如何监听webview的滚动事件

package com.jcodecraeer.mobile.view;

import android.content.Context;

import android.util.AttributeSet;

import android.webkit.WebView;

/**
* Created by jianghejie on 15/7/16.
*/
public class ObservableWebView extends WebView {
private OnScrollChangedCallback mOnScrollChangedCallback;

public ObservableWebView(final Context context) {
super(context);
}

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

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

@Override
protected void onScrollChanged(final int l, final int t, final int oldl,
final int oldt) {
super.onScrollChanged(l, t, oldl, oldt);

if (mOnScrollChangedCallback != null) {
mOnScrollChangedCallback.onScroll(l - oldl, t - oldt);
}
}

public OnScrollChangedCallback getOnScrollChangedCallback() {
return mOnScrollChangedCallback;
}

public void setOnScrollChangedCallback(
final OnScrollChangedCallback onScrollChangedCallback) {
mOnScrollChangedCallback = onScrollChangedCallback;
}

/**
* Impliment in the activity/fragment/view that you want to listen to the webview
*/
public static interface OnScrollChangedCallback {
public void onScroll(int dx, int dy);
}
}