XMPP传入和传出文件委托不调用

I am trying to implement file transfer using XMPP. Below is a snippet of my code. Now the problem is, delegate methods of XMPPOutGoingFileTransfer and XMPPIncomingFileTransfer is not getting called. I dig out the framework for a solution but got no success. I don't know where i'm going wrong. Any type of help would be appreciated. I am trying to implement file transfer using XM