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Quick Tip: Peeking into objects

Hi everyone!

Remember this¬†post from way back about creating custom LLDB summaries for your objects so that you can conveniently inspect objects while debugging? Well the folks at Cupertino have been hard at work and have shipped an awesome feature with the new XCode 5.1 – “Debug Quick Look” which renders my previous post pretty much useless (and I mean useless in the sense that it is now a magnitude easier to accomplish something even better).

So basically this is how it works- You can now provide a visual “meaning” to objects you instantiate by implementing the following method:

- (id)debugQuickLookObject{
    //TODO: Return anything you want to see while debugging

Let’s take a look at the following example – I’ve created a data type that maps an NSString to a UIColor (don’t ask me why). It’s really easy to think of how you’d like this visualized for debugging purposes, so rather than seeing something like this:

   Coffee = "UIDeviceRGBColorSpace 0.6 0.4 0.2 1";
   Deep = "UIDeviceRGBColorSpace 0.5 0 0.5 1";
   Ocean = "UIDeviceRGBColorSpace 0 0 1 1";
   Tree = "UIDeviceRGBColorSpace 0 1 0 1";

…we can now use the debugQuickLookObject and see something like this:
Screen Shot 2014-02-08 at 6.50.37 PM
Here’s the code that is responsible for generating this wonder:

- (id)debugQuickLookObject
    NSMutableAttributedString *attrStr = [[NSMutableAttributedString alloc] init];
    [self.theDict enumerateKeysAndObjectsUsingBlock:^(NSString *str, UIColor *color, BOOL *stop) {
        NSAttributedString *colorAttrStr = [[NSAttributedString alloc] initWithString:[str stringByAppendingString:@"\n"] attributes:@{NSForegroundColorAttributeName : color}];
        [attrStr appendAttributedString:colorAttrStr];
    return attrStr;

Now remember, this is code that actually runs while your process is paused so you want to:

  1. Run as little code as possible (creating a caching mechanism is recommended)
  2. Avoid changing your application’s internal state – anything that runs in the¬†debugQuickLookObject method actually interacts with your application’s classes, instances and general state so don’t mess with anything (think of this as Marty avoiding his parents in Back to the Future).

The cool thing about this feature is that you can basically return anything that has visual representation (Strings, Attributed strings, images, bezier paths, views, urls, etc…) and see them while debugging. To access the provided quick look object, all you need to do is hover over the instance and click the little eye icon.

I can already think of dozens of places in my code where everyone collaborating with me could benefit from this visualization (including myself). Also, it would be nice to see this becoming a community thing – Developers writing open source projects could enrich documentation using custom quick looks and generally anyone could write quick looks for existing complex and hard-to-debug code.