title: write plist files incrementally
body: <p>I have a <code>NSMutableArray</code>, and I write it to a plist file using <code>writeToFile: atomically:</code>. The elements in the array are all property list objects.</p>
  
  <p>Then in my iPhone App, I meet a case: when the <code>NSMutableArray</code> needs to add an object and I also want to update the plist file. Now what I do is to call <code>writeToFile: atomically:</code> when an new one is added. </p>
  
  <p>However when the code>NSMutableArray has a large mount of data, it must be in poor performance. SO I wonder how to write the newly added element to the existed plist file.</p>
  
  <p>Thanks.</p>
  
  <p>PS: the business logic is:</p>
  
  <p>When the user does some action,like sending a new message or add a new photo, I'll make a record of user,message/image data and time(all is property list objects). The Action can be happen everywhere, and I didn't create the array to check the actions always (and it is in no need). Of course I use sqilte to do it, but I also need a plist file, so that user's friends can download it whenever they want. The friends need no request from the user and just download it. So I plan to keep an up-to-date plist file. 
  That's all why I need it. </p>