browser/renderer_host/chrome_render_widget_host_view_mac_history_swiper.h

// 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.

#ifndef CHROME_BROWSER_RENDERER_HOST_CHROME_RENDER_WIDGET_HOST_VIEW_MAC_HISTORY_SWIPER_
#define CHROME_BROWSER_RENDERER_HOST_CHROME_RENDER_WIDGET_HOST_VIEW_MAC_HISTORY_SWIPER_

#import <Cocoa/Cocoa.h>

namespace blink {
class WebMouseWheelEvent;
}

@class HistorySwiper;
@protocol HistorySwiperDelegate
// Return NO from this method is the view/render_widget_host should not
// allow history swiping.
- (BOOL)shouldAllowHistorySwiping;
// The history overlay is added to the view returning from this method.
- (NSView*)viewThatWantsHistoryOverlay;
@end

namespace history_swiper {
enum NavigationDirection {
  kBackwards = 0,
  kForwards,
};
enum RecognitionState {
  // Waiting to see whether the renderer will handle the event with phase
  // NSEventPhaseBegan. The state machine will also stay in this state if
  // external conditions prohibit the initialization of history swiping. New
  // gestures always start in this state.
  // Events are forwarded to the renderer.
  kPending,
  // The gesture looks like the beginning of a history swipe.
  // Events are forwarded to the renderer.
  // The history overlay is visible.
  kPotential,
  // The gesture is definitely a history swipe.
  // Events are not forwarded to the renderer.
  // The history overlay is visible.
  kTracking,
  // The history swipe gesture has finished.
  // Events are not forwarded to the renderer.
  kCompleted,
  // The history swipe gesture was cancelled.
  // Events are forwarded to the renderer.
  kCancelled,
};
} // history_swiper

// History swiping is the feature wherein a horizontal 2-finger swipe of of a
// trackpad causes the browser to navigate forwards or backwards.
// Unfortunately, the act of 2-finger swiping is overloaded, and has 3 possible
// effects. In descending order of priority, the swipe should:
//   1. Scroll the content on the web page.
//   2. Perform a history swipe.
//   3. Rubberband/overscroll the content past the edge of the window.
// Effects (1) and (3) are managed by the renderer, whereas effect (2) is
// managed by this class.
//
// Touches on the trackpad enter the run loop as NSEvents, grouped into
// gestures. The phases of NSEvents within a gesture follow a well defined
// order.
//   1. NSEventPhaseMayBegin. (exactly 1 event with this phase)
//   2. NSEventPhaseBegan. (exactly 1 event with this phase)
//   3. NSEventPhaseMoved. (many events with this phase)
//   4. NSEventPhaseEnded. (exactly 1 event with this phase)
// Events with the phase NSEventPhaseCancelled may come in at any time, and
// generally mean that an entity within the Cocoa framework has consumed the
// gesture, and wants to "cancel" previous NSEvents that have been passed to
// this class.
//
// The event handling stack in Chrome passes all events to this class, which is
// given the opportunity to process and consume the event. If the event is not
// consumed, it is passed to the renderer via IPC. The renderer returns an IPC
// indicating whether the event was consumed. To prevent spamming the renderer
// with IPCs, the browser waits for an ACK from the renderer from the previous
// event before sending the next one. While waiting for an ACK, the browser
// coalesces NSEvents with the same phase. It is common for dozens of events
// with the phase NSEventPhaseMoved to be coalesced.
//
// It is difficult to determine from the initial events in a gesture whether
// the gesture was intended to be a history swipe. The loss of information from
// the coalescing of events with phase NSEventPhaseMoved before they are passed
// to the renderer is also problematic. The general approach is as follows:
//   1. Wait for the renderer to return an ACK for the event with phase
//   NSEventPhaseBegan. If that event was not consumed, change the state to
//   kPotential.  If the renderer is not certain about whether the event should
//   be consumed, it tries to not consume the event.
//   2. In the state kPotential, this class will process events and update its
//   internal state machine, but it will also continue to pass events to the
//   renderer. This class tries to aggressively cancel history swiping to make
//   up for the fact that the renderer errs on the side of allowing history
//   swiping to occur.
//   3. As more events come in, if the gesture continues to appear horizontal,
//   then this class will transition to the state kTracking. Events are
//   consumed, and not passed to the renderer.
//
// There are multiple APIs that provide information about gestures on the
// trackpad. This class uses two different set of APIs.
//   1. The -[NSView touches*WithEvent:] APIs provide detailed information
//   about the touches within a gesture. The callbacks happen with more
//   frequency, and have higher accuracy. These APIs are used to transition
//   between all state, exception for kPending -> kPotential.
//   2. The -[NSView scrollWheel:] API provides less information, but the
//   events are passed to the renderer. This class must process these events so
//   that it can decide whether to consume the events and prevent them from
//   being passed to the renderer. This API is used to transition from kPending
//   -> kPotential.
@class HistoryOverlayController;
@interface HistorySwiper : NSObject {
 @private
  // This controller will exist if and only if the UI is in history swipe mode.
  HistoryOverlayController* historyOverlay_;
  // Each gesture received by the window is given a unique id.
  // The id is monotonically increasing.
  int currentGestureId_;
  // The location of the fingers when the gesture started.
  NSPoint gestureStartPoint_;
  // The current location of the fingers in the gesture.
  NSPoint gestureCurrentPoint_;
  // A flag that indicates that there is an ongoing gesture.
  // The method [NSEvent touchesMatchingPhase:inView:] is only valid for events
  // that are part of a gesture.
  BOOL inGesture_;
  // Each time a new gesture begins, we must get a new start point.
  // This ivar determines whether the start point is valid.
  int gestureStartPointValid_;
  // The id of the last gesture that we processed as a history swipe.
  int lastProcessedGestureId_;
  // A flag that indicates the user's intended direction with the history swipe.
  history_swiper::NavigationDirection historySwipeDirection_;
  // A flag that indicates whether the gesture has its direction inverted.
  BOOL historySwipeDirectionInverted_;

  // Whether the event with phase NSEventPhaseBegan was not consumed by the
  // renderer. This variables defaults to NO for new gestures.
  BOOL beganEventUnconsumed_;
  history_swiper::RecognitionState recognitionState_;

  id<HistorySwiperDelegate> delegate_;

  // Magic mouse and touchpad swipe events are identical except magic mouse
  // events do not generate NSTouch callbacks. Since we rely on NSTouch
  // callbacks to determine vertical scrolling, magic mouse swipe events use an
  // entirely different set of logic.
  //
  // The two event types do not play well together. Just calling the API
  // `[NSEvent trackSwipeEventWithOptions:]` will block touches{Began, Moved, *}
  // callbacks for a non-deterministic period of time (even after the swipe has
  // completed).
  BOOL receivedTouch_;
  // Cumulative scroll delta since scroll gesture start. Only valid during
  // scroll gesture handling. Used for history swiping.
  NSSize mouseScrollDelta_;
}

// Many event types are passed in, but the only one we care about is
// NSScrollWheel. We look at the phase to determine whether to trigger history
// swiping
- (BOOL)handleEvent:(NSEvent*)event;
- (void)rendererHandledWheelEvent:(const blink::WebMouseWheelEvent&)event
                         consumed:(BOOL)consumed;

// The event passed in is a gesture event, and has touch data associated with
// the trackpad.
- (void)touchesBeganWithEvent:(NSEvent*)event;
- (void)touchesMovedWithEvent:(NSEvent*)event;
- (void)touchesCancelledWithEvent:(NSEvent*)event;
- (void)touchesEndedWithEvent:(NSEvent*)event;
- (void)beginGestureWithEvent:(NSEvent*)event;
- (void)endGestureWithEvent:(NSEvent*)event;

// These methods control whether a given view is allowed to rubberband in the
// given direction. This is inversely related to whether the view is allowed to
// 2-finger history swipe in the given direction.
- (BOOL)canRubberbandLeft:(NSView*)view;
- (BOOL)canRubberbandRight:(NSView*)view;

// Designated initializer.
- (id)initWithDelegate:(id<HistorySwiperDelegate>)delegate;

@property (nonatomic, assign) id<HistorySwiperDelegate> delegate;

@end

// Exposed only for unit testing, do not call directly.
@interface HistorySwiper (PrivateExposedForTesting)
+ (void)resetMagicMouseState;
@end

#endif // CHROME_BROWSER_RENDERER_HOST_CHROME_RENDER_WIDGET_HOST_VIEW_MAC_HISTORY_SWIPER_