[gnome-shell] appDisplay: Make page panning smoother on touch

[Jasper St. Pierre <jstpierre src gnome org>]

commit 28c1f81f4a16034109214929e56ddfe650bdd920
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Sun Apr 27 11:18:04 2014 -0400

    appDisplay: Make page panning smoother on touch
    
    Currently, our logic for page panning isn't great. If the user starts a
    pan upwards and hesitates over a new page, we'll go to the *next* page
    on release, since the difference is greater, but the velocity wound down
    to 0.
    
    Instead of trying to treat it like page down or scrolls, simply do the
    math to find the page where the user scrolled to.
    
    This is unfortunately broken for fast swipes, since the user doesn't get
    far enough into the new page to make a difference. I'm getting the
    impression we'll need a gesture recognizer for this, though, however
    crude. Simple hacks I tried, like a velocity multiplier, didn't work
    properly.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=729064

 js/ui/appDisplay.js |   26 ++++++++++++++------------
 js/ui/iconGrid.js   |    4 ++++
 2 files changed, 18 insertions(+), 12 deletions(-)
---
diff --git a/js/ui/appDisplay.js b/js/ui/appDisplay.js
index 0f5baf8..e04c59a 100644
--- a/js/ui/appDisplay.js
+++ b/js/ui/appDisplay.js
@@ -41,9 +41,7 @@ const MIN_FREQUENT_APPS_COUNT = 3;
 const INDICATORS_BASE_TIME = 0.25;
 const INDICATORS_ANIMATION_DELAY = 0.125;
 const INDICATORS_ANIMATION_MAX_TIME = 0.75;
-// Fraction of page height the finger or mouse must reach
-// to change page
-const PAGE_SWITCH_TRESHOLD = 0.2;
+
 const PAGE_SWITCH_TIME = 0.3;
 
 const VIEWS_SWITCH_TIME = 0.4;
@@ -528,15 +526,19 @@ const AllView = new Lang.Class({
     _onPanEnd: function(action) {
          if (this._displayingPopup)
             return;
-        let diffCurrentPage = this._diffToPage(this._currentPage);
-        if (diffCurrentPage > this._scrollView.height * PAGE_SWITCH_TRESHOLD) {
-            if (action.get_velocity(0)[2] > 0)
-                this.goToPage(this._currentPage - 1);
-            else
-                this.goToPage(this._currentPage + 1);
-        } else {
-            this.goToPage(this._currentPage);
-        }
+
+        let pageHeight = this._grid.getPageHeight();
+
+        // Calculate the scroll value we'd be at, which is our current
+        // scroll plus any velocity the user had when they released
+        // their finger.
+
+        let velocity = -action.get_velocity(0)[2];
+        let endPanValue = this._adjustment.value + velocity;
+
+        let closestPage = Math.round(endPanValue / pageHeight);
+        this.goToPage(closestPage);
+
         this._panning = false;
     },
 
diff --git a/js/ui/iconGrid.js b/js/ui/iconGrid.js
index 6bbbcb7..16c58e9 100644
--- a/js/ui/iconGrid.js
+++ b/js/ui/iconGrid.js
@@ -645,6 +645,10 @@ const PaginatedIconGrid = new Lang.Class({
         return this._nPages;
     },
 
+    getPageHeight: function() {
+        return this._availableHeightPerPageForItems();
+    },
+
     getPageY: function(pageNumber) {
         if (!this._nPages)
             return 0;