+
+}  // namespace
+
+GestureEventQueue::GestureEventQueue(GestureEventQueueClient* client)
+    : client_(client),
+      outstanding_tap_(false),
+      outstanding_fling_(false) {
+  DCHECK(client_);
+}
+
+GestureEventQueue::~GestureEventQueue() {}
+
+void GestureEventQueue::OnGestureEventPacket(const GestureEventPacket& packet) {
+  switch (packet.gesture_source()) {
+    case GestureEventPacket::TOUCH_BEGIN:
+      sequences_.push_back(GestureSequence());
+      break;
+    case GestureEventPacket::TOUCH:
+      break;
+    case GestureEventPacket::TOUCH_TIMEOUT:
+      // Touches preceding the timeout event are still awaiting an ack.
+      if (!Tail().IsEmpty())
+        break;
+      // Touches preceding the timeout were handled and gestures are allowed.
+      if (Tail().IsGestureAllowed())
+        Forward(packet);
+      // The timeout gesture should be dropped.
+      return;
+    case GestureEventPacket::SYNTHETIC:

[tdresser, 2014/01/16 14:31:45]

Couldn't this violate some ordering constraints?

[jdduke (slow), 2014/01/16 17:17:53]

Yeah, though on Android that constraint is already violated (e.g., synthetic
gestures sent down while the CVGH is awaiting a touch ack will already be
processed out-of-order).

One thought is to simply intersperse synthetic packets with an existing
sequence; they'd be processed in-order, but they'd simply ignore the disposition
of the current sequence and always be sent. wdyt? 

[tdresser, 2014/01/16 18:28:29]

SGTM.

+      Forward(packet);
+      return;
+  }
+
+  Tail().Push(packet);
+}
+
+void GestureEventQueue::OnTouchEventAck(InputEventAckState ack_state) {
+  // Transition to the next touch sequence.  This transition is deferred
+  // until the ack in the event that the sequence receives a timeout event.

[tdresser, 2014/01/16 14:31:45]

This is the only place where you're transitioning to the next touch sequence,
isn't it? I'm confused by the comment.

[jdduke (slow), 2014/01/16 17:17:53]

It's a terrible comment.  I was thinking of adding a TOUCH_END marker so that
we'd pop a sequence then, though in-practice the result should be the same. 
Regardless, it's a bad comment.

+  if (Head().IsEmpty()) {
+    CancelTapIfNecessary();
+    CancelFlingIfNecessary();
+    sequences_.pop_front();
+  }
+
+  Forward(Head().OnTouchEventAck(ack_state));
+
+  // Immediately cancel a TapDown if TouchStart went unconsumed, but a
+  // subsequent TouchMove is consumed.
+  if (!Head().IsGestureAllowed())
+    CancelTapIfNecessary();
+}
+
+void GestureEventQueue::Forward(const GestureEventPacket& packet) {
+  for (size_t i = 0; i < packet.gesture_count(); ++i)
+    Forward(packet.gesture(i));
+}
+
+void GestureEventQueue::Forward(const WebGestureEvent& event) {
+  switch (event.type) {
+    case WebInputEvent::GestureScrollBegin:
+      CancelTapIfNecessary();
+      break;
+    case WebInputEvent::GestureTapDown:
+      outstanding_tap_ = true;
+      break;
+    case WebInputEvent::GestureTapCancel:
+    case WebInputEvent::GestureTap:
+    case WebInputEvent::GestureTapUnconfirmed:
+    case WebInputEvent::GestureDoubleTap:
+      outstanding_tap_ = false;
+      break;
+    case WebInputEvent::GestureFlingStart:
+      outstanding_fling_ = true;
+      break;
+    case WebInputEvent::GestureFlingCancel:
+      outstanding_fling_ = false;
+      break;
+    default:
+      break;
+  }
+  client_->ForwardGestureEvent(event);
+}
+
+void GestureEventQueue::CancelTapIfNecessary() {
+  if (!outstanding_tap_)
+    return;
+
+  Forward(CreateGesture(WebInputEvent::GestureTapCancel));
+  DCHECK(!outstanding_tap_);
+}
+
+void GestureEventQueue::CancelFlingIfNecessary() {
+  if (!outstanding_fling_)
+    return;
+
+  Forward(CreateGesture(WebInputEvent::GestureFlingCancel));
+  DCHECK(!outstanding_fling_);
+}
+
+GestureEventQueue::GestureSequence& GestureEventQueue::Head() {
+  DCHECK(!sequences_.empty());
+  return sequences_.front();
+}
+
+GestureEventQueue::GestureSequence& GestureEventQueue::Tail() {
+  DCHECK(!sequences_.empty());
+  return sequences_.back();
+}
+
+
+GestureEventQueue::GestureSequence::GestureSequence()
+    : ack_state_(INPUT_EVENT_ACK_STATE_UNKNOWN) {}
+
+GestureEventQueue::GestureSequence::~GestureSequence() {}
+
+void GestureEventQueue::GestureSequence::Push(
+    const GestureEventPacket& packet) {
+  DCHECK_NE(GestureEventPacket::SYNTHETIC, packet.gesture_source());
+  if (packet.gesture_source() == GestureEventPacket::TOUCH_TIMEOUT) {
+    // Timeout-derived gestures should have beeen forwarded or dropped

[tdresser, 2014/01/16 14:31:45]

beeen -> been

[jdduke (slow), 2014/01/16 17:17:53]

Done.

+    // immediately if the sequence was empty.
+    DCHECK(!IsEmpty());
+
+    // Merge the timeout packet with the previous packet, simplifying dispatch
+    // when the touch ack preceding the timeout is received.
+    for (size_t i = 0; i < packet.gesture_count(); ++i)
+      sequence_.back().Push(packet.gesture(i));
+    return;
+  }
+  sequence_.push_back(packet);
+}
+
+GestureEventPacket GestureEventQueue::GestureSequence::OnTouchEventAck(
+    InputEventAckState new_ack_state) {
+  DCHECK(!IsEmpty());
+
+  ack_state_ = TransitionToMostRestrictiveState(ack_state_, new_ack_state);
+  DCHECK_NE(ack_state_, INPUT_EVENT_ACK_STATE_UNKNOWN);
+
+  GestureEventPacket gestures =
+      IsGestureAllowed() ? sequence_.front() : GestureEventPacket();
+  sequence_.pop_front();
+  return gestures;
+}
+
+bool GestureEventQueue::GestureSequence::IsGestureAllowed() const {
+  return ack_state_ == INPUT_EVENT_ACK_STATE_NO_CONSUMER_EXISTS ||
+         ack_state_ == INPUT_EVENT_ACK_STATE_NOT_CONSUMED;
+}
+
+bool GestureEventQueue::GestureSequence::IsEmpty() const {
+  return sequence_.empty();
+}
+
+}  // namespace content