Support longpress drag selection

ui/touch_selection/longpress_drag_selector.h

Index: ui/touch_selection/longpress_drag_selector.h
diff --git a/ui/touch_selection/longpress_drag_selector.h b/ui/touch_selection/longpress_drag_selector.h
new file mode 100644
index 0000000000000000000000000000000000000000..922ce6fe2ae505efdc6ef531a67efe88cbae3493
--- /dev/null
+++ b/ui/touch_selection/longpress_drag_selector.h
@@ -0,0 +1,72 @@
+// Copyright 2015 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 UI_TOUCH_SELECTION_LONGPRESS_DRAG_SELECTOR_H_
+#define UI_TOUCH_SELECTION_LONGPRESS_DRAG_SELECTOR_H_
+
+#include "base/time/time.h"
+#include "ui/gfx/geometry/point_f.h"
+#include "ui/gfx/geometry/vector2d_f.h"
+#include "ui/touch_selection/touch_selection_draggable.h"
+#include "ui/touch_selection/ui_touch_selection_export.h"
+
+namespace ui {
+
+class MotionEvent;
+
+// Supports text selection via touch dragging after a longpress-initiated
+// selection.
+class UI_TOUCH_SELECTION_EXPORT LongPressDragSelector
+    : public TouchSelectionDraggable {
+ public:
+  LongPressDragSelector(TouchSelectionDraggableClient* client,
+                        float slop_length);

[mfomitchev, 2015/04/23 21:05:53]

Can we put GetTapSlop() into TouchSelectionDraggableClient and use that instead
of passing to the constructor?

[jdduke (slow), 2015/04/27 20:24:25]

Done (added a |IsWithinTapSlop| client routine).

+  ~LongPressDragSelector() override;
+
+  // Allows selection dragging after a long-press initiated selection. Returns
+  // true if the event was consumed, in which case the caller should cease
+  // further handling.
+  bool WillHandleTouchEvent(const MotionEvent& event);
+
+  // Called when a longpress event is observed, before any of its side effects
+  // (e.g., selection activation) have been realized.
+  void OnLongPressEvent(base::TimeTicks event_time,
+                        const gfx::PointF& position);
+
+  // Called when the active selection changes.
+  void OnSelectionActivated(const gfx::PointF& start, const gfx::PointF& end);
+  void OnSelectionDeactivated();
+
+  // Whether a drag is active or being detected for the current touch sequence.
+  bool IsActive() const;
+
+ private:
+  enum SelectionState {
+    INACTIVE,
+    LONGPRESS_PENDING,
+    SELECTION_PENDING,
+    DRAG_PENDING,
+    DRAGGING
+  };
+
+  void SetState(SelectionState state);
+
+  TouchSelectionDraggableClient* const client_;
+  const float slop_length_;
+
+  SelectionState state_;
+
+  gfx::PointF selection_start_;

[mfomitchev, 2015/04/23 21:05:53]

Hmm.. this is not DRY (http://en.wikipedia.org/wiki/Don%27t_repeat_yourself).
Any good reason we don't add GetSelectionStart()/GetSelectionEnd() to 
TouchSelectionDraggableClient instead?

[jdduke (slow), 2015/04/27 20:24:25]

"DRY" is a guideline, not a religion. I consider this change to be of the
caching type, which doesn't violate "DRY"; there's still one true source of
truth and the ancestry is clear. Moreover, I'm not sure polling the selection is
any more clear than getting a "push", as we're really interested in the
coordinates when the selection is first shown. In general, I tend to prefer
"KISS" (I too can quote wikipedia...
http://en.wikipedia.org/wiki/KISS_principle) =/.

What I can do is put the getters on a LongPressDragSelector-specific interface
(I wanted to avoid creating one, but maybe that's just laziness).

[mfomitchev, 2015/04/28 02:17:30]

Sorry if I came off as a design snob, I guess it's hard not to do it when
linking to wiki on design terms.
I don't think it was just laziness :) I am definitely not a fan of adding an
extra interface here if it could be avoided. If we can get rid of
OnLongPressDragDetectionStateChanged() (I posted a comment on it separately), I
think we should nuke LongPressDragSelectorClient. 

Assuming we can get rid of OnLongPressDragDetectionStateChanged(), IMHO we can
put the getters into TouchSelectionDraggableClient. I mean if you are okay with
getting those through LongPressDragSelectorClient, getting them though
TouchSelectionDraggableClient should be okay as well? If you don't like it - I
am fine with caching.

[jdduke (slow), 2015/04/28 20:37:48]

Too late :) It's possible we already needed
OnLongPressDragDetectionStateChanged() as its semantics are slightly different
than OnDragBegin/End, and otherwise we'd have to poll the drag detection state
in random places. So maybe the new client interface isn't such a bad thing.

+  gfx::PointF selection_end_;
+
+  gfx::PointF longpress_drag_start_position_;
+  gfx::Vector2dF longpress_drag_selection_offset_;
+
+  base::TimeTicks touch_down_time_;
+  gfx::PointF touch_down_position_;
+};
+
+}  // namespace ui
+
+#endif  // UI_TOUCH_SELECTION_LONGPRESS_DRAG_SELECTOR_H_