FlexScrollViewTest.java

package com.example.app.myapplication;

import android.content.Context;
import android.util.AttributeSet;
import android.view.View;

/*
res/layout/fsvt.xml test layout file:

<?xml version="1.0" encoding="utf-8"?>
<com.example.app.myapplication.FlexScrollViewTest
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/image"
        android:src="@drawable/ic_launcher"
        />
    <TextView
        android:layout_width="match_parent"
        android:layout_height="100sp"
        android:id="@+id/header"
        android:textSize="32sp"
        android:text="drag/swipe anywhere up and down"
        android:gravity="center"
        android:background="#af00"
        />
    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/content"
        android:textSize="32sp"
        android:text="32\n31\n30\n29\n28\n27\n26\n25\n24\n23\n22\n21\n20\n19\n18\n17\n16\n15\n14\n13\n12\n11\n10\n9\n8\n7\n6\n5\n4\n3\n2\n1"
        android:background="#a00f"
        />

</com.example.app.myapplication.FlexScrollViewTest>
 */

public class FlexScrollViewTest extends FlexScrollView {
    private static final String TAG = "FlexScrollViewTest";

    private View image;
    private View header;
    private View content;

    public FlexScrollViewTest(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    protected void onFinishInflate() {
        image = findViewById(R.id.image);
        header = findViewById(R.id.header);
        content = findViewById(R.id.content);
    }

    @Override
    public int getMaxHeight() {
        int w = MeasureSpec.makeMeasureSpec(getMeasuredWidth(), MeasureSpec.EXACTLY);
        int h = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
        content.measure(w, h);
        return getMeasuredHeight() + content.getMeasuredHeight();
    }

    @Override
    public void layoutChildren(int yOffset, int yDelta) {
//        Log.d(TAG, "layoutChild " + yOffset + " " + yDelta);
        int w = getWidth();
        int h = getHeight();
        int hh = header.getMeasuredHeight();
        int y1, y2;
        if (yOffset < h - hh) {
            y1 = h - yOffset - hh;
            y2 = y1 + hh;
            image.layout(0, -yOffset / 3, w, y1 + yOffset / 3);
            image.setAlpha(1 - yOffset / (float) (h - hh));
            header.layout(0, y1, w, y2);
            content.layout(0, y2, w, h);
            content.scrollTo(0, 0);
        } else {
            y1 = 0;
            y2 = hh;
            image.layout(0, 0, 0, y1);
            header.layout(0, y1, w, y2);
            content.layout(0, y2, w, h);
            content.scrollTo(0, yOffset - (h - hh));
        }
    }
}

Exception raised during rendering: com.android.layoutlib.bridge.MockView cannot be cast to android.view.ViewGroup