Expanding Outline Views in Cocoa

Thanks to a lengthy commute last week, I’ve been making a toy in Cocoa. It reads an URL, parses some XML and displays it in an NSOutlineView. Simple stuff, but it will hopefully make my life better at work, where I need to do this a fair bit.

By default, when I load the XML into the NSOutlineView, everything is closed up. So all you’re presented with is the root element. I’d like to expand that so it automatically includes all children of the root element. Nice and simple—there’s an expandItem: method.

Except that when I call it from the action that puts the XML into the NSOutlineView, it doesn’t work. Bugger.

After instrumenting my XML data source, I can see that nothing is really happening until after my action. My suspicion is that the NSOutlineView isn’t realising that it has any data until after the first call to display.

I tested this by hooking up the call to expandItem into a secondary action (on another button). And it works great.

So, I need a way to say “call this code back in the next idle period”. And this is where I start to get upset that Objective-C doesn’t have closures.

How to execute code soon isn’t easily determined from the apple docs. My guess is that you use an NSTimer with a very small NSTimeInterval . Let’s try that…

  NSTimer *idle = [NSTimer scheduledTimerWithTimeInterval:0.0
                                                   target:self
                                                 selector:@selector(expandRoot)
                                                 userInfo:nil
                                                  repeats:NO];
  [[NSRunLoop currentRunLoop] addTimer:idle forMode:NSDefaultRunLoopMode];

Well, it works. But! Further investigation finally reveals Deferring the Execution of Operations. This suggests that I should use an NSNotification instead, but posted with a NSPostWhenIdle flag. This means getting involved with the cocoa notifications system…

The code now ends up looking like this:

  - (void)awakeFromNib
  {
    // Listen out for notification's we've posted to ourselves.
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(expandRoot:)
                                                 name:@"expandRoot"
                                               object:nil];
  }

  -(IBAction)fetch: (id)sender
  {
    // …
    NSNotification* todo = [NSNotification notificationWithName:@"expandRoot"
                                                         object:self];
    [[NSNotificationQueue defaultQueue] enqueueNotification:todo
                                               postingStyle:NSPostWhenIdle];
  }

  - (void)expandRoot:(NSNotification *)notification
  {
    [outlineView expandItem:[[outlineViewDataSource doc] rootElement]];
  }

Which is quite a bit more code. But it feels more robust doing it this way.

The big take away from all this is how difficult it is to use a non-Open-Source framework. If I had the source to Cocoa, I’d be able to look inside and see what I needed to do simply and quickly. Instead, it took me three train journeys. But there’s still enough to like in Cocoa that I intend to carry on.