Lower touch slop in ViewConfiguration.

Our typical touchscreens are now good enough that we don't need the
touch slop to be as large as it once was. Lower it to increase
responsiveness while scrolling.

Move the touch slop constant to the config resource
config_viewConfigurationTouchSlop so that it may be more easily
tweaked in device-specific overlays.

Change-Id: I7c11d8affcb98e91654203beb13a26cfeec18f7c

Author: adamp

core/java/android/view/ViewConfiguration.java

@@ -139,13 +139,29 @@ public class ViewConfiguration {
     private static final int EDGE_SLOP = 12;
 
     /**
-     * Distance a touch can wander before we think the user is scrolling in pixels
+     * Distance a touch can wander before we think the user is scrolling in dips.
+     * Note that this value defined here is only used as a fallback by legacy/misbehaving
+     * applications that do not provide a Context for determining density/configuration-dependent
+     * values.
+     *
+     * To alter this value, see the configuration resource config_viewConfigurationTouchSlop
+     * in frameworks/base/core/res/res/values/config.xml or the appropriate device resource overlay.
+     * It may be appropriate to tweak this on a device-specific basis in an overlay based on
+     * the characteristics of the touch panel and firmware.
      */
-    private static final int TOUCH_SLOP = 16;
+    private static final int TOUCH_SLOP = 4;
 
     /**
      * Distance a touch can wander before we think the user is attempting a paged scroll
      * (in dips)
+     *
+     * Note that this value defined here is only used as a fallback by legacy/misbehaving
+     * applications that do not provide a Context for determining density/configuration-dependent
+     * values.
+     *
+     * See the note above on {@link #TOUCH_SLOP} regarding the dimen resource
+     * config_viewConfigurationTouchSlop. ViewConfiguration will report a paging touch slop of
+     * config_viewConfigurationTouchSlop * 2 when provided with a Context.
      */
     private static final int PAGING_TOUCH_SLOP = TOUCH_SLOP * 2;
 
@@ -277,8 +293,6 @@ private ViewConfiguration(Context context) {
         mMinimumFlingVelocity = (int) (density * MINIMUM_FLING_VELOCITY + 0.5f);
         mMaximumFlingVelocity = (int) (density * MAXIMUM_FLING_VELOCITY + 0.5f);
         mScrollbarSize = (int) (density * SCROLL_BAR_SIZE + 0.5f);
-        mTouchSlop = (int) (sizeAndDensity * TOUCH_SLOP + 0.5f);
-        mPagingTouchSlop = (int) (sizeAndDensity * PAGING_TOUCH_SLOP + 0.5f);
         mDoubleTapSlop = (int) (sizeAndDensity * DOUBLE_TAP_SLOP + 0.5f);
         mScaledTouchExplorationTapSlop = (int) (density * TOUCH_EXPLORATION_TAP_SLOP + 0.5f);
         mWindowTouchSlop = (int) (sizeAndDensity * WINDOW_TOUCH_SLOP + 0.5f);
@@ -301,6 +315,9 @@ private ViewConfiguration(Context context) {
 
         mFadingMarqueeEnabled = res.getBoolean(
                 com.android.internal.R.bool.config_ui_enableFadingMarquee);
+        mTouchSlop = res.getDimensionPixelSize(
+                com.android.internal.R.dimen.config_viewConfigurationTouchSlop);
+        mPagingTouchSlop = mTouchSlop * 2;
     }
 
     /**

core/res/res/values/config.xml

@@ -741,4 +741,8 @@
     <string name="config_wimaxServiceClassname"></string>
     <!-- Name of the wimax state tracker clas -->
     <string name="config_wimaxStateTrackerClassname"></string>
+
+    <!-- Base "touch slop" value used by ViewConfiguration as a
+         movement threshold where scrolling should begin. -->
+    <dimen name="config_viewConfigurationTouchSlop">4dp</dimen>
 </resources>