Wrap scrollable childs of viewpager in NestedScrollableHost

app/src/main/java/de/danoeh/antennapod/view/NestedScrollableHost.java

@@ -0,0 +1,133 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Source: https://github.com/android/views-widgets-samples/blob/87e58d1/ViewPager2/app/src/main/java/androidx/viewpager2/integration/testapp/NestedScrollableHost.kt
+ */
+
+package de.danoeh.antennapod.view;
+
+import android.content.Context;
+import android.util.AttributeSet;
+import android.view.MotionEvent;
+import android.view.View;
+import android.view.ViewConfiguration;
+import android.widget.FrameLayout;
+
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+import androidx.viewpager2.widget.ViewPager2;
+
+import static androidx.viewpager2.widget.ViewPager2.ORIENTATION_HORIZONTAL;
+
+/**
+ * Layout to wrap a scrollable component inside a ViewPager2. Provided as a solution to the problem
+ * where pages of ViewPager2 have nested scrollable elements that scroll in the same direction as
+ * ViewPager2. The scrollable element needs to be the immediate and only child of this host layout.
+ *
+ * <p>This solution has limitations when using multiple levels of nested scrollable elements
+ * (e.g. a horizontal RecyclerView in a vertical RecyclerView in a horizontal ViewPager2).</p>
+ */
+class NestedScrollableHost extends FrameLayout {
+
+    public NestedScrollableHost(@NonNull Context context) {
+        super(context);
+        touchSlop = ViewConfiguration.get(context).getScaledTouchSlop();
+    }
+
+    public NestedScrollableHost(@NonNull Context context, @Nullable AttributeSet attrs) {
+        super(context, attrs);
+        touchSlop = ViewConfiguration.get(context).getScaledTouchSlop();
+    }
+
+    private int touchSlop = 0;
+    private float initialX = 0f;
+    private float initialY = 0f;
+
+    private ViewPager2 getParentViewPager() {
+        View v = (View) getParent();
+        while (v != null && !(v instanceof ViewPager2)) {
+            v = (View) v.getParent();
+        }
+        return v == null ? null : (ViewPager2) v;
+    }
+
+    private View getChild() {
+        return getChildCount() > 0 ? getChildAt(0) : null;
+    }
+
+    private boolean canChildScroll(int orientation, float delta) {
+        int direction = (int) -Math.copySign(1, delta);
+        View child = getChild();
+        if (child == null) {
+            return false;
+        }
+        switch (orientation) {
+            case 0:
+                return child.canScrollHorizontally(direction);
+            case 1:
+                return child.canScrollVertically(direction);
+            default:
+                return false;
+        }
+    }
+
+    public boolean onInterceptTouchEvent(MotionEvent e) {
+        handleInterceptTouchEvent(e);
+        return super.onInterceptTouchEvent(e);
+    }
+
+    private void handleInterceptTouchEvent(MotionEvent e) {
+        ViewPager2 parentViewPager = getParentViewPager();
+        if (parentViewPager == null) {
+            return;
+        }
+        int orientation = parentViewPager.getOrientation();
+
+        // Early return if child can't scroll in same direction as parent
+        if (!canChildScroll(orientation, -1f) && !canChildScroll(orientation, 1f)) {
+            return;
+        }
+
+        if (e.getAction() == MotionEvent.ACTION_DOWN) {
+            initialX = e.getX();
+            initialY = e.getY();
+            getParent().requestDisallowInterceptTouchEvent(true);
+        } else if (e.getAction() == MotionEvent.ACTION_MOVE) {
+            int dx = (int) (e.getX() - initialX);
+            int dy = (int) (e.getY() - initialY);
+            boolean isVpHorizontal = orientation == ORIENTATION_HORIZONTAL;
+
+            // assuming ViewPager2 touch-slop is 2x touch-slop of child
+            float scaledDx = Math.abs(dx) * (isVpHorizontal ? .5f : 1f);
+            float scaledDy = Math.abs(dy) * (isVpHorizontal ? 1f : .5f);
+
+            if (scaledDx > touchSlop || scaledDy > touchSlop) {
+                if (isVpHorizontal == (scaledDy > scaledDx)) {
+                    // Gesture is perpendicular, allow all parents to intercept
+                    getParent().requestDisallowInterceptTouchEvent(false);
+                } else {
+                    // Gesture is parallel, query child if movement in that direction is possible
+                    if (canChildScroll(orientation, isVpHorizontal ? dx : dy)) {
+                        // Child can scroll, disallow all parents to intercept
+                        getParent().requestDisallowInterceptTouchEvent(true);
+                    } else {
+                        // Child cannot scroll, allow all parents to intercept
+                        getParent().requestDisallowInterceptTouchEvent(false);
+                    }
+                }
+            }
+        }
+    }
+}

app/src/main/res/layout/feeditem_fragment.xml

@@ -171,12 +171,18 @@
 
     </LinearLayout>
 
-    <de.danoeh.antennapod.view.ShownotesWebView
-        android:id="@+id/webvDescription"
-        android:layout_width="match_parent"
+    <de.danoeh.antennapod.view.NestedScrollableHost
         android:layout_below="@id/header"
-        android:layout_height="match_parent"
-        android:foreground="?android:windowContentOverlay" />
+        android:layout_width="match_parent"
+        android:layout_height="match_parent">
+
+        <de.danoeh.antennapod.view.ShownotesWebView
+            android:id="@+id/webvDescription"
+            android:layout_width="match_parent"
+            android:layout_height="match_parent"
+            android:foreground="?android:windowContentOverlay" />
+
+    </de.danoeh.antennapod.view.NestedScrollableHost>
 
     <FrameLayout
         android:layout_width="match_parent"

app/src/main/res/layout/item_description_fragment.xml

@@ -1,11 +1,18 @@
 <?xml version="1.0" encoding="utf-8"?>
-<androidx.core.widget.NestedScrollView
-xmlns:android="http://schemas.android.com/apk/res/android"
-android:layout_width="match_parent"
-android:layout_height="match_parent"
-android:fillViewport="false">
-    <de.danoeh.antennapod.view.ShownotesWebView
+<androidx.core.widget.NestedScrollView xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent"
+    android:fillViewport="false">
+
+    <de.danoeh.antennapod.view.NestedScrollableHost
+        android:layout_width="match_parent"
+        android:layout_height="match_parent">
+
+        <de.danoeh.antennapod.view.ShownotesWebView
             android:id="@+id/webview"
             android:layout_width="match_parent"
-            android:layout_height="wrap_content"/>
+            android:layout_height="wrap_content" />
+
+    </de.danoeh.antennapod.view.NestedScrollableHost>
+
 </androidx.core.widget.NestedScrollView>