Revision control

Line Code
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set sw=2 ts=8 et tw=80 : */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef mozilla_layers_AsyncPanZoomController_h
#define mozilla_layers_AsyncPanZoomController_h

#include "CrossProcessMutex.h"
#include "mozilla/layers/GeckoContentController.h"
#include "mozilla/layers/APZCTreeManager.h"
#include "mozilla/layers/AsyncPanZoomAnimation.h"
#include "mozilla/Attributes.h"
#include "mozilla/EventForwards.h"
#include "mozilla/Monitor.h"
#include "mozilla/ReentrantMonitor.h"
#include "mozilla/RefPtr.h"
#include "mozilla/UniquePtr.h"
#include "mozilla/Atomics.h"
#include "InputData.h"
#include "Axis.h"
#include "InputQueue.h"
#include "APZUtils.h"
#include "Layers.h"                     // for Layer::ScrollDirection
#include "LayersTypes.h"
#include "mozilla/gfx/Matrix.h"
#include "nsIScrollableFrame.h"
#include "nsRegion.h"
#include "nsTArray.h"
#include "PotentialCheckerboardDurationTracker.h"

#include "base/message_loop.h"

namespace mozilla {

namespace ipc {

class SharedMemoryBasic;

} // namespace ipc

namespace layers {

class AsyncDragMetrics;
struct ScrollableLayerGuid;
class CompositorController;
class MetricsSharingController;
class GestureEventListener;
struct AsyncTransform;
class AsyncPanZoomAnimation;
class AndroidFlingAnimation;
class GenericFlingAnimation;
class InputBlockState;
class TouchBlockState;
class PanGestureBlockState;
class OverscrollHandoffChain;
class StateChangeNotificationBlocker;
class CheckerboardEvent;
class OverscrollEffectBase;
class WidgetOverscrollEffect;
class GenericOverscrollEffect;
class AndroidSpecificState;

// Base class for grouping platform-specific APZC state variables.
class PlatformSpecificStateBase {
public:
  virtual ~PlatformSpecificStateBase() {}
  virtual AndroidSpecificState* AsAndroidSpecificState() { return nullptr; }
};

/**
 * Controller for all panning and zooming logic. Any time a user input is
 * detected and it must be processed in some way to affect what the user sees,
 * it goes through here. Listens for any input event from InputData and can
 * optionally handle WidgetGUIEvent-derived touch events, but this must be done
 * on the main thread. Note that this class completely cross-platform.
 *
 * Input events originate on the UI thread of the platform that this runs on,
 * and are then sent to this class. This class processes the event in some way;
 * for example, a touch move will usually lead to a panning of content (though
 * of course there are exceptions, such as if content preventDefaults the event,
 * or if the target frame is not scrollable). The compositor interacts with this
 * class by locking it and querying it for the current transform matrix based on
 * the panning and zooming logic that was invoked on the UI thread.
 *
 * Currently, each outer DOM window (i.e. a website in a tab, but not any
 * subframes) has its own AsyncPanZoomController. In the future, to support
 * asynchronously scrolled subframes, we want to have one AsyncPanZoomController
 * per frame.
 */
class AsyncPanZoomController {
  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AsyncPanZoomController)

  typedef mozilla::MonitorAutoLock MonitorAutoLock;
  typedef mozilla::gfx::Matrix4x4 Matrix4x4;

public:
  enum GestureBehavior {
    // The platform code is responsible for forwarding gesture events here. We
    // will not attempt to generate gesture events from MultiTouchInputs.
    DEFAULT_GESTURES,
    // An instance of GestureEventListener is used to detect gestures. This is
    // handled completely internally within this class.
    USE_GESTURE_DETECTOR
  };

  /**
   * Constant describing the tolerance in distance we use, multiplied by the
   * device DPI, before we start panning the screen. This is to prevent us from
   * accidentally processing taps as touch moves, and from very short/accidental
   * touches moving the screen.
   * Note: It's an abuse of the 'Coord' class to use it to represent a 2D
   *       distance, but it's the closest thing we currently have.
   */
  static ScreenCoord GetTouchStartTolerance();

  AsyncPanZoomController(uint64_t aLayersId,
                         APZCTreeManager* aTreeManager,
                         const RefPtr<InputQueue>& aInputQueue,
                         GeckoContentController* aController,
                         GestureBehavior aGestures = DEFAULT_GESTURES);

  // --------------------------------------------------------------------------
  // These methods must only be called on the gecko thread.
  //

  /**
   * Read the various prefs and do any global initialization for all APZC instances.
   * This must be run on the gecko thread before any APZC instances are actually
   * used for anything meaningful.
   */
  static void InitializeGlobalState();

  // --------------------------------------------------------------------------
  // These methods must only be called on the controller/UI thread.
  //

  /**
   * Kicks an animation to zoom to a rect. This may be either a zoom out or zoom
   * in. The actual animation is done on the compositor thread after being set
   * up.
   */
  void ZoomToRect(CSSRect aRect, const uint32_t aFlags);

  /**
   * Updates any zoom constraints contained in the <meta name="viewport"> tag.
   */
  void UpdateZoomConstraints(const ZoomConstraints& aConstraints);

  /**
   * Return the zoom constraints last set for this APZC (in the constructor
   * or in UpdateZoomConstraints()).
   */
  ZoomConstraints GetZoomConstraints() const;

  /**
   * Schedules a runnable to run on the controller/UI thread at some time
   * in the future.
   */
  void PostDelayedTask(already_AddRefed<Runnable> aTask, int aDelayMs);

  // --------------------------------------------------------------------------
  // These methods must only be called on the compositor thread.
  //

  /**
   * Advances any animations currently running to the given timestamp.
   * This may be called multiple times with the same timestamp.
   *
   * The return value indicates whether or not any currently running animation
   * should continue. If true, the compositor should schedule another composite.
   */
  bool AdvanceAnimations(const TimeStamp& aSampleTime);

  bool UpdateAnimation(const TimeStamp& aSampleTime,
                       nsTArray<RefPtr<Runnable>>* aOutDeferredTasks);

  /**
   * A shadow layer update has arrived. |aScrollMetdata| is the new ScrollMetadata
   * for the container layer corresponding to this APZC.
   * |aIsFirstPaint| is a flag passed from the shadow
   * layers code indicating that the scroll metadata being sent with this call are
   * the initial metadata and the initial paint of the frame has just happened.
   */
  void NotifyLayersUpdated(const ScrollMetadata& aScrollMetadata, bool aIsFirstPaint,
                           bool aThisLayerTreeUpdated);

  /**
   * The platform implementation must set the compositor controller so that we can
   * request composites.
   */
  void SetCompositorController(CompositorController* aCompositorController);

  /**
   * If we need to share the frame metrics with some other thread, this controller
   * needs to be set and provides relevant information/APIs.
   */
  void SetMetricsSharingController(MetricsSharingController* aMetricsSharingController);

  // --------------------------------------------------------------------------
  // These methods can be called from any thread.
  //

  /**
   * Shut down the controller/UI thread state and prepare to be
   * deleted (which may happen from any thread).
   */
  void Destroy();

  /**
   * Returns true if Destroy() has already been called on this APZC instance.
   */
  bool IsDestroyed() const;

  /**
   * Returns the transform to take something from the coordinate space of the
   * last thing we know gecko painted, to the coordinate space of the last thing
   * we asked gecko to paint. In cases where that last request has not yet been
   * processed, this is needed to transform input events properly into a space
   * gecko will understand.
   */
  Matrix4x4 GetTransformToLastDispatchedPaint() const;

  /**
   * Returns the number of CSS pixels of checkerboard according to the metrics
   * in this APZC.
   */
  uint32_t GetCheckerboardMagnitude() const;

  /**
   * Report the number of CSSPixel-milliseconds of checkerboard to telemetry.
   */
  void ReportCheckerboard(const TimeStamp& aSampleTime);

  /**
   * Flush any active checkerboard report that's in progress. This basically
   * pretends like any in-progress checkerboard event has terminated, and pushes
   * out the report to the checkerboard reporting service and telemetry. If the
   * checkerboard event has not really finished, it will start a new event
   * on the next composite.
   */
  void FlushActiveCheckerboardReport();

  /**
   * Returns whether or not the APZC is currently in a state of checkerboarding.
   * This is a simple computation based on the last-painted content and whether
   * the async transform has pushed it so far that it doesn't fully contain the
   * composition bounds.
   */
  bool IsCurrentlyCheckerboarding() const;

  /**
   * Recalculates the displayport. Ideally, this should paint an area bigger
   * than the composite-to dimensions so that when you scroll down, you don't
   * checkerboard immediately. This includes a bunch of logic, including
   * algorithms to bias painting in the direction of the velocity.
   */
  static const ScreenMargin CalculatePendingDisplayPort(
    const FrameMetrics& aFrameMetrics,
    const ParentLayerPoint& aVelocity);

  nsEventStatus HandleDragEvent(const MouseInput& aEvent,
                                const AsyncDragMetrics& aDragMetrics);

  /**
   * Handler for events which should not be intercepted by the touch listener.
   */
  nsEventStatus HandleInputEvent(const InputData& aEvent,
                                 const ScreenToParentLayerMatrix4x4& aTransformToApzc);

  /**
   * Handler for gesture events.
   * Currently some gestures are detected in GestureEventListener that calls
   * APZC back through this handler in order to avoid recursive calls to
   * APZC::HandleInputEvent() which is supposed to do the work for
   * ReceiveInputEvent().
   */
  nsEventStatus HandleGestureEvent(const InputData& aEvent);

  /**
   * Handler for touch velocity.
   * Sometimes the touch move event will have a velocity even though no scrolling
   * is occurring such as when the toolbar is being hidden/shown in Fennec.
   * This function can be called to have the y axis' velocity queue updated.
   */
  void HandleTouchVelocity(uint32_t aTimesampMs, float aSpeedY);

  /**
   * Populates the provided object (if non-null) with the scrollable guid of this apzc.
   */
  void GetGuid(ScrollableLayerGuid* aGuidOut) const;

  /**
   * Returns the scrollable guid of this apzc.
   */
  ScrollableLayerGuid GetGuid() const;

  /**
   * Returns true if this APZC instance is for the layer identified by the guid.
   */
  bool Matches(const ScrollableLayerGuid& aGuid);

  /**
   * Returns true if the tree manager of this APZC is the same as the one
   * passed in.
   */
  bool HasTreeManager(const APZCTreeManager* aTreeManager) const;

  void StartAnimation(AsyncPanZoomAnimation* aAnimation);

  /**
   * Cancels any currently running animation.
   * aFlags is a bit-field to provide specifics of how to cancel the animation.
   * See CancelAnimationFlags.
   */
  void CancelAnimation(CancelAnimationFlags aFlags = Default);

  /**
   * Adjusts the scroll position to compensate for a shift in the surface, such
   * that the content appears to remain visually in the same position. i.e. if
   * the surface moves up by 10 screenpixels, the scroll position should also
   * move up by 10 pixels so that what used to be at the top of the surface is
   * now 10 pixels down the surface.
   */
  void AdjustScrollForSurfaceShift(const ScreenPoint& aShift);

  /**
   * Clear any overscroll on this APZC.
   */
  void ClearOverscroll();

  /**
   * Returns whether this APZC is for an element marked with the 'scrollgrab'
   * attribute.
   */
  bool HasScrollgrab() const { return mScrollMetadata.GetHasScrollgrab(); }

  /**
   * Returns whether this APZC has room to be panned (in any direction).
   */
  bool IsPannable() const;

  /**
   * Returns true if the APZC has been flung with a velocity greater than the
   * stop-on-tap fling velocity threshold (which is pref-controlled).
   */
  bool IsFlingingFast() const;

  /**
   * Returns the identifier of the touch in the last touch event processed by
   * this APZC. This should only be called when the last touch event contained
   * only one touch.
   */
  int32_t GetLastTouchIdentifier() const;

  /**
   * Returns the matrix that transforms points from global screen space into
   * this APZC's ParentLayer space.
   * To respect the lock ordering, mMonitor must NOT be held when calling
   * this function (since this function acquires the tree lock).
   */
  ScreenToParentLayerMatrix4x4 GetTransformToThis() const;

  /**
   * Convert the vector |aVector|, rooted at the point |aAnchor|, from
   * this APZC's ParentLayer coordinates into screen coordinates.
   * The anchor is necessary because with 3D tranforms, the location of the
   * vector can affect the result of the transform.
   * To respect the lock ordering, mMonitor must NOT be held when calling
   * this function (since this function acquires the tree lock).
   */
  ScreenPoint ToScreenCoordinates(const ParentLayerPoint& aVector,
                                  const ParentLayerPoint& aAnchor) const;

  /**
   * Convert the vector |aVector|, rooted at the point |aAnchor|, from
   * screen coordinates into this APZC's ParentLayer coordinates.
   * The anchor is necessary because with 3D tranforms, the location of the
   * vector can affect the result of the transform.
   * To respect the lock ordering, mMonitor must NOT be held when calling
   * this function (since this function acquires the tree lock).
   */
  ParentLayerPoint ToParentLayerCoordinates(const ScreenPoint& aVector,
                                            const ScreenPoint& aAnchor) const;

  // Return whether or not a wheel event will be able to scroll in either
  // direction.
  bool CanScroll(const InputData& aEvent) const;

  // Return whether or not a scroll delta will be able to scroll in either
  // direction.
  bool CanScrollWithWheel(const ParentLayerPoint& aDelta) const;

  // Return whether or not there is room to scroll this APZC
  // in the given direction.
  bool CanScroll(ScrollDirection aDirection) const;

  void NotifyMozMouseScrollEvent(const nsString& aString) const;

protected:
  // Protected destructor, to discourage deletion outside of Release():
  virtual ~AsyncPanZoomController();

  // Returns the cached current frame time.
  TimeStamp GetFrameTime() const;

  /**
   * Helper method for touches beginning. Sets everything up for panning and any
   * multitouch gestures.
   */
  nsEventStatus OnTouchStart(const MultiTouchInput& aEvent);

  /**
   * Helper method for touches moving. Does any transforms needed when panning.
   */
  nsEventStatus OnTouchMove(const MultiTouchInput& aEvent);

  /**
   * Helper method for touches ending. Redraws the screen if necessary and does
   * any cleanup after a touch has ended.
   */
  nsEventStatus OnTouchEnd(const MultiTouchInput& aEvent);

  /**
   * Helper method for touches being cancelled. Treated roughly the same as a
   * touch ending (OnTouchEnd()).
   */
  nsEventStatus OnTouchCancel(const MultiTouchInput& aEvent);

  /**
   * Helper method for scales beginning. Distinct from the OnTouch* handlers in
   * that this implies some outside implementation has determined that the user
   * is pinching.
   */
  nsEventStatus OnScaleBegin(const PinchGestureInput& aEvent);

  /**
   * Helper method for scaling. As the user moves their fingers when pinching,
   * this changes the scale of the page.
   */
  nsEventStatus OnScale(const PinchGestureInput& aEvent);

  /**
   * Helper method for scales ending. Redraws the screen if necessary and does
   * any cleanup after a scale has ended.
   */
  nsEventStatus OnScaleEnd(const PinchGestureInput& aEvent);

  /**
   * Helper methods for handling pan events.
   */
  nsEventStatus OnPanMayBegin(const PanGestureInput& aEvent);
  nsEventStatus OnPanCancelled(const PanGestureInput& aEvent);
  nsEventStatus OnPanBegin(const PanGestureInput& aEvent);
  nsEventStatus OnPan(const PanGestureInput& aEvent, bool aFingersOnTouchpad);
  nsEventStatus OnPanEnd(const PanGestureInput& aEvent);
  nsEventStatus OnPanMomentumStart(const PanGestureInput& aEvent);
  nsEventStatus OnPanMomentumEnd(const PanGestureInput& aEvent);
  nsEventStatus HandleEndOfPan();

  /**
   * Helper methods for handling scroll wheel events.
   */
  nsEventStatus OnScrollWheel(const ScrollWheelInput& aEvent);

  ParentLayerPoint GetScrollWheelDelta(const ScrollWheelInput& aEvent) const;

  /**
   * Helper methods for long press gestures.
   */
  nsEventStatus OnLongPress(const TapGestureInput& aEvent);
  nsEventStatus OnLongPressUp(const TapGestureInput& aEvent);

  /**
   * Helper method for single tap gestures.
   */
  nsEventStatus OnSingleTapUp(const TapGestureInput& aEvent);

  /**
   * Helper method for a single tap confirmed.
   */
  nsEventStatus OnSingleTapConfirmed(const TapGestureInput& aEvent);

  /**
   * Helper method for double taps.
   */
  nsEventStatus OnDoubleTap(const TapGestureInput& aEvent);

  /**
   * Helper method for double taps where the double-tap gesture is disabled.
   */
  nsEventStatus OnSecondTap(const TapGestureInput& aEvent);

  /**
   * Helper method to cancel any gesture currently going to Gecko. Used
   * primarily when a user taps the screen over some clickable content but then
   * pans down instead of letting go (i.e. to cancel a previous touch so that a
   * new one can properly take effect.
   */
  nsEventStatus OnCancelTap(const TapGestureInput& aEvent);

  /**
   * Scrolls the viewport by an X,Y offset.
   */
  void ScrollBy(const CSSPoint& aOffset);

  /**
   * Scales the viewport by an amount (note that it multiplies this scale in to
   * the current scale, it doesn't set it to |aScale|). Also considers a focus
   * point so that the page zooms inward/outward from that point.
   */
  void ScaleWithFocus(float aScale,
                      const CSSPoint& aFocus);

  /**
   * Schedules a composite on the compositor thread.
   */
  void ScheduleComposite();

  /**
   * Schedules a composite, and if enough time has elapsed since the last
   * paint, a paint.
   */
  void ScheduleCompositeAndMaybeRepaint();

  /**
   * Gets the displacement of the current touch since it began. That is, it is
   * the distance between the current position and the initial position of the
   * current touch (this only makes sense if a touch is currently happening and
   * OnTouchMove() or the equivalent for pan gestures is being invoked).
   * Note: It's an abuse of the 'Coord' class to use it to represent a 2D
   *       distance, but it's the closest thing we currently have.
   */
  ScreenCoord PanDistance() const;

  /**
   * Gets the start point of the current touch.
   * Like PanDistance(), this only makes sense if a touch is currently
   * happening and OnTouchMove() or the equivalent for pan gestures is
   * being invoked.
   */
  ParentLayerPoint PanStart() const;

  /**
   * Gets a vector of the velocities of each axis.
   */
  const ParentLayerPoint GetVelocityVector() const;

  /**
   * Sets the velocities of each axis.
   */
  void SetVelocityVector(const ParentLayerPoint& aVelocityVector);

  /**
   * Gets the first touch point from a MultiTouchInput.  This gets only
   * the first one and assumes the rest are either missing or not relevant.
   */
  ParentLayerPoint GetFirstTouchPoint(const MultiTouchInput& aEvent);

  /**
   * Sets the panning state basing on the pan direction angle and current touch-action value.
   */
  void HandlePanningWithTouchAction(double angle);

  /**
   * Sets the panning state ignoring the touch action value.
   */
  void HandlePanning(double angle);

  /**
   * Update the panning state and axis locks.
   */
  void HandlePanningUpdate(const ScreenPoint& aDelta);

  /**
   * Sets up anything needed for panning. This takes us out of the "TOUCHING"
   * state and starts actually panning us.
   */
  nsEventStatus StartPanning(const ParentLayerPoint& aStartPoint);

  /**
   * Wrapper for Axis::UpdateWithTouchAtDevicePoint(). Calls this function for
   * both axes and factors in the time delta from the last update.
   */
  void UpdateWithTouchAtDevicePoint(const MultiTouchInput& aEvent);

  /**
   * Does any panning required due to a new touch event.
   */
  void TrackTouch(const MultiTouchInput& aEvent);

  /**
   * Utility function to send updated FrameMetrics to Gecko so that it can paint
   * the displayport area. Calls into GeckoContentController to do the actual
   * work. This call will use the current metrics. If this function is called
   * from a non-main thread, it will redispatch itself to the main thread, and
   * use the latest metrics during the redispatch.
   */
  void RequestContentRepaint(bool aUserAction = true);

  /**
   * Send the provided metrics to Gecko to trigger a repaint. This function
   * may filter duplicate calls with the same metrics. This function must be
   * called on the main thread.
   */
  void RequestContentRepaint(const FrameMetrics& aFrameMetrics,
                             const ParentLayerPoint& aVelocity);

  /**
   * Gets the current frame metrics. This is *not* the Gecko copy stored in the
   * layers code.
   */
  const FrameMetrics& GetFrameMetrics() const;

  /**
   * Gets the pointer to the apzc tree manager. All the access to tree manager
   * should be made via this method and not via private variable since this method
   * ensures that no lock is set.
   */
  APZCTreeManager* GetApzcTreeManager() const;

  /**
   * Convert ScreenPoint relative to the screen to LayoutDevicePoint relative
   * to the parent document. This excludes the transient compositor transform.
   * NOTE: This must be converted to LayoutDevicePoint relative to the child
   * document before sending over IPC to a child process.
   */
  bool ConvertToGecko(const ScreenIntPoint& aPoint, LayoutDevicePoint* aOut);

  enum AxisLockMode {
    FREE,     /* No locking at all */
    STANDARD, /* Default axis locking mode that remains locked until pan ends*/
    STICKY,   /* Allow lock to be broken, with hysteresis */
  };

  static AxisLockMode GetAxisLockMode();

  // Helper function for OnSingleTapUp(), OnSingleTapConfirmed(), and
  // OnLongPressUp().
  nsEventStatus GenerateSingleTap(GeckoContentController::TapType aType,
                                  const ScreenIntPoint& aPoint,
                                  mozilla::Modifiers aModifiers);

  // Common processing at the end of a touch block.
  void OnTouchEndOrCancel();

  uint64_t mLayersId;
  RefPtr<CompositorController> mCompositorController;
  RefPtr<MetricsSharingController> mMetricsSharingController;

  /* Access to the following two fields is protected by the mRefPtrMonitor,
     since they are accessed on the UI thread but can be cleared on the
     compositor thread. */
  RefPtr<GeckoContentController> mGeckoContentController;
  RefPtr<GestureEventListener> mGestureEventListener;
  mutable Monitor mRefPtrMonitor;

  // This is a raw pointer to avoid introducing a reference cycle between
  // AsyncPanZoomController and APZCTreeManager. Since these objects don't
  // live on the main thread, we can't use the cycle collector with them.
  // The APZCTreeManager owns the lifetime of the APZCs, so nulling this
  // pointer out in Destroy() will prevent accessing deleted memory.
  Atomic<APZCTreeManager*> mTreeManager;

  /* Utility functions that return a addrefed pointer to the corresponding fields. */
  already_AddRefed<GeckoContentController> GetGeckoContentController() const;
  already_AddRefed<GestureEventListener> GetGestureEventListener() const;

  PlatformSpecificStateBase* GetPlatformSpecificState();

protected:
  // Both |mFrameMetrics| and |mLastContentPaintMetrics| are protected by the
  // monitor. Do not read from or modify either of them without locking.
  ScrollMetadata mScrollMetadata;
  FrameMetrics& mFrameMetrics;  // for convenience, refers to mScrollMetadata.mMetrics

  // Protects |mFrameMetrics|, |mLastContentPaintMetrics|, and |mState|.
  // Before manipulating |mFrameMetrics| or |mLastContentPaintMetrics|, the
  // monitor should be held. When setting |mState|, either the SetState()
  // function can be used, or the monitor can be held and then |mState| updated.
  // IMPORTANT: See the note about lock ordering at the top of APZCTreeManager.h.
  // This is mutable to allow entering it from 'const' methods; doing otherwise
  // would significantly limit what methods could be 'const'.
  mutable ReentrantMonitor mMonitor;

private:
  // Metadata of the container layer corresponding to this APZC. This is
  // stored here so that it is accessible from the UI/controller thread.
  // These are the metrics at last content paint, the most recent
  // values we were notified of in NotifyLayersUpdate(). Since it represents
  // the Gecko state, it should be used as a basis for untransformation when
  // sending messages back to Gecko.
  ScrollMetadata mLastContentPaintMetadata;
  FrameMetrics& mLastContentPaintMetrics;  // for convenience, refers to mLastContentPaintMetadata.mMetrics
  // The last metrics used for a content repaint request.
  FrameMetrics mLastPaintRequestMetrics;
  // The metrics that we expect content to have. This is updated when we
  // request a content repaint, and when we receive a shadow layers update.
  // This allows us to transform events into Gecko's coordinate space.
  FrameMetrics mExpectedGeckoMetrics;

  AxisX mX;
  AxisY mY;

  // This flag is set to true when we are in a axis-locked pan as a result of
  // the touch-action CSS property.
  bool mPanDirRestricted;

  // Most up-to-date constraints on zooming. These should always be reasonable
  // values; for example, allowing a min zoom of 0.0 can cause very bad things
  // to happen.
  ZoomConstraints mZoomConstraints;

  // The last time the compositor has sampled the content transform for this
  // frame.
  TimeStamp mLastSampleTime;

  // The last sample time at which we submitted a checkerboarding report.
  TimeStamp mLastCheckerboardReport;

  // Stores the previous focus point if there is a pinch gesture happening. Used
  // to allow panning by moving multiple fingers (thus moving the focus point).
  ParentLayerPoint mLastZoomFocus;

  RefPtr<AsyncPanZoomAnimation> mAnimation;

  UniquePtr<OverscrollEffectBase> mOverscrollEffect;

  // Groups state variables that are specific to a platform.
  // Initialized on first use.
  UniquePtr<PlatformSpecificStateBase> mPlatformSpecificState;

  friend class Axis;


  /* ===================================================================
   * The functions and members in this section are used to expose
   * the current async transform state to callers.
   */
public:
  /**
   * Allows callers to specify which type of async transform they want:
   * NORMAL provides the actual async transforms of the APZC, whereas
   * RESPECT_FORCE_DISABLE will provide empty async transforms if and only if
   * the metrics has the mForceDisableApz flag set. In general the latter should
   * only be used by call sites that are applying the transform to update
   * a layer's position.
   */
  enum AsyncMode {
    NORMAL,
    RESPECT_FORCE_DISABLE,
  };

  /**
   * Get the current scroll offset of the scrollable frame corresponding
   * to this APZC, including the effects of any asynchronous panning and
   * zooming, in ParentLayer pixels.
   */
  ParentLayerPoint GetCurrentAsyncScrollOffset(AsyncMode aMode) const;

  /**
   * Get the current scroll offset of the scrollable frame corresponding
   * to this APZC, including the effects of any asynchronous panning, in
   * CSS pixels.
   */
  CSSPoint GetCurrentAsyncScrollOffsetInCssPixels(AsyncMode aMode) const;

  /**
   * Return a visual effect that reflects this apzc's
   * overscrolled state, if any.
   */
  AsyncTransformComponentMatrix GetOverscrollTransform(AsyncMode aMode) const;

  /**
   * Returns the incremental transformation corresponding to the async pan/zoom
   * in progress. That is, when this transform is multiplied with the layer's
   * existing transform, it will make the layer appear with the desired pan/zoom
   * amount.
   */
  AsyncTransform GetCurrentAsyncTransform(AsyncMode aMode) const;

  /**
   * Returns the same transform as GetCurrentAsyncTransform(), but includes
   * any transform due to axis over-scroll.
   */
  AsyncTransformComponentMatrix GetCurrentAsyncTransformWithOverscroll(AsyncMode aMode) const;


  /* ===================================================================
   * The functions and members in this section are used to manage
   * the state that tracks what this APZC is doing with the input events.
   */
protected:
  enum PanZoomState {
    NOTHING,                  /* no touch-start events received */
    FLING,                    /* all touches removed, but we're still scrolling page */
    TOUCHING,                 /* one touch-start event received */

    PANNING,                  /* panning the frame */
    PANNING_LOCKED_X,         /* touch-start followed by move (i.e. panning with axis lock) X axis */
    PANNING_LOCKED_Y,         /* as above for Y axis */

    PAN_MOMENTUM,             /* like PANNING, but controlled by momentum PanGestureInput events */

    PINCHING,                 /* nth touch-start, where n > 1. this mode allows pan and zoom */
    ANIMATING_ZOOM,           /* animated zoom to a new rect */
    OVERSCROLL_ANIMATION,     /* Spring-based animation used to relieve overscroll once
                                 the finger is lifted. */
    SMOOTH_SCROLL,            /* Smooth scrolling to destination. Used by
                                 CSSOM-View smooth scroll-behavior */
    WHEEL_SCROLL              /* Smooth scrolling to a destination for a wheel event. */
  };

  // This is in theory protected by |mMonitor|; that is, it should be held whenever
  // this is updated. In practice though... see bug 897017.
  PanZoomState mState;

private:
  friend class StateChangeNotificationBlocker;
  /**
   * A counter of how many StateChangeNotificationBlockers are active.
   * A non-zero count will prevent state change notifications from
   * being dispatched. Only code that holds mMonitor should touch this.
   */
  int mNotificationBlockers;

  /**
   * Helper to set the current state. Holds the monitor before actually setting
   * it and fires content controller events based on state changes. Always set
   * the state using this call, do not set it directly.
   */
  void SetState(PanZoomState aState);
  /**
   * Fire content controller notifications about state changes, assuming no
   * StateChangeNotificationBlocker has been activated.
   */
  void DispatchStateChangeNotification(PanZoomState aOldState, PanZoomState aNewState);
  /**
   * Internal helpers for checking general state of this apzc.
   */
  static bool IsTransformingState(PanZoomState aState);

  /* ===================================================================
   * The functions and members in this section are used to manage
   * blocks of touch events and the state needed to deal with content
   * listeners.
   */
public:
  /**
   * Flush a repaint request if one is needed, without throttling it with the
   * paint throttler.
   */
  void FlushRepaintForNewInputBlock();

  /**
   * Given the number of touch points in an input event and touch block they
   * belong to, check if the event can result in a panning/zooming behavior.
   * This is primarily used to figure out when to dispatch the pointercancel
   * event for the pointer events spec.
   */
  bool ArePointerEventsConsumable(TouchBlockState* aBlock, uint32_t aTouchPoints);

  /**
   * Clear internal state relating to touch input handling.
   */
  void ResetTouchInputState();

  /**
   * Gets a ref to the input queue that is shared across the entire tree manager.
   */
  const RefPtr<InputQueue>& GetInputQueue() const;

private:
  void CancelAnimationAndGestureState();

  RefPtr<InputQueue> mInputQueue;
  CancelableBlockState* GetCurrentInputBlock() const;
  TouchBlockState* GetCurrentTouchBlock() const;
  bool HasReadyTouchBlock() const;

  PanGestureBlockState* GetCurrentPanGestureBlock() const;

private:
  /* ===================================================================
   * The functions and members in this section are used to manage
   * fling animations, smooth scroll animations, and overscroll
   * during a fling or smooth scroll.
   */
public:
  /**
   * Attempt a fling with the velocity specified in |aHandoffState|.
   * If we are not pannable, the fling is handed off to the next APZC in
   * the handoff chain via mTreeManager->DispatchFling().
   * Returns true iff. the entire velocity of the fling was consumed by
   * this APZC. |aHandoffState.mVelocity| is modified to contain any
   * unused, residual velocity.
   * |aHandoffState.mIsHandoff| should be true iff. the fling was handed off
   * from a previous APZC, and determines whether acceleration is applied
   * to the fling.
   */
  bool AttemptFling(FlingHandoffState& aHandoffState);

private:
  friend class AndroidFlingAnimation;
  friend class GenericFlingAnimation;
  friend class OverscrollAnimation;
  friend class SmoothScrollAnimation;
  friend class WheelScrollAnimation;

  friend class GenericOverscrollEffect;
  friend class WidgetOverscrollEffect;

  // The initial velocity of the most recent fling.
  ParentLayerPoint mLastFlingVelocity;
  // The time at which the most recent fling started.
  TimeStamp mLastFlingTime;
  // Indicates if the repaint-during-pinch timer is currently set
  bool mPinchPaintTimerSet;

  // Deal with overscroll resulting from a fling animation. This is only ever
  // called on APZC instances that were actually performing a fling.
  // The overscroll is handled by trying to hand the fling off to an APZC
  // later in the handoff chain, or if there are no takers, continuing the
  // fling and entering an overscrolled state.
  void HandleFlingOverscroll(const ParentLayerPoint& aVelocity,
                             const RefPtr<const OverscrollHandoffChain>& aOverscrollHandoffChain,
                             const RefPtr<const AsyncPanZoomController>& aScrolledApzc);

  void HandleSmoothScrollOverscroll(const ParentLayerPoint& aVelocity);

  // Helper function used by AttemptFling().
  void AcceptFling(FlingHandoffState& aHandoffState);

  // Start an overscroll animation with the given initial velocity.
  void StartOverscrollAnimation(const ParentLayerPoint& aVelocity);

  void SmoothScrollTo(const CSSPoint& aDestination);

  // Returns whether overscroll is allowed during an event.
  bool AllowScrollHandoffInCurrentBlock() const;

  // Invoked by the pinch repaint timer.
  void DoDelayedRequestContentRepaint();

  /* ===================================================================
   * The functions and members in this section are used to make ancestor chains
   * out of APZC instances. These chains can only be walked or manipulated
   * while holding the lock in the associated APZCTreeManager instance.
   */
public:
  void SetParent(AsyncPanZoomController* aParent) {
    mParent = aParent;
  }

  AsyncPanZoomController* GetParent() const {
    return mParent;
  }

  /* Returns true if there is no APZC higher in the tree with the same
   * layers id.
   */
  bool HasNoParentWithSameLayersId() const {
    return !mParent || (mParent->mLayersId != mLayersId);
  }

  bool IsRootForLayersId() const {
    ReentrantMonitorAutoEnter lock(mMonitor);
    return mScrollMetadata.IsLayersIdRoot();
  }

  bool IsRootContent() const {
    ReentrantMonitorAutoEnter lock(mMonitor);
    return mFrameMetrics.IsRootContent();
  }

private:
  // |mTreeManager| belongs in this section but it's declaration is a bit
  // further above due to initialization-order constraints.

  RefPtr<AsyncPanZoomController> mParent;


  /* ===================================================================
   * The functions and members in this section are used for scrolling,
   * including handing off scroll to another APZC, and overscrolling.
   */
public:
  FrameMetrics::ViewID GetScrollHandoffParentId() const {
    return mScrollMetadata.GetScrollParentId();
  }

  /**
   * Attempt to scroll in response to a touch-move from |aStartPoint| to
   * |aEndPoint|, which are in our (transformed) screen coordinates.
   * Due to overscroll handling, there may not actually have been a touch-move
   * at these points, but this function will scroll as if there had been.
   * If this attempt causes overscroll (i.e. the layer cannot be scrolled
   * by the entire amount requested), the overscroll is passed back to the
   * tree manager via APZCTreeManager::DispatchScroll(). If the tree manager
   * does not find an APZC further in the handoff chain to accept the
   * overscroll, and this APZC is pannable, this APZC enters an overscrolled
   * state.
   * |aOverscrollHandoffChain| and |aOverscrollHandoffChainIndex| are used by
   * the tree manager to keep track of which APZC to hand off the overscroll
   * to; this function increments the chain and the index and passes it on to
   * APZCTreeManager::DispatchScroll() in the event of overscroll.
   * Returns true iff. this APZC, or an APZC further down the
   * handoff chain, accepted the scroll (possibly entering an overscrolled
   * state). If this returns false, the caller APZC knows that it should enter
   * an overscrolled state itself if it can.
   * aStartPoint and aEndPoint are modified depending on how much of the
   * scroll gesture was consumed by APZCs in the handoff chain.
   */
  bool AttemptScroll(ParentLayerPoint& aStartPoint,
                     ParentLayerPoint& aEndPoint,
                     OverscrollHandoffState& aOverscrollHandoffState);

  void FlushRepaintForOverscrollHandoff();

  /**
   * If overscrolled, start a snap-back animation and return true.
   * Otherwise return false.
   */
  bool SnapBackIfOverscrolled();

  /**
   * Build the chain of APZCs along which scroll will be handed off when
   * this APZC receives input events.
   *
   * Notes on lifetime and const-correctness:
   *   - The returned handoff chain is |const|, to indicate that it cannot be
   *     changed after being built.
   *   - When passing the chain to a function that uses it without storing it,
   *     pass it by reference-to-const (as in |const OverscrollHandoffChain&|).
   *   - When storing the chain, store it by RefPtr-to-const (as in
   *     |RefPtr<const OverscrollHandoffChain>|). This ensures the chain is
   *     kept alive. Note that queueing a task that uses the chain as an
   *     argument constitutes storing, as the task may outlive its queuer.
   *   - When passing the chain to a function that will store it, pass it as
   *     |const RefPtr<const OverscrollHandoffChain>&|. This allows the
   *     function to copy it into the |RefPtr<const OverscrollHandoffChain>|
   *     that will store it, while avoiding an unnecessary copy (and thus
   *     AddRef() and Release()) when passing it.
   */
  RefPtr<const OverscrollHandoffChain> BuildOverscrollHandoffChain();

private:
  /**
   * A helper function for calling APZCTreeManager::DispatchScroll().
   * Guards against the case where the APZC is being concurrently destroyed
   * (and thus mTreeManager is being nulled out).
   */
  void CallDispatchScroll(ParentLayerPoint& aStartPoint,
                          ParentLayerPoint& aEndPoint,
                          OverscrollHandoffState& aOverscrollHandoffState);

  /**
   * A helper function for overscrolling during panning. This is a wrapper
   * around OverscrollBy() that also implements restrictions on entering
   * overscroll based on the pan angle.
   */
  void OverscrollForPanning(ParentLayerPoint& aOverscroll,
                            const ScreenPoint& aPanDistance);

  /**
   * Try to overscroll by 'aOverscroll'.
   * If we are pannable on a particular axis, that component of 'aOverscroll'
   * is transferred to any existing overscroll.
   */
  void OverscrollBy(ParentLayerPoint& aOverscroll);


  /* ===================================================================
   * The functions and members in this section are used to maintain the
   * area that this APZC instance is responsible for. This is used when
   * hit-testing to see which APZC instance should handle touch events.
   */
public:
  void SetAncestorTransform(const Matrix4x4& aTransformToLayer) {
    mAncestorTransform = aTransformToLayer;
  }

  Matrix4x4 GetAncestorTransform() const {
    return mAncestorTransform;
  }

  // Returns whether or not this apzc contains the given screen point within
  // its composition bounds.
  bool Contains(const ScreenIntPoint& aPoint) const;

  bool IsOverscrolled() const {
    return mX.IsOverscrolled() || mY.IsOverscrolled();
  }

  bool IsInPanningState() const;

private:
  /* This is the cumulative CSS transform for all the layers from (and including)
   * the parent APZC down to (but excluding) this one, and excluding any
   * perspective transforms. */
  Matrix4x4 mAncestorTransform;


  /* ===================================================================
   * The functions and members in this section are used for sharing the
   * FrameMetrics across processes for the progressive tiling code.
   */
private:
  /* Unique id assigned to each APZC. Used with ViewID to uniquely identify
   * shared FrameMeterics used in progressive tile painting. */
  const uint32_t mAPZCId;

  RefPtr<ipc::SharedMemoryBasic> mSharedFrameMetricsBuffer;
  CrossProcessMutex* mSharedLock;
  /**
   * Called when ever mFrameMetrics is updated so that if it is being
   * shared with the content process the shared FrameMetrics may be updated.
   */
  void UpdateSharedCompositorFrameMetrics();
  /**
   * Create a shared memory buffer for containing the FrameMetrics and
   * a CrossProcessMutex that may be shared with the content process
   * for use in progressive tiled update calculations.
   */
  void ShareCompositorFrameMetrics();


  /* ===================================================================
   * The functions and members in this section are used for testing
   * and assertion purposes only.
   */
public:
  /**
   * Set an extra offset for testing async scrolling.
   */
  void SetTestAsyncScrollOffset(const CSSPoint& aPoint)
  {
    mTestAsyncScrollOffset = aPoint;
  }
  /**
   * Set an extra offset for testing async scrolling.
   */
  void SetTestAsyncZoom(const LayerToParentLayerScale& aZoom)
  {
    mTestAsyncZoom = aZoom;
  }

  void MarkAsyncTransformAppliedToContent()
  {
    mAsyncTransformAppliedToContent = true;
  }

  bool GetAsyncTransformAppliedToContent() const
  {
    return mAsyncTransformAppliedToContent;
  }

  uint64_t GetLayersId() const
  {
    return mLayersId;
  }

private:
  // Extra offset to add to the async scroll position for testing
  CSSPoint mTestAsyncScrollOffset;
  // Extra zoom to include in the aync zoom for testing
  LayerToParentLayerScale mTestAsyncZoom;
  // Flag to track whether or not the APZ transform is not used. This
  // flag is recomputed for every composition frame.
  bool mAsyncTransformAppliedToContent;


  /* ===================================================================
   * The functions and members in this section are used for checkerboard
   * recording.
   */
private:
  // Helper function to update the in-progress checkerboard event, if any.
  void UpdateCheckerboardEvent(const MutexAutoLock& aProofOfLock,
                               uint32_t aMagnitude);

  // Mutex protecting mCheckerboardEvent
  Mutex mCheckerboardEventLock;
  // This is created when this APZC instance is first included as part of a
  // composite. If a checkerboard event takes place, this is destroyed at the
  // end of the event, and a new one is created on the next composite.
  UniquePtr<CheckerboardEvent> mCheckerboardEvent;
  // This is used to track the total amount of time that we could reasonably
  // be checkerboarding. Combined with other info, this allows us to meaningfully
  // say how frequently users actually encounter checkerboarding.
  PotentialCheckerboardDurationTracker mPotentialCheckerboardTracker;


  /* ===================================================================
   * The functions in this section are used for CSS scroll snapping.
   */

  // If |aEvent| should trigger scroll snapping, adjust |aDelta| to reflect
  // the snapping (that is, make it a delta that will take us to the desired
  // snap point). The delta is interpreted as being relative to
  // |aStartPosition|, and if a target snap point is found, |aStartPosition|
  // is also updated, to the value of the snap point.
  // Returns true iff. a target snap point was found.
  bool MaybeAdjustDeltaForScrollSnapping(const ScrollWheelInput& aEvent,
                                         ParentLayerPoint& aDelta,
                                         CSSPoint& aStartPosition);

  // Snap to a snap position nearby the current scroll position, if appropriate.
  void ScrollSnap();

  // Snap to a snap position nearby the destination predicted based on the
  // current velocity, if appropriate.
  void ScrollSnapToDestination();

  // Snap to a snap position nearby the provided destination, if appropriate.
  void ScrollSnapNear(const CSSPoint& aDestination);

  // Find a snap point near |aDestination| that we should snap to.
  // Returns the snap point if one was found, or an empty Maybe otherwise.
  // |aUnit| affects the snapping behaviour (see ScrollSnapUtils::
  // GetSnapPointForDestination). It should generally be determined by the
  // type of event that's triggering the scroll.
  Maybe<CSSPoint> FindSnapPointNear(const CSSPoint& aDestination,
                                    nsIScrollableFrame::ScrollUnit aUnit);
};

} // namespace layers
} // namespace mozilla

#endif // mozilla_layers_PanZoomController_h