android仿知乎ScrollView滚动改变标题栏透明度

import com.example.d_changealphabyscroll.widget.ObservableScrollView;
import com.example.d_changealphabyscroll.widget.ObservableScrollView.ScrollViewListener;
import android.app.Activity;
import android.graphics.Color;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewTreeObserver;
import android.view.ViewTreeObserver.OnGlobalLayoutListener;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.ImageView;
import android.widget.ScrollView;
public class MainActivity extends Activity implements ScrollViewListener{
private View layoutHead;
private ObservableScrollView scrollView;
private ImageView imageView;
private WebView webView;
private int height ;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initView();
}
private void initView() {
webView = (WebView) findViewById(R.id.webview1);
scrollView = (ObservableScrollView) findViewById(R.id.scrollview);
layoutHead = findViewById(R.id.view_head);
imageView = (ImageView) findViewById(R.id.imageView1);
layoutHead.setBackgroundColor(Color.argb(0, 0xfd, 0x91, 0x5b));
//初始化webview
//启用支持javascript
WebSettings settings = webView.getSettings();
settings.setJavaScriptEnabled(true);
webView.loadUrl("http://www.topit.me/");
//覆盖WebView默认使用第三方或系统默认浏览器打开网页的行为，使网页用WebView打开
webView.setWebViewClient(new WebViewClient(){
@Override
public boolean shouldOverrideUrlLoading(WebView view, String url) {
//返回值是true的时候控制去WebView打开，为false调用系统浏览器或第三方浏览器
view.loadUrl(url);
return true;
}
});
//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);
height = imageView.getHeight();
imageView.getWidth();
scrollView.setScrollViewListener(MainActivity.this);
} 
});
}
@Override
public void onScrollChanged(ObservableScrollView scrollView, int x, int y,
int oldx, int oldy) {
// Log.i("TAG","y--- "+y+" height-- "+height);
if(y<=height){
float scale =(float) y /height;
float alpha = (255 * scale);
// Log.i("TAG","alpha--- "+alpha);
//layout全部透明
// layoutHead.setAlpha(scale);
//只是layout背景透明(仿知乎滑动效果)
layoutHead.setBackgroundColor(Color.argb((int) alpha, 0xfd, 0x91, 0x5b));
}
}
}

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