[AppBarLayout] Use a uniform way to determine the target scrolling view

Author: pubiqq

docs/components/TopAppBar.md

@@ -645,7 +645,7 @@ In the layout:
 within another view (e.g., a `SwipeRefreshLayout`), you should make sure to set
 `app:liftOnScrollTargetViewId` on your `AppBarLayout` to the id of the scrolling
 view. This will ensure that the `AppBarLayout` is using the right view to
-determine whether it should lift or not, and it will help avoid flicker issues.
+determine whether it should lift or not.
 
 The following example shows the top app bar disappearing upon scrolling up, and
 appearing upon scrolling down.

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

@@ -82,9 +82,11 @@
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.ref.WeakReference;
+import java.util.ArrayDeque;
 import java.util.ArrayList;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Queue;
 
 /**
  * AppBarLayout is a vertical {@link LinearLayout} which implements many of the features of material
@@ -227,7 +229,7 @@ public abstract void onUpdate(
   private boolean liftOnScroll;
   @Nullable private ColorStateList liftOnScrollColor;
   @IdRes private int liftOnScrollTargetViewId;
-  @Nullable private WeakReference<View> liftOnScrollTargetView;
+  @Nullable private WeakReference<View> liftOnScrollTargetViewRef;
   @Nullable private ValueAnimator liftOnScrollColorAnimator;
   @Nullable private AnimatorUpdateListener liftOnScrollColorUpdateListener;
   private final List<LiftOnScrollListener> liftOnScrollListeners = new ArrayList<>();
@@ -841,7 +843,7 @@ protected LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) {
   protected void onDetachedFromWindow() {
     super.onDetachedFromWindow();
 
-    clearLiftOnScrollTargetView();
+    clearLiftOnScrollTargetViewRef();
   }
 
   boolean hasChildWithInterpolator() {
@@ -1171,9 +1173,9 @@ public boolean isLiftOnScroll() {
   public void setLiftOnScrollTargetView(@Nullable View liftOnScrollTargetView) {
     this.liftOnScrollTargetViewId = View.NO_ID;
     if (liftOnScrollTargetView == null) {
-      clearLiftOnScrollTargetView();
+      clearLiftOnScrollTargetViewRef();
     } else {
-      this.liftOnScrollTargetView = new WeakReference<>(liftOnScrollTargetView);
+      this.liftOnScrollTargetViewRef = new WeakReference<>(liftOnScrollTargetView);
     }
   }
 
@@ -1184,7 +1186,7 @@ public void setLiftOnScrollTargetView(@Nullable View liftOnScrollTargetView) {
   public void setLiftOnScrollTargetViewId(@IdRes int liftOnScrollTargetViewId) {
     this.liftOnScrollTargetViewId = liftOnScrollTargetViewId;
     // Invalidate cached target view so it will be looked up on next scroll.
-    clearLiftOnScrollTargetView();
+    clearLiftOnScrollTargetViewRef();
   }
 
   /** Sets the color of the {@link AppBarLayout} when it is fully lifted. */
@@ -1207,39 +1209,88 @@ public int getLiftOnScrollTargetViewId() {
     return liftOnScrollTargetViewId;
   }
 
-  boolean shouldLift(@Nullable View defaultScrollingView) {
-    View scrollingView = findLiftOnScrollTargetView(defaultScrollingView);
-    if (scrollingView == null) {
-      scrollingView = defaultScrollingView;
-    }
+  boolean shouldBeLifted() {
+    final View scrollingView = findLiftOnScrollTargetView();
     return scrollingView != null
         && (scrollingView.canScrollVertically(-1) || scrollingView.getScrollY() > 0);
   }
 
   @Nullable
-  private View findLiftOnScrollTargetView(@Nullable View defaultScrollingView) {
+  private View findLiftOnScrollTargetView() {
+    View liftOnScrollTargetView = liftOnScrollTargetViewRef != null
+        ? liftOnScrollTargetViewRef.get()
+        : null;
+
+    final ViewGroup parent = (ViewGroup) getParent();
+
     if (liftOnScrollTargetView == null && liftOnScrollTargetViewId != View.NO_ID) {
-      View targetView = null;
-      if (defaultScrollingView != null) {
-        targetView = defaultScrollingView.findViewById(liftOnScrollTargetViewId);
+      liftOnScrollTargetView = parent.findViewById(liftOnScrollTargetViewId);
+      if (liftOnScrollTargetView != null) {
+        clearLiftOnScrollTargetViewRef();
+        liftOnScrollTargetViewRef = new WeakReference<>(liftOnScrollTargetView);
       }
-      if (targetView == null && getParent() instanceof ViewGroup) {
-        // Assumes the scrolling view is a child of the AppBarLayout's parent,
-        // which should be true due to the CoordinatorLayout pattern.
-        targetView = ((ViewGroup) getParent()).findViewById(liftOnScrollTargetViewId);
+    }
+
+    return liftOnScrollTargetView != null
+        ? liftOnScrollTargetView
+        : getDefaultLiftOnScrollTargetView(parent);
+  }
+
+  private View getDefaultLiftOnScrollTargetView(@NonNull ViewGroup parent) {
+    for (int i = 0, z = parent.getChildCount(); i < z; i++) {
+      final View child = parent.getChildAt(i);
+      if (hasScrollingBehavior(child)) {
+        final View scrollableView = findClosestScrollableView(child);
+        if (scrollableView != null) {
+          return scrollableView;
+        }
       }
-      if (targetView != null) {
-        liftOnScrollTargetView = new WeakReference<>(targetView);
+    }
+    return null;
+  }
+
+  private boolean hasScrollingBehavior(@NonNull View view) {
+    if (view.getLayoutParams() instanceof CoordinatorLayout.LayoutParams) {
+      CoordinatorLayout.LayoutParams lp = (CoordinatorLayout.LayoutParams) view.getLayoutParams();
+      return lp.getBehavior() instanceof ScrollingViewBehavior;
+    }
+
+    return false;
+  }
+
+  @Nullable
+  private View findClosestScrollableView(@NonNull View rootView) {
+    final Queue<View> queue = new ArrayDeque<>();
+    queue.add(rootView);
+
+    while (!queue.isEmpty()) {
+      final View view = queue.remove();
+      if (isScrollableView(view)) {
+        return view;
+      } else {
+        if (view instanceof ViewGroup) {
+          final ViewGroup viewGroup = (ViewGroup) view;
+          for (int i = 0, count = viewGroup.getChildCount(); i < count; i++) {
+            queue.add(viewGroup.getChildAt(i));
+          }
+        }
       }
     }
-    return liftOnScrollTargetView != null ? liftOnScrollTargetView.get() : null;
+
+    return null;
   }
 
-  private void clearLiftOnScrollTargetView() {
-    if (liftOnScrollTargetView != null) {
-      liftOnScrollTargetView.clear();
+  private boolean isScrollableView(@NonNull View view) {
+    return view instanceof NestedScrollingChild
+        || view instanceof AbsListView
+        || view instanceof ScrollView;
+  }
+
+  private void clearLiftOnScrollTargetViewRef() {
+    if (liftOnScrollTargetViewRef != null) {
+      liftOnScrollTargetViewRef.clear();
     }
-    liftOnScrollTargetView = null;
+    liftOnScrollTargetViewRef = null;
   }
 
   /**
@@ -1655,12 +1706,12 @@ private boolean canScrollChildren(
 
     @Override
     public void onNestedPreScroll(
-        CoordinatorLayout coordinatorLayout,
+        @NonNull CoordinatorLayout coordinatorLayout,
         @NonNull T child,
-        View target,
+        @NonNull View target,
         int dx,
         int dy,
-        int[] consumed,
+        @NonNull int[] consumed,
         int type) {
       if (dy != 0) {
         int min;
@@ -1679,7 +1730,7 @@ public void onNestedPreScroll(
         }
       }
       if (child.isLiftOnScroll()) {
-        child.setLiftedState(child.shouldLift(target));
+        child.setLiftedState(child.shouldBeLifted());
       }
     }
 
@@ -1710,7 +1761,10 @@ public void onNestedScroll(
 
     @Override
     public void onStopNestedScroll(
-        CoordinatorLayout coordinatorLayout, @NonNull T abl, View target, int type) {
+        @NonNull CoordinatorLayout coordinatorLayout,
+        @NonNull T abl,
+        @NonNull View target,
+        int type) {
       // onStartNestedScroll for a fling will happen before onStopNestedScroll for the scroll. This
       // isn't necessarily guaranteed yet, but it should be in the future. We use this to our
       // advantage to check if a fling (ViewCompat.TYPE_NON_TOUCH) will start after the touch scroll
@@ -1719,7 +1773,7 @@ public void onStopNestedScroll(
         // If we haven't been flung, or a fling is ending
         snapToChildIfNeeded(coordinatorLayout, abl);
         if (abl.isLiftOnScroll()) {
-          abl.setLiftedState(abl.shouldLift(target));
+          abl.setLiftedState(abl.shouldBeLifted());
         }
       }
 
@@ -2114,7 +2168,7 @@ void onFlingFinished(@NonNull CoordinatorLayout parent, @NonNull T layout) {
       // At the end of a manual fling, check to see if we need to snap to the edge-child
       snapToChildIfNeeded(parent, layout);
       if (layout.isLiftOnScroll()) {
-        layout.setLiftedState(layout.shouldLift(findFirstScrollingChild(parent)));
+        layout.setLiftedState(layout.shouldBeLifted());
       }
     }
 
@@ -2281,9 +2335,7 @@ private void updateAppBarLayoutDrawableState(
       }
 
       if (layout.isLiftOnScroll()) {
-        // Use first scrolling child as default scrolling view for updating lifted state because
-        // it represents the content that would be scrolled beneath the app bar.
-        lifted = layout.shouldLift(findFirstScrollingChild(parent));
+        lifted = layout.shouldBeLifted();
       }
 
       final boolean changed = layout.setLiftedState(lifted);
@@ -2333,19 +2385,6 @@ private static View getAppBarChildOnOffset(
       return null;
     }
 
-    @Nullable
-    private View findFirstScrollingChild(@NonNull CoordinatorLayout parent) {
-      for (int i = 0, z = parent.getChildCount(); i < z; i++) {
-        final View child = parent.getChildAt(i);
-        if (child instanceof NestedScrollingChild
-            || child instanceof AbsListView
-            || child instanceof ScrollView) {
-          return child;
-        }
-      }
-      return null;
-    }
-
     @Override
     int getTopBottomOffsetForScrollingSibling() {
       return getTopAndBottomOffset() + offsetDelta;
@@ -2482,7 +2521,7 @@ public boolean layoutDependsOn(CoordinatorLayout parent, View child, View depend
     public boolean onDependentViewChanged(
         @NonNull CoordinatorLayout parent, @NonNull View child, @NonNull View dependency) {
       offsetChildAsNeeded(child, dependency);
-      updateLiftedStateIfNeeded(child, dependency);
+      updateLiftedStateIfNeeded(dependency);
       return false;
     }
 
@@ -2587,11 +2626,11 @@ int getScrollRange(View v) {
       }
     }
 
-    private void updateLiftedStateIfNeeded(View child, View dependency) {
+    private void updateLiftedStateIfNeeded(@NonNull View dependency) {
       if (dependency instanceof AppBarLayout) {
         AppBarLayout appBarLayout = (AppBarLayout) dependency;
         if (appBarLayout.isLiftOnScroll()) {
-          appBarLayout.setLiftedState(appBarLayout.shouldLift(child));
+          appBarLayout.setLiftedState(appBarLayout.shouldBeLifted());
         }
       }
     }