title: Create my own completion blocks in iOS
body: <p>I have an object which takes a long time to do some stuff (it downloads data from a server).</p>
  
  <p>How can I write my own completion block so that I can run...</p>
  
  <pre><code>[downloader doSomeLongThing:^(void) {
      //do something when it is finished
  }];
  </code></pre>
  
  <p>I'm not sure how to save this block in the downloader object.</p>