1 files changed, 92 insertions, 0 deletions

diff --git a/tab-layout-helper/src/main/java/it/niedermann/android/tablayouthelper/FixedTabLayoutOnPageChangeListener.java b/tab-layout-helper/src/main/java/it/niedermann/android/tablayouthelper/FixedTabLayoutOnPageChangeListener.java
new file mode 100644
index 000000000..c3d33d083
--- /dev/null
+++ b/tab-layout-helper/src/main/java/it/niedermann/android/tablayouthelper/FixedTabLayoutOnPageChangeListener.java
@@ -0,0 +1,92 @@
+package it.niedermann.android.tablayouthelper;
+
+import android.util.Log;
+
+import androidx.viewpager2.widget.ViewPager2;
+
+import com.google.android.material.tabs.TabLayout;
+
+import java.lang.ref.WeakReference;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+
+class FixedTabLayoutOnPageChangeListener extends ViewPager2.OnPageChangeCallback {
+    private final WeakReference<TabLayout> mTabLayoutRef;
+    private int mPreviousScrollState;
+    private int mScrollState;
+
+    FixedTabLayoutOnPageChangeListener(TabLayout tabLayout) {
+        mTabLayoutRef = new WeakReference<>(tabLayout);
+    }
+
+    @Override
+    public void onPageScrollStateChanged(int state) {
+        mPreviousScrollState = mScrollState;
+        mScrollState = state;
+    }
+
+    @Override
+    public void onPageScrolled(int position, float positionOffset,
+                               int positionOffsetPixels) {
+        final TabLayout tabLayout = mTabLayoutRef.get();
+        if (tabLayout != null && shouldUpdateScrollPosition()) {
+            // Update the scroll position, only update the text selection if we're being
+            // dragged (or we're settling after a drag)
+            tabLayout.setScrollPosition(position, positionOffset, true);
+        }
+    }
+
+    @Override
+    public void onPageSelected(int position) {
+        final TabLayout tabLayout = mTabLayoutRef.get();
+        if (tabLayout != null && tabLayout.getSelectedTabPosition() != position) {
+            // Select the tab, only updating the indicator if we're not being dragged/settled
+            // (since onPageScrolled will handle that).
+            Internal.selectTab(tabLayout, tabLayout.getTabAt(position),
+                    mScrollState == ViewPager2.SCROLL_STATE_IDLE);
+        }
+    }
+
+    private boolean shouldUpdateScrollPosition() {
+        return (mScrollState == ViewPager2.SCROLL_STATE_DRAGGING) ||
+                ((mScrollState == ViewPager2.SCROLL_STATE_SETTLING) && (mPreviousScrollState == ViewPager2.SCROLL_STATE_DRAGGING));
+    }
+
+    private static class Internal {
+        private static final Method mMethodSelectTab;
+
+        static {
+            mMethodSelectTab = getAccessiblePrivateMethod(TabLayout.class, "selectTab", TabLayout.Tab.class, boolean.class);
+        }
+
+        @SuppressWarnings("SameParameterValue")
+        private static Method getAccessiblePrivateMethod(Class<?> targetClass, String methodName, Class<?>... params) throws RuntimeException {
+            try {
+                Method m = targetClass.getDeclaredMethod(methodName, params);
+                m.setAccessible(true);
+                return m;
+            } catch (NoSuchMethodException e) {
+                throw new IllegalStateException(e);
+            }
+        }
+
+        private static void selectTab(TabLayout tabLayout, TabLayout.Tab tab, boolean updateIndicator) {
+            try {
+                mMethodSelectTab.invoke(tabLayout, tab, updateIndicator);
+            } catch (IllegalAccessException e) {
+                Log.e(TabLayoutHelper.class.getCanonicalName(), e.getMessage(), new IllegalStateException(e));
+            } catch (InvocationTargetException e) {
+                throw handleInvocationTargetException(e);
+            }
+        }
+
+        private static RuntimeException handleInvocationTargetException(InvocationTargetException e) {
+            Throwable targetException = e.getTargetException();
+            if (targetException instanceof RuntimeException) {
+                throw (RuntimeException) targetException;
+            } else {
+                throw new IllegalStateException(targetException);
+            }
+        }
+    }
+}
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