[gnome-shell/wip/exalm/long-swipes: 4/7] swipeTracker: Calculate velocity using scroll history

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 92bc21c9c59c33d33b8344aa18042bf9d8c46a90
Author: Alexander Mikhaylenko <exalm7659 gmail com>
Date:   Sun Feb 7 22:46:49 2021 +0500

    swipeTracker: Calculate velocity using scroll history
    
    In some cases we may get anevent with very low delta at the end of the
    swipe. While we can't completely ignore them, we can smooth them using a
    scroll history, similarly to what GTK kinetic scrolling does: keep track
    of the last 150ms of events, and sum their deltas when calculating the
    velocity.
    
    The logic is based on what GTK does in GtkGestureSwipe and
    GtkEventControllerScroll.

 js/ui/swipeTracker.js | 75 ++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 56 insertions(+), 19 deletions(-)
---
diff --git a/js/ui/swipeTracker.js b/js/ui/swipeTracker.js
index 2ce054ab19..bf990b07e2 100644
--- a/js/ui/swipeTracker.js
+++ b/js/ui/swipeTracker.js
@@ -13,6 +13,8 @@ const Params = imports.misc.params;
 const TOUCHPAD_BASE_HEIGHT = 300;
 const TOUCHPAD_BASE_WIDTH = 400;
 
+const EVENT_HISTORY_THRESHOLD_MS = 150;
+
 const SCROLL_MULTIPLIER = 10;
 const SWIPE_MULTIPLIER = 0.5;
 
@@ -30,6 +32,45 @@ const State = {
     SCROLLING: 1,
 };
 
+const EventHistory = class {
+    constructor() {
+        this.reset();
+    }
+
+    reset() {
+        this._data = [];
+    }
+
+    trim(time) {
+        const thresholdTime = time - EVENT_HISTORY_THRESHOLD_MS;
+        const index = this._data.findIndex(r => r.time >= thresholdTime);
+
+        this._data.splice(0, index);
+    }
+
+    append(time, delta) {
+        this.trim(time);
+
+        this._data.push({ time, delta });
+    }
+
+    calculateVelocity() {
+        if (this._data.length < 2)
+            return 0;
+
+        const firstTime = this._data[0].time;
+        const lastTime = this._data[this._data.length - 1].time;
+
+        if (firstTime === lastTime)
+            return 0;
+
+        const totalDelta = this._data.slice(1).map(a => a.delta).reduce((a, b) => a + b);
+        const period = lastTime - firstTime;
+
+        return totalDelta / period;
+    }
+};
+
 const TouchpadSwipeGesture = GObject.registerClass({
     Properties: {
         'enabled': GObject.ParamSpec.boolean(
@@ -362,7 +403,7 @@ var SwipeTracker = GObject.registerClass({
         this._allowedModes = allowedModes;
         this._enabled = true;
         this._distance = global.screen_height;
-
+        this._history = new EventHistory();
         this._reset();
 
         this._touchpadGesture = new TouchpadSwipeGesture(allowedModes);
@@ -464,10 +505,9 @@ var SwipeTracker = GObject.registerClass({
         this._prevOffset = 0;
         this._progress = 0;
 
-        this._prevTime = 0;
-        this._velocity = 0;
-
         this._cancelled = false;
+
+        this._history.reset();
     }
 
     _interrupt() {
@@ -486,7 +526,7 @@ var SwipeTracker = GObject.registerClass({
         if (this._state === State.SCROLLING)
             return;
 
-        this._prevTime = time;
+        this._history.append(time, 0);
 
         let rect = new Meta.Rectangle({ x, y, width: 1, height: 1 });
         let monitor = global.display.get_monitor_index_for_rect(rect);
@@ -508,9 +548,7 @@ var SwipeTracker = GObject.registerClass({
             delta = -delta;
 
         this._progress += delta;
-
-        if (time !== this._prevTime)
-            this._velocity = delta / (time - this._prevTime);
+        this._history.append(time, delta);
 
         let firstPoint = this._snapPoints[0];
         let lastPoint = this._snapPoints[this._snapPoints.length - 1];
@@ -519,8 +557,6 @@ var SwipeTracker = GObject.registerClass({
             this._initialProgress - 1, this._initialProgress + 1);
 
         this.emit('update', this._progress);
-
-        this._prevTime = time;
     }
 
     _getClosestSnapPoints() {
@@ -529,7 +565,7 @@ var SwipeTracker = GObject.registerClass({
         return [lower, upper];
     }
 
-    _getEndProgress() {
+    _getEndProgress(velocity) {
         if (this._cancelled)
             return this._cancelProgress;
 
@@ -537,15 +573,15 @@ var SwipeTracker = GObject.registerClass({
         let middle = (upper + lower) / 2;
 
         if (this._progress > middle) {
-            let thresholdMet = this._velocity * this._distance > -VELOCITY_THRESHOLD;
+            const thresholdMet = velocity * this._distance > -VELOCITY_THRESHOLD;
             return thresholdMet || this._initialProgress > upper ? upper : lower;
         } else {
-            let thresholdMet = this._velocity * this._distance < VELOCITY_THRESHOLD;
+            const thresholdMet = velocity * this._distance < VELOCITY_THRESHOLD;
             return thresholdMet || this._initialProgress < lower ? lower : upper;
         }
     }
 
-    _endGesture(_gesture, _time) {
+    _endGesture(_gesture, time) {
         if (this._state !== State.SCROLLING)
             return;
 
@@ -554,11 +590,13 @@ var SwipeTracker = GObject.registerClass({
             return;
         }
 
-        let endProgress = this._getEndProgress();
+        this._history.trim(time);
+
+        let velocity = this._history.calculateVelocity();
+        const endProgress = this._getEndProgress(velocity);
 
-        let velocity = ANIMATION_BASE_VELOCITY;
-        if ((endProgress - this._progress) * this._velocity > 0)
-            velocity = this._velocity;
+        if ((endProgress - this._progress) * velocity <= 0)
+            velocity = ANIMATION_BASE_VELOCITY;
 
         let duration = Math.abs((this._progress - endProgress) / velocity * DURATION_MULTIPLIER);
         if (duration > 0) {
@@ -600,7 +638,6 @@ var SwipeTracker = GObject.registerClass({
         this._progress = currentProgress;
         this._cancelProgress = cancelProgress;
 
-        this._velocity = 0;
         this._state = State.SCROLLING;
     }