Momentum scrolling in flow mode

src/pyj/read_book/flow_mode.pyj

@@ -1,6 +1,6 @@
 # vim:fileencoding=utf-8
 # License: GPL v3 Copyright: 2016, Kovid Goyal <kovid at kovidgoyal.net>
-from __python__ import hash_literals
+from __python__ import hash_literals, bound_methods
 
 from dom import set_css
 from read_book.globals import get_boss
@@ -99,7 +99,56 @@ def flow_onkeydown(evt):
 def layout(is_single_page):
     set_css(document.body, margin='0', border_width='0', padding='0')
 
+class FlickAnimator:
+
+    SPEED_FACTOR = 0.04
+    DECEL_TIME_CONSTANT = 325  # milliseconds
+    VELOCITY_HISTORY = 300  # milliseconds
+    MIMUMUM_VELOCITY = 100  # pixels/sec
+
+    def __init__(self):
+        self.animation_id = None
+
+    def start(self, gesture):
+        self.vertical = gesture.axis is 'vertical'
+        now = window.performance.now()
+        points = times = None
+        for i, t in enumerate(gesture.times):
+            if now - t < self.VELOCITY_HISTORY:
+                points, times = gesture.points[i:], gesture.times[i:]
+                break
+        if times and times.length > 1:
+            elapsed = (times[-1] - times[0]) / 1000
+            if elapsed > 0 and points.length > 1:
+                delta = points[0] - points[-1]
+                velocity = delta / elapsed
+                if abs(velocity) > self.MIMUMUM_VELOCITY:
+                    self.amplitude = self.SPEED_FACTOR * velocity
+                    self.start_time = now
+                    self.animation_id = window.requestAnimationFrame(self.auto_scroll)
+
+    def auto_scroll(self, ts):
+        if self.animation_id is None:
+            return
+        elapsed = window.performance.now() - self.start_time
+        delta = self.amplitude * Math.exp(-elapsed / self.DECEL_TIME_CONSTANT)
+        if abs(delta) >= 1:
+            delta = Math.round(delta)
+            if self.vertical:
+                window.scrollBy(0, delta)
+            else:
+                window.scrollBy(delta, 0)
+            self.animation_id = window.requestAnimationFrame(self.auto_scroll)
+
+    def stop(self):
+        if self.animation_id is not None:
+            window.cancelAnimationFrame(self.animation_id)
+            self.animation_id = None
+
+flick_animator = FlickAnimator()
+
 def handle_gesture(gesture):
+    flick_animator.stop()
     if gesture.type is 'swipe':
         if gesture.points.length > 1 and not gesture.is_held:
             delta = gesture.points[-2] - gesture.points[-1]
@@ -107,5 +156,5 @@ def handle_gesture(gesture):
                 scroll_by(delta)
             else:
                 window.scrollBy(delta, 0)
-        if not gesture.active:
-            print('TODO: Implement momentum scrolling')
+        if not gesture.active and not gesture.is_held:
+            flick_animator.start(gesture)

src/pyj/read_book/touch.pyj

@@ -16,7 +16,7 @@ def copy_touch(t):
     return {
         'identifier':t.identifier,
         'page_x':v'[t.pageX]', 'page_y':v'[t.pageY]', 'viewport_x':v'[t.clientX]', 'viewport_y':v'[t.clientY]',
-        'active':True, 'mtime':now, 'ctime':now, 'is_held':False
+        'active':True, 'mtimes':v'[now]', 'ctime':now, 'is_held':False, 'x_velocity': 0, 'y_velocity': 0
     }
 
 def max_displacement(points):
@@ -47,11 +47,14 @@ def interpret_single_gesture(touch, gesture_id):
         ans.type = 'swipe'
         ans.axis = 'vertical' if delta_y > delta_x else 'horizontal'
         ans.points = pts = touch.viewport_y if ans.axis is 'vertical' else touch.viewport_x
+        ans.times = touch.mtimes
         positive = pts[-1] > pts[0]
         if ans.axis is 'vertical':
             ans.direction = 'down' if positive else 'up'
+            ans.velocity = touch.y_velocity
         else:
             ans.direction = 'right' if positive else 'left'
+            ans.velocity = touch.x_velocity
         return ans
     return ans
 
@@ -97,7 +100,7 @@ class TouchHandler:
         for tid in self.ongoing_touches:
             t = self.ongoing_touches[tid]
             if t.active:
-                if now - t.mtime > 3000:
+                if now - t.mtimes[-1] > 3000:
                     expired.push(t.identifier)
         for tid in expired:
             v'delete self.ongoing_touches[tid]'
@@ -125,7 +128,7 @@ class TouchHandler:
             found_hold = False
             for touchid in self.ongoing_touches:
                 touch = self.ongoing_touches[touchid]
-                if touch.active and now - touch.mtime > HOLD_THRESHOLD:
+                if touch.active and now - touch.mtimes[-1] > HOLD_THRESHOLD:
                     touch.is_held = True
                     found_hold = True
             if found_hold:
@@ -146,7 +149,8 @@ class TouchHandler:
             self.start_hold_timer()
 
     def update_touch(self, t, touch):
-        t.mtime = window.performance.now()
+        now = window.performance.now()
+        t.mtimes.push(now)
         t.page_x.push(touch.pageX), t.page_y.push(touch.pageY)
         t.viewport_x.push(touch.clientX), t.viewport_y.push(touch.clientY)