Cool Stuff in Cocoa and Objective-C

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In my travels through Objective C and Cocoa Programming, I've come across some random bits of coolness.
Last updated April 2, 2002.

• PoseAsClass is really cool. You can have your own class become any other class, allowing you to do some work, and then forward up the inheritance chain. Big deal? When you pose as, your class gets used whenever the other class would be used. For example - to peek at a textview when they're drawn anywhere in your program (even from the font and color panels)

  @interface TextViewPoser : NSTextView
  @end // TextViewPoser
  
  @implementation TextViewPoser
  
  - (void) drawRect: (NSRect) bounds
  {
      NSLog (@"drawRect");
      [super drawRect: bounds];
  
  } // drawRect
  
  @end // TextViewPoser
  

  Then in your main.m (or somewhere convenient)

  [[TextViewPoser class] poseAsClass: [NSTextView class]];

• Using categories, you can put methods on other classes, including the pre-canned toolkit classes. For example, this makes NSViews draw their backgrounds in yellow by default, which will cause windows to take on a yellow cast:

  @implementation NSView (BWYellowView)
  
  - (void) drawRect: (NSRect) rect
  {
      NSColor *transparentYellow;
  
      rect = [self bounds];
  
      transparentYellow = [NSColor colorWithCalibratedRed: 1.0
                                   green: 1.0
                                   blue: 0.0
                                   alpha: 0.333];
      [transparentYellow set];
      [NSBezierPath fillRect: rect];
  
  } // rect
  

• NSObject's "valueForKey" and "takeValue" methods. These let you set and get values in an arbitrary object by name
  

      NSString *identifier = [aTableColumn identifier];
      Person *person = [employees objectAtIndex: rowIndex];
      
      return ([person valueForKey: identifier]);
  

  Why is this cool? By changing the value of the identifier, you can change the field of the Person object you'll be accessing. e.g. you can change which part of the object you're manipulating in Interface Builder and not have to recompile code. Also, it lets you avoid having to write (and maintain) the nasty idiom of

      if (identifier is "oop") {
          return (person.oop);
      } else if (identifier is "ack") {
          return (person.ack);
      } ...
  

• Delegation in objective C uses the 'respondsToSelector:' method of NSObject. If you want to see all the different selectors your object is being asked about, add this to your class:

      - (BOOL)respondsToSelector: (SEL)aSelector
      {
          NSString *methodName = NSStringFromSelector (aSelector);
          NSLog (@"respondsToSelector: %@", methodName);
          return ([super respondsToSelector: aSelector]);
      } // respondsToSelector