[AppBarLayout] Save and restore scroll state during scroll range recalculation

When scroll range changes, the current scroll position may not make sense anymore. Therefore we need to save the scroll state and restore it after the scroll range is invalidated. This change reuses and refactors the existing saving instance state logic to support this need. Also adds a flag to denote "fully expanded" state to avoid improper scroll position calculation when views are still being initialized.

PiperOrigin-RevId: 421348135

Author: drchen

lib/java/com/google/android/material/appbar/AppBarLayout.java

@@ -68,6 +68,7 @@
 import androidx.core.view.accessibility.AccessibilityViewCommand;
 import androidx.customview.view.AbsSavedState;
 import com.google.android.material.animation.AnimationUtils;
+import com.google.android.material.appbar.AppBarLayout.BaseBehavior.SavedState;
 import com.google.android.material.internal.ThemeEnforcement;
 import com.google.android.material.shape.MaterialShapeDrawable;
 import com.google.android.material.shape.MaterialShapeUtils;
@@ -203,6 +204,8 @@ public interface LiftOnScrollListener {
 
   @Nullable private Drawable statusBarForeground;
 
+  private Behavior behavior;
+
   public AppBarLayout(@NonNull Context context) {
     this(context, null);
   }
@@ -533,10 +536,20 @@ private boolean hasCollapsibleChild() {
   }
 
   private void invalidateScrollRanges() {
+    // Saves the current scrolling state when we need to recalculate scroll ranges
+    SavedState savedState = behavior == null || totalScrollRange == INVALID_SCROLL_RANGE
+            ? null : behavior.saveScrollState(AbsSavedState.EMPTY_STATE, this);
     // Invalidate the scroll ranges
     totalScrollRange = INVALID_SCROLL_RANGE;
     downPreScrollRange = INVALID_SCROLL_RANGE;
     downScrollRange = INVALID_SCROLL_RANGE;
+    // Restores the previous scrolling state. Don't override if there's a previously saved state
+    // which has not be restored yet. Multiple re-measuring can happen before the scroll state
+    // is actually restored. We don't want to restore the state in-between those re-measuring,
+    // since they can be incorrect.
+    if (savedState != null) {
+      behavior.restoreScrollState(savedState, false);
+    }
   }
 
   @Override
@@ -558,7 +571,8 @@ protected void onAttachedToWindow() {
   @Override
   @NonNull
   public CoordinatorLayout.Behavior<AppBarLayout> getBehavior() {
-    return new AppBarLayout.Behavior();
+    behavior = new AppBarLayout.Behavior();
+    return behavior;
   }
 
   @RequiresApi(VERSION_CODES.LOLLIPOP)
@@ -1651,6 +1665,9 @@ public boolean onLayoutChild(
         if (savedState.fullyScrolled) {
           // Keep fully scrolled.
           setHeaderTopBottomOffset(parent, abl, -abl.getTotalScrollRange());
+        } else if (savedState.fullyExpanded) {
+          // Keep fully expanded.
+          setHeaderTopBottomOffset(parent, abl, 0);
         } else {
           // Not fully scrolled, restore the visible percetage of child layout.
           View child = abl.getChildAt(savedState.firstVisibleChildIndex);
@@ -2042,7 +2059,25 @@ int getTopBottomOffsetForScrollingSibling() {
 
     @Override
     public Parcelable onSaveInstanceState(@NonNull CoordinatorLayout parent, @NonNull T abl) {
-      final Parcelable superState = super.onSaveInstanceState(parent, abl);
+      Parcelable superState = super.onSaveInstanceState(parent, abl);
+      SavedState scrollState = saveScrollState(superState, abl);
+      return scrollState == null ? superState : scrollState;
+    }
+
+    @Override
+    public void onRestoreInstanceState(
+        @NonNull CoordinatorLayout parent, @NonNull T appBarLayout, Parcelable state) {
+      if (state instanceof SavedState) {
+        restoreScrollState((SavedState) state, true);
+        super.onRestoreInstanceState(parent, appBarLayout, savedState.getSuperState());
+      } else {
+        super.onRestoreInstanceState(parent, appBarLayout, state);
+        savedState = null;
+      }
+    }
+
+    @Nullable
+    SavedState saveScrollState(@Nullable Parcelable superState, @NonNull T abl) {
       final int offset = getTopAndBottomOffset();
 
       // Try and find the first visible child...
@@ -2051,42 +2086,38 @@ public Parcelable onSaveInstanceState(@NonNull CoordinatorLayout parent, @NonNul
         final int visBottom = child.getBottom() + offset;
 
         if (child.getTop() + offset <= 0 && visBottom >= 0) {
-          final SavedState ss = new SavedState(superState);
-          ss.fullyScrolled = -getTopAndBottomOffset() >= abl.getTotalScrollRange();
+          final SavedState ss =
+              new SavedState(superState == null ? AbsSavedState.EMPTY_STATE : superState);
+          ss.fullyExpanded = offset == 0;
+          ss.fullyScrolled = !ss.fullyExpanded && -offset >= abl.getTotalScrollRange();
           ss.firstVisibleChildIndex = i;
           ss.firstVisibleChildAtMinimumHeight =
               visBottom == (ViewCompat.getMinimumHeight(child) + abl.getTopInset());
           ss.firstVisibleChildPercentageShown = visBottom / (float) child.getHeight();
           return ss;
         }
       }
-
-      // Else we'll just return the super state
-      return superState;
+      return null;
     }
 
-    @Override
-    public void onRestoreInstanceState(
-        @NonNull CoordinatorLayout parent, @NonNull T appBarLayout, Parcelable state) {
-      if (state instanceof SavedState) {
-        savedState = (SavedState) state;
-        super.onRestoreInstanceState(parent, appBarLayout, savedState.getSuperState());
-      } else {
-        super.onRestoreInstanceState(parent, appBarLayout, state);
-        savedState = null;
+    void restoreScrollState(@Nullable SavedState state, boolean force) {
+      if (savedState == null || force) {
+        savedState = state;
       }
     }
 
     /** A {@link Parcelable} implementation for {@link AppBarLayout}. */
     protected static class SavedState extends AbsSavedState {
       boolean fullyScrolled;
+      boolean fullyExpanded;
       int firstVisibleChildIndex;
       float firstVisibleChildPercentageShown;
       boolean firstVisibleChildAtMinimumHeight;
 
       public SavedState(@NonNull Parcel source, ClassLoader loader) {
         super(source, loader);
         fullyScrolled = source.readByte() != 0;
+        fullyExpanded = source.readByte() != 0;
         firstVisibleChildIndex = source.readInt();
         firstVisibleChildPercentageShown = source.readFloat();
         firstVisibleChildAtMinimumHeight = source.readByte() != 0;
@@ -2100,6 +2131,7 @@ public SavedState(Parcelable superState) {
       public void writeToParcel(@NonNull Parcel dest, int flags) {
         super.writeToParcel(dest, flags);
         dest.writeByte((byte) (fullyScrolled ? 1 : 0));
+        dest.writeByte((byte) (fullyExpanded ? 1 : 0));
         dest.writeInt(firstVisibleChildIndex);
         dest.writeFloat(firstVisibleChildPercentageShown);
         dest.writeByte((byte) (firstVisibleChildAtMinimumHeight ? 1 : 0));