Android ListView滑动改变标题栏背景渐变效果

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

 <ListView
 android:id="@+id/listvew"
 android:layout_width="match_parent"
 android:layout_height="match_parent"/>

 <!--标题栏，这里简单写个textview-->
 <TextView
 android:id="@+id/title_tv"
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:background="#00000000"
 android:gravity="center"
 android:orientation="vertical"
 android:padding="5dp"
 android:textSize="30sp"/>

</RelativeLayout>

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:orientation="vertical">

 <ImageView
 android:id="@+id/head_iv"
 android:layout_width="match_parent"
 android:layout_height="150dp"
 android:background="@mipmap/ch"/>


</LinearLayout>

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
 xmlns:android="http://schemas.android.com/apk/res/android"
 android:layout_width="match_parent"
 android:layout_height="50dp">

 <TextView
 android:id="@+id/item_tv"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:gravity="center"
 android:textSize="15sp">

 </TextView>

</FrameLayout>

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AbsListView;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;
import java.util.ArrayList;

public class MainActivity10 extends Activity {

 private TextView title_tv;

 private ListView listvew;

 //listView的头
 private View headView;

 //listView头中包含的布局。这里仅仅是一个ImageView
 private ImageView head_iv;

 private ArrayList<String> dataList;

 private MyAdapter myAdapter;

 //listview的头部（这里是ImageView）顶部距离屏幕顶部（包含状态栏）的距离
 //注：这个高度，是头布局在屏幕里才会计算的，出了屏幕，就不会变了
 private int height;

 //listView的头部的真实高度。头布局的整体高度，因为这个demo只简单写了个ImageView作为头部，所以ImageView的高度，就是头部的高度
 private int headViewHeight;

 private Context context;

 @Override
 protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.activity_main_10);

 context = this;

 title_tv = (TextView) findViewById(R.id.title_tv);

 listvew = (ListView) findViewById(R.id.listvew);

 headView = LayoutInflater.from(this).inflate(R.layout.head_layout, null);

 head_iv = (ImageView) headView.findViewById(R.id.head_iv);

 //添加listView的头布局
 listvew.addHeaderView(headView);

 dataList = new ArrayList<>();

 for (int i = 0; i < 50; i++) {
  dataList.add("==" + i + "==");
 }

 myAdapter = new MyAdapter();

 listvew.setAdapter(myAdapter);

 listvew.setOnScrollListener(new AbsListView.OnScrollListener() {
  @Override
  public void onScrollStateChanged(AbsListView view, int scrollState) {

  }

  @Override
  public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {

  int location[] = new int[2];

  /**
   * public void getLocationInWindow(@Size(2) int[] location)
   *
   * <p>Computes 计算 the coordinates 坐标 of this view in its window. The argument 参数
   * must be an array of two integers. After the method returns, the array
   * contains 包含 the x and y location in that order.</p>
   *
   * @param location an array of two integers in which to hold the coordinates
   */
  head_iv.getLocationInWindow(location);

  //listview的头部（这里是ImageView）顶部距离屏幕顶部（包含状态栏）的距离
  height = location[1];
  headViewHeight = head_iv.getHeight();

  Utils.printLogData("==location==0==" + location[0]);
  Utils.printLogData("==location==1==" + location[1]);
  Utils.printLogData("==height==" + height);
  Utils.printLogData("==headViewHeight==" + headViewHeight);

  //在head_layout.xml中，固定设置了150dp的高度，用于和上面测量的对比
  Utils.printLogData("==setHeigth==" + dip2px(150));

  handleTitleBarColorEvaluate();

  }
 });

 }

 // 处理标题栏颜色渐变
 private void handleTitleBarColorEvaluate() {
 //比例
 float fraction;
 if (height > 0) {

  fraction = 1f - height * 1f / 60;
  if (fraction < 0f) {
  fraction = 0f;
  }
  title_tv.setAlpha(fraction);
  return;
 }

 //高度值是负数，但是负号仅仅是表示方向，取绝对值
 float space = Math.abs(height) * 1f;
 // 标题栏的高度
 fraction = space / headViewHeight;
 if (fraction < 0f)
  fraction = 0f;
 if (fraction > 1f)
  fraction = 1f;
 title_tv.setAlpha(1f);

 if (fraction >= 1f) {
  title_tv.setBackgroundColor(0xffec434b);
 } else {
  //根据比例，生成一个按比例的颜色值
  title_tv.setBackgroundColor(ChenColorUtils.getNewColorByStartEndColor(context, fraction, R.color.transparent, R.color.red));
 }

 if (fraction >= 0.8f) {
  title_tv.setTextColor(ChenColorUtils.getNewColorByStartEndColor(context, fraction, R.color.transparent, R.color.black));
  title_tv.setText("标题栏");
 } else {
  title_tv.setText("");
 }
 }


 private class MyAdapter extends BaseAdapter {

 @Override
 public int getCount() {
  return dataList.size();
 }

 @Override
 public Object getItem(int position) {
  return null;
 }

 @Override
 public long getItemId(int position) {
  return 0;
 }

 @Override
 public View getView(int position, View convertView, ViewGroup parent) {
  ViewHolder holder;
  if (convertView == null) {
  convertView = LayoutInflater.from(context).inflate(R.layout.listview_item_layout, null);
  holder = new ViewHolder();

  holder.item_tv = (TextView) convertView.findViewById(R.id.item_tv);

  convertView.setTag(holder);
  } else {
  holder = (ViewHolder) convertView.getTag();
  }

  holder.item_tv.setText(dataList.get(position));


  return convertView;
 }

 private class ViewHolder {
  private TextView item_tv;
 }

 }


 /**
 * dip转换px
 */
 public int dip2px(int dip) {
 final float scale = context.getResources().getDisplayMetrics().density;
 return (int) (dip * scale + 0.5f);
 }


}

<color name="red">#ec434b</color>
<color name="transparent">#00000000</color>
<color name="black">#FF000000</color>

import android.util.Log;

public class Utils {

 public static void printLogData(String data) {
 Log.e("chen", data);
 }

}

import android.content.Context;

public class ChenColorUtils {

 // 成新的颜色值
 public static int getNewColorByStartEndColor(Context context, float fraction, int startValue, int endValue) {
 return evaluate(fraction, context.getResources().getColor(startValue), context.getResources().getColor(endValue));
 }
 /**
 * 成新的颜色值
 * @param fraction 颜色取值的级别 (0.0f ~ 1.0f)
 * @param startValue 开始显示的颜色
 * @param endValue 结束显示的颜色
 * @return 返回生成新的颜色值
 */
 public static int evaluate(float fraction, int startValue, int endValue) {
 int startA = (startValue >> 24) & 0xff;
 int startR = (startValue >> 16) & 0xff;
 int startG = (startValue >> 8) & 0xff;
 int startB = startValue & 0xff;

 int endA = (endValue >> 24) & 0xff;
 int endR = (endValue >> 16) & 0xff;
 int endG = (endValue >> 8) & 0xff;
 int endB = endValue & 0xff;

 return ((startA + (int) (fraction * (endA - startA))) << 24) |
  ((startR + (int) (fraction * (endR - startR))) << 16) |
  ((startG + (int) (fraction * (endG - startG))) << 8) |
  ((startB + (int) (fraction * (endB - startB))));
 }

}