title: Popover not show up by tapping UIButton
body: <p>I may need to show up a popover containing my own view controller when tapping a UIButton in a view, here is my code in a UIButton click method.</p>
  
  <pre><code>self.assetPickerPopover=[[UIPopoverController alloc]initWithContentViewController:avc];
  
  self.assetPickerPopover.delegate=self;
  
  //[self.imagePickerPopover presentPopoverFromBarButtonItem:sender permittedArrowDirections:UIPopoverArrowDirectionAny animated:YES];
  [self.imagePickerPopover setPopoverContentSize:CGSizeMake(400, 2000) animated:NO];
  [self.imagePickerPopover presentPopoverFromRect:self.assetTypeButton.frame inView:self.view permittedArrowDirections:UIPopoverArrowDirectionAny animated:YES];
  </code></pre>
  
  <p>as you may see that I tried both methods to call out popover but both failed, even I set size of popover explicitly.</p>