Implemented auto select on scroll feature.

demo/src/main/java/com/ogaclejapan/smarttablayout/demo/Demo.java

@@ -25,6 +25,8 @@ public int[] tabs() {
     }
   },
 
+  AUTOSELECT_ON_SCROLL(R.string.demo_title_autoselect_on_scroll, R.layout.demo_autoselect_on_scroll),
+
   ALWAYS_IN_CENTER(R.string.demo_title_always_in_center, R.layout.demo_always_in_center),
 
   CUSTOM_TAB(R.string.demo_title_custom_tab_text, R.layout.demo_custom_tab_text),

demo/src/main/res/layout/demo_autoselect_on_scroll.xml

@@ -0,0 +1,18 @@
+<com.ogaclejapan.smarttablayout.SmartTabLayout
+  xmlns:android="http://schemas.android.com/apk/res/android"
+  xmlns:app="http://schemas.android.com/apk/res-auto"
+  android:id="@id/viewpagertab"
+  android:layout_width="match_parent"
+  android:layout_height="@dimen/tab_height"
+  android:background="@color/primary"
+  app:stl_defaultTabTextColor="@color/white"
+  app:stl_defaultTabTextHorizontalPadding="24dp"
+  app:stl_indicatorColor="@color/accent"
+  app:stl_indicatorInterpolation="linear"
+  app:stl_indicatorThickness="3dp"
+  app:stl_underlineThickness="1dp"
+  app:stl_autoSelectOnScroll="true"
+  app:stl_indicatorAlwaysInCenter="true"
+  />
+
+

demo/src/main/res/values/strings.xml

@@ -10,6 +10,7 @@
   <string name="demo_title_basic2">Basic(Title Offset Auto Center)</string>
   <string name="demo_title_smart_indicator">Smart Indicator</string>
   <string name="demo_title_distribute_evenly">Distribute Evenly</string>
+  <string name="demo_title_autoselect_on_scroll">Auto Select On Scroll</string>
   <string name="demo_title_always_in_center">Always In Center</string>
   <string name="demo_title_custom_tab_text">Custom Tab Text</string>
   <string name="demo_title_custom_tab_margin">Custom Tab Margin (since 1.2~)</string>

library/src/main/java/com/ogaclejapan/smarttablayout/SmartTabLayout.java

@@ -26,9 +26,11 @@
 import android.support.v4.view.ViewPager;
 import android.util.AttributeSet;
 import android.util.DisplayMetrics;
+import android.util.SparseIntArray;
 import android.util.TypedValue;
 import android.view.Gravity;
 import android.view.LayoutInflater;
+import android.view.MotionEvent;
 import android.view.View;
 import android.view.ViewGroup;
 import android.widget.HorizontalScrollView;
@@ -85,6 +87,8 @@ public class SmartTabLayout extends HorizontalScrollView {
   private InternalTabClickListener internalTabClickListener;
   private OnTabClickListener onTabClickListener;
   private boolean distributeEvenly;
+  private boolean autoSelectOnScroll;
+  private SparseIntArray offsetArray = new SparseIntArray();
 
   public SmartTabLayout(Context context) {
     this(context, null);
@@ -114,6 +118,7 @@ public SmartTabLayout(Context context, AttributeSet attrs, int defStyle) {
     int customTabLayoutId = NO_ID;
     int customTabTextViewId = NO_ID;
     boolean clickable = TAB_CLICKABLE;
+    boolean autoSelectOnScroll = false;
     int titleOffset = (int) (TITLE_OFFSET_DIPS * density);
 
     TypedArray a = context.obtainStyledAttributes(
@@ -140,6 +145,8 @@ public SmartTabLayout(Context context, AttributeSet attrs, int defStyle) {
         R.styleable.stl_SmartTabLayout_stl_clickable, clickable);
     titleOffset = a.getLayoutDimension(
         R.styleable.stl_SmartTabLayout_stl_titleOffset, titleOffset);
+    autoSelectOnScroll = a.getBoolean(
+            R.styleable.stl_SmartTabLayout_stl_autoSelectOnScroll, autoSelectOnScroll);
     a.recycle();
 
     this.titleOffset = titleOffset;
@@ -153,6 +160,7 @@ public SmartTabLayout(Context context, AttributeSet attrs, int defStyle) {
     this.tabViewTextMinWidth = textMinWidth;
     this.internalTabClickListener = clickable ? new InternalTabClickListener() : null;
     this.distributeEvenly = distributeEvenly;
+    this.autoSelectOnScroll = autoSelectOnScroll;
 
     if (customTabLayoutId != NO_ID) {
       setCustomTabView(customTabLayoutId, customTabTextViewId);
@@ -165,13 +173,39 @@ public SmartTabLayout(Context context, AttributeSet attrs, int defStyle) {
           "'distributeEvenly' and 'indicatorAlwaysInCenter' both use does not support");
     }
 
+    if (autoSelectOnScroll && !tabStrip.isIndicatorAlwaysInCenter()) {
+      throw new UnsupportedOperationException(
+              "'autoSelectOnScroll' and 'indicatorAlwaysInCenter' both should be enabled");
+    }
+
     // Make sure that the Tab Strips fills this View
     setFillViewport(!tabStrip.isIndicatorAlwaysInCenter());
 
     addView(tabStrip, LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
 
   }
 
+  @Override
+  public boolean onTouchEvent(MotionEvent ev) {
+    if (autoSelectOnScroll) {
+      if (ev.getAction() == MotionEvent.ACTION_UP || ev.getAction() == MotionEvent.ACTION_CANCEL) {
+        int scrollX = getScrollX();
+        int currPos = viewPager.getCurrentItem();
+        int featureWidth = getTabAt(currPos).getMeasuredWidth();
+        // Calculate the compensation to add/remove to the scroll for tabs with different widths
+        for (int i = 0; i < currPos; i++) {
+          scrollX += (featureWidth - getTabAt(i).getMeasuredWidth());
+        }
+        int activeTab = ((scrollX + (featureWidth / 2)) / featureWidth);
+        int scrollTo = offsetArray.get(activeTab);
+        smoothScrollTo(scrollTo, 0);
+        viewPager.setCurrentItem(activeTab);
+        return true;
+      }
+    }
+    return super.onTouchEvent(ev);
+  }
+
   @Override
   protected void onScrollChanged(int l, int t, int oldl, int oldt) {
     super.onScrollChanged(l, t, oldl, oldt);
@@ -201,6 +235,14 @@ protected void onLayout(boolean changed, int l, int t, int r, int b) {
     if (changed && viewPager != null) {
       scrollToTab(viewPager.getCurrentItem(), 0);
     }
+    if (autoSelectOnScroll) {
+      int totalWidth = 0;
+      offsetArray.put(0, totalWidth);
+      for (int i = 1; i < tabStrip.getChildCount(); i++) {
+        totalWidth += getTabAt(i - 1).getMeasuredWidth();
+        offsetArray.put(i, totalWidth);
+      }
+    }
   }
 
   /**
@@ -299,7 +341,7 @@ public void setOnTabClickListener(OnTabClickListener listener) {
    * Set the custom layout to be inflated for the tab views.
    *
    * @param layoutResId Layout id to be inflated
-   * @param textViewId id of the {@link android.widget.TextView} in the inflated view
+   * @param textViewId id of the {@link TextView} in the inflated view
    */
   public void setCustomTabView(int layoutResId, int textViewId) {
     tabProvider = new SimpleTabProvider(getContext(), layoutResId, textViewId);

library/src/main/res/values/attrs.xml

@@ -41,5 +41,6 @@
       <enum name="auto_center" value="-1"/>
     </attr>
     <attr name="stl_drawDecorationAfterTab" format="boolean"/>
+    <attr name="stl_autoSelectOnScroll" format="boolean"/>
   </declare-styleable>
 </resources>