监测database的改变--notifyChange
我们在ContentProvider的insert,update,delete等改变之后调用getContext().getContentResolver().notifyChange(uri, null);这样就通知那些监测databases变化的observer了，而你的observer可以在一个service里面注册。

以Downloadmanger为例子：
定义ContentObserver，并且在onChange里做你想做的事情。

/**
     * Receives notifications when the data in the content provider changes
     */
    private class DownloadManagerContentObserver extends ContentObserver {

        public DownloadManagerContentObserver() {
            super(new Handler());
        }

        /**
         * Receives notification when the data in the observed content
         * provider changes.
         */
        public void onChange(final boolean selfChange) {
            if (Constants.LOGVV) {
                Log.v(Constants.TAG, "Service ContentObserver received notification");
            }
            updateFromProvider();
        }

    }

在DownloadService的onCreate中注册：

 public void onCreate() {
        super.onCreate();
        if (Constants.LOGVV) {
            Log.v(Constants.TAG, "Service onCreate");
        }

        mDownloads = Lists.newArrayList();

        mObserver = new DownloadManagerContentObserver();
        getContentResolver().registerContentObserver(Downloads.CONTENT_URI,
                true, mObserver);
......}

 /**
     * Cleans up when the service is destroyed
     */
    public void onDestroy() {
        getContentResolver().unregisterContentObserver(mObserver);
        if (Constants.LOGVV) {
            Log.v(Constants.TAG, "Service onDestroy");
        }
        super.onDestroy();
    }

可以参考以下文章：
http://hi.baidu.com/lck0502/blog/item/a818258f304b61e0f01f3691.html