From 6b30e21f4ee4d21d6e549a86c6726dba399e2b03 Mon Sep 17 00:00:00 2001
From: Markus Webel <markus.webel@tum.de>
Date: Sun, 6 Sep 2020 06:57:29 +0200
Subject: [PATCH] Kinetic scrolling (#20)

* Fixed typo in function description

* Added vertical kinetic scrolling

* Checked off kinetic scrolling from TODO

Co-authored-by: Markus Webel <m@rkus.online>
---
 TODO.md                            |  2 +-
 egui/src/containers/area.rs        |  2 +-
 egui/src/containers/scroll_area.rs | 25 ++++++++++++++++++++++++-
 3 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/TODO.md b/TODO.md
index fffd75e2..a35c9975 100644
--- a/TODO.md
+++ b/TODO.md
@@ -18,7 +18,7 @@ TODO-list for the Egui project. If you looking for something to do, look here.
         * [x] Scroll-wheel input
         * [x] Drag background to scroll
         * [ ] Horizontal scrolling
-        * [ ] Kinetic scrolling
+        * [X] Kinetic scrolling
 * [ ] Text
     * [ ] Unicode
         * [ ] Shared mutable expanding texture map?
diff --git a/egui/src/containers/area.rs b/egui/src/containers/area.rs
index 4ec5f93d..cb0d81c3 100644
--- a/egui/src/containers/area.rs
+++ b/egui/src/containers/area.rs
@@ -32,7 +32,7 @@ impl State {
     }
 }
 
-/// An area on the screen that can be move by dragging.
+/// An area on the screen that can be moved by dragging.
 ///
 /// This forms the base of the `Window` container.
 #[derive(Clone, Copy, Debug)]
diff --git a/egui/src/containers/scroll_area.rs b/egui/src/containers/scroll_area.rs
index 105240e4..c29c622b 100644
--- a/egui/src/containers/scroll_area.rs
+++ b/egui/src/containers/scroll_area.rs
@@ -8,6 +8,10 @@ pub(crate) struct State {
     offset: Vec2,
 
     show_scroll: bool,
+
+    /// Momentum, used for kinetic scrolling
+    #[cfg_attr(feature = "serde", serde(skip))]
+    pub vel: Vec2,
 }
 
 impl Default for State {
@@ -15,6 +19,7 @@ impl Default for State {
         Self {
             offset: Vec2::zero(),
             show_scroll: false,
+            vel: Vec2::zero(),
         }
     }
 }
@@ -153,8 +158,26 @@ impl Prepared {
         if content_is_too_small {
             // Drag contents to scroll (for touch screens mostly):
             let content_response = ui.interact(inner_rect, id.with("area"), Sense::drag());
+
+            let input = ui.input();
             if content_response.active {
-                state.offset.y -= ui.input().mouse.delta.y;
+                state.offset.y -= input.mouse.delta.y;
+                state.vel = input.mouse.velocity;
+            } else {
+                let stop_speed = 20.0; // Pixels per second.
+                let friction_coeff = 1000.0; // Pixels per second squared.
+                let dt = input.unstable_dt;
+
+                let friction = friction_coeff * dt;
+                if friction > state.vel.length() || state.vel.length() < stop_speed {
+                    state.vel = Vec2::zero();
+                } else {
+                    state.vel -= friction * state.vel.normalized();
+                    // Offset has an inverted coordinate system compared to
+                    // the velocity, so we subtract it instead of adding it
+                    state.offset.y -= state.vel.y * dt;
+                    ui.ctx().request_repaint();
+                }
             }
         }