title: DidfinshLoad for imageData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:srcAddres]];?
body: <pre><code>imageData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:srcAddres]];
  </code></pre>
  
  <p>Downloading jpeg to my imagedata  will spend some time,for example: 2 second, so I want to show an activity indicator until the downloading finished. How can I know when the downloading has finished??</p>