ContentObserver有点类似于BroadcastReceiver,当某类事件发生时进行调用.ContentObserver一般和系统或第三方程序提供的Provider一起使用.

当为某个URI注册了ContentObserver后,对其进行操作后都会调用注册的回调函数,以监视未接电话为例（未读短信和未接电话类似,只需将URI改为短信的URI就可以了）：

// 注册ContentObserver   
getContentResolver().registerContentObserver(CallLog.Calls.CONTENT_URI, false, new MissedCallContentObserver(this,    
  
new Handler()));   
  
// 当有call log时,就会对这个类进行回调   
public class MissedCallContentObserver extends ContentObserver {   
       
    private Context ctx;   
       
    private static final String TAG = "MissedCallContentObserver";   
       
    public MissedCallContentObserver(Context context, Handler handler) {   
        super(handler);   
        ctx = context;   
    }   
  
    @Override  
    public void onChange(boolean selfChange) {   
           
        Cursor csr = ctx.getContentResolver().query(Calls.CONTENT_URI, new String[] {Calls.NUMBER,    
  
Calls.TYPE, Calls.NEW}, null, null, Calls.DEFAULT_SORT_ORDER);   
           
        if (csr != null) {   
            if (csr.moveToFirst()) {   
                int type = csr.getInt(csr.getColumnIndex(Calls.TYPE));   
                switch (type) {   
                case Calls.MISSED_TYPE:   
                    Log.v(TAG, "missed type");   
                    if (csr.getInt(csr.getColumnIndex(Calls.NEW)) == 1) {   
                        Log.v(TAG, "you have a missed call");   
                    }   
                    break;   
                case Calls.INCOMING_TYPE:   
                    Log.v(TAG, "incoming type");   
                    break;   
                case Calls.OUTGOING_TYPE:   
                    Log.v(TAG, "outgoing type");   
                    break;   
                }   
            }   
            // release resource   
            csr.close();   
        }   
    }   
       
    @Override  
    public boolean deliverSelfNotifications() {   
        return super.deliverSelfNotifications();   
    }   
}