android仿知乎标题栏随ScrollView滚动变色

package com.jukopro.titlebarcolor;

import android.content.Context;
import android.util.AttributeSet;
import android.widget.ScrollView;

/**
 * 带滚动监听的scrollview
 */
public class ObservableScrollView extends ScrollView {

  public interface ScrollViewListener {
    void onScrollChanged(ObservableScrollView scrollView, int x, int y, int oldx, int oldy);
  }

  private ScrollViewListener scrollViewListener = null;

  public ObservableScrollView(Context context) {
    super(context);
  }

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

  public ObservableScrollView(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
  }


  public void setScrollViewListener(ScrollViewListener scrollViewListener) {
    this.scrollViewListener = scrollViewListener;
  }

  @Override
  protected void onScrollChanged(int x, int y, int oldx, int oldy) {
    super.onScrollChanged(x, y, oldx, oldy);
    if (scrollViewListener != null) {
      scrollViewListener.onScrollChanged(this, x, y, oldx, oldy);
    }
  }

}

public class MyListview extends ListView {

  public MyListview(Context context) {
    super(context);
  }

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

  public MyListview(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
  }

  @Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int expandSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2, MeasureSpec.AT_MOST);
    super.onMeasure(widthMeasureSpec, expandSpec);
  }
} 

package com.jukopro.titlebarcolor;

import android.app.Activity;
import android.graphics.Color;
import android.os.Bundle;
import android.view.ViewTreeObserver;
import android.view.ViewTreeObserver.OnGlobalLayoutListener;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;

import com.jukopro.titlebarcolor.ObservableScrollView.ScrollViewListener;

public class MainActivity extends Activity implements ScrollViewListener {
  private ObservableScrollView scrollView;
  private ListView listView;
  private ImageView imageView;
  private TextView textView;
  private int imageHeight;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    scrollView = (ObservableScrollView) findViewById(R.id.scrollview);
    listView = (ListView) findViewById(R.id.listview);
    imageView = (ImageView) findViewById(R.id.imageview);
    textView = (TextView) findViewById(R.id.textview);
    initListeners();
    initData();
  }

  private void initListeners() {
    // 获取顶部图片高度后，设置滚动监听
    ViewTreeObserver vto = imageView.getViewTreeObserver();
    vto.addOnGlobalLayoutListener(new OnGlobalLayoutListener() {
      @Override
      public void onGlobalLayout() {
        imageView.getViewTreeObserver().removeGlobalOnLayoutListener(this);
        imageHeight = imageView.getHeight();

        scrollView.setScrollViewListener(MainActivity.this);
      }
    });
  }


  private void initData() {
    ArrayAdapter<String> adapter = new ArrayAdapter<String>(MainActivity.this, 
      android.R.layout.simple_list_item_1, getResources().getStringArray(R.array.data));
    listView.setAdapter(adapter);
  }


  /**
   * ScrollView滚动监听
   *
   * @param scrollView：滚动控件
   * @param x：x轴坐标
   * @param y：y轴坐标
   * @param oldx：上一个x轴坐标
   * @param oldy：上一个y轴坐标
   */
  @Override
  public void onScrollChanged(ObservableScrollView scrollView, int x, int y, int oldx, int oldy) {
    if (y <= 0) {
      textView.setBackgroundColor(Color.argb((int) 0, 227, 29, 26));//AGB由相关工具获得，或者美工提供
    } else if (y > 0 && y <= imageHeight) {
      float scale = (float) y / imageHeight;
      float alpha = (255 * scale);
      // 只是layout背景透明(仿知乎滑动效果)
      textView.setBackgroundColor(Color.argb((int) alpha, 227, 29, 26));
    } else {
      textView.setBackgroundColor(Color.argb((int) 255, 227, 29, 26));
    }
  }
}

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent">

  <com.jukopro.titlebarcolor.ObservableScrollView
    android:id="@+id/scrollview"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:scrollbars="none">

    <LinearLayout
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:orientation="vertical">

      <ImageView
        android:id="@+id/imageview"
        android:layout_width="match_parent"
        android:layout_height="200dp"
        android:background="@drawable/ic_launcher" />

      <com.jukopro.titlebarcolor.MyListview
        android:id="@+id/listview"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />
    </LinearLayout>
  </com.jukopro.titlebarcolor.ObservableScrollView>

  <TextView
    android:id="@+id/textview"
    android:layout_width="match_parent"
    android:layout_height="48dp"
    android:background="#00000000"
    android:gravity="center"
    android:text="我是标题"
    android:textColor="@android:color/white"
    android:textSize="18sp" />

</RelativeLayout>