android 在service 启动activity

我想我们一般在Service里想启动Activity一定会这样写： 

Intent intentv = new Intent(Intent.ACTION_VIEW);
            intentv.setData(uri);
            intentv.putExtra("keepTitle", true);
            startActivity(intentv);

这样写就会报错的： 

03-11 02:37:09.737: ERROR/AndroidRuntime(7881): android.util.AndroidRuntimeException: Calling startActivity() from outside of an Activity  context requires the FLAG_ACTIVITY_NEW_TASK flag. Is this really what you want? 

提示要加上FLAG_ACTIVITY_NEW_TASK，所以加上 intentv.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);这一行就可以了。 

但若看的源码够的话，还会发现有另外一种方法在Service里面启动Activity的另外一种方法：通过PendingIntent,下面就以Tag中TagView(Activity)和TagService为例子： 

TagView里启动TagService并且把待会在TagService里面启动的Activity用Pending将其封装好并且传给TagService: 

TagService.saveMessages(this, msgs, false, getPendingIntent());private PendingIntent getPendingIntent() {
        Intent callback = new Intent();
        callback.setClass(this, TagViewer.class);
        callback.setAction(Intent.ACTION_VIEW);
        callback.setFlags(Intent. FLAG_ACTIVITY_CLEAR_TOP);
        callback.putExtra(EXTRA_KEEP_TITLE, true);

        return PendingIntent.getActivity(this, 0, callback, PendingIntent.FLAG_CANCEL_CURRENT);
    }

public static void saveMessages(Context context, NdefMessage[] msgs, boolean starred,
            PendingIntent pending) {
        Intent intent = new Intent(context, TagService.class);
        intent.putExtra(TagService.EXTRA_SAVE_MSGS, msgs);
        intent.putExtra(TagService.EXTRA_STARRED, starred);
        intent.putExtra(TagService.EXTRA_PENDING_INTENT, pending);
        context.startService(intent);
    }

@Override
    public void onHandleIntent(Intent intent) {
        if (intent.hasExtra(EXTRA_SAVE_MSGS)) {
            Parcelable[] msgs = intent.getParcelableArrayExtra(EXTRA_SAVE_MSGS);
            NdefMessage msg = (NdefMessage) msgs[0];

            ContentValues values = NdefMessages.toValues(this, msg, false, System.currentTimeMillis());
            Uri uri = getContentResolver().insert(NdefMessages.CONTENT_URI, values);
            if (intent.hasExtra(EXTRA_PENDING_INTENT)) {
                Intent result = new Intent();
                result.setData(uri);

                PendingIntent pending = (PendingIntent) intent.getParcelableExtra(EXTRA_PENDING_INTENT);

                try {
                    pending.send(this, 0, result);
                } catch (CanceledException e) {
                    if (DEBUG) Log.d(TAG, "Pending intent was canceled.");
                }
            }

            return;
        }
.....
}

通过pending.send(this, 0, result);启动了对应的Activity. 

这里也是PendingIntent的用法之一。 
我不太明白这两种启动Activity的方法有什么不同之处虽然效果都是一样的，对开销会怎样，希望知道的朋友能和我分享。呵呵..