The Eau theme uses method swizzling to augment existing GNUstep classes without completely replacing their implementations. This ensures that original behavior is preserved while adding theme-specific enhancements.

Why Method Swizzling?

Direct category overrides (using @implementation ClassName (Category)) replace the original method entirely, which can break inheritance chains and skip important superclass logic. Method swizzling allows you to "wrap" the original implementation with custom logic while ensuring the original code still runs.

Example: NSButton+Eau.m

In NSButton+Eau.m, we need to handle keyboard events (spacebar and Enter/Return) for button activation while preserving the original NSButton behavior for other keys.

Incorrect Approach (Direct Override):

@implementation NSButton (EauKeyboardHandling)

- (void) keyDown: (NSEvent*)theEvent
{
  // Custom logic...
  [super keyDown: theEvent];  // This skips NSButton's logic!
}

@end

Correct Approach (Method Swizzling):

#import <objc/runtime.h>

@implementation NSButton (EauKeyboardHandling)

+ (void) load
{
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    Class cls = [NSButton class];
    SEL origSelector = @selector(keyDown:);
    SEL swizSelector = @selector(eau_keyDown:);

    Method origMethod = class_getInstanceMethod(cls, origSelector);
    Method swizMethod = class_getInstanceMethod(cls, swizSelector);

    // Attempt to add the method first in case NSButton doesn't implement it directly
    BOOL didAddMethod = class_addMethod(cls,
                                        origSelector,
                                        method_getImplementation(swizMethod),
                                        method_getTypeEncoding(swizMethod));

    if (didAddMethod)
      {
        class_replaceMethod(cls,
                            swizSelector,
                            method_getImplementation(origMethod),
                            method_getTypeEncoding(origMethod));
      }
    else
      {
        method_exchangeImplementations(origMethod, swizMethod);
      }
  });
}

- (void) eau_keyDown: (NSEvent*)theEvent
{
  // Custom logic for spacebar/Enter handling...

  // Call the original implementation (now points to eau_keyDown)
  [self eau_keyDown: theEvent];
}

@end

Key Benefits:

Preserves Original Behavior: The original NSButton's keyDown: logic is preserved under a new name (eau_keyDown:)
Avoids Superclass Conflicts: Calling [self eau_keyDown: theEvent] invokes the original NSButton implementation, not skipping to NSControl
Predictable Load Order: Using +load ensures swizzling happens as soon as the theme bundle loads
Safe for Inheritance: The class_addMethod check handles cases where NSButton inherits keyDown: from its parent

When to Use Swizzling vs. Theme Engine Hooks

Use Swizzling: For standard AppKit classes like NSButton where the GSTheme engine doesn't provide override hooks
Use Theme Engine: For classes like NSPopUpButton that provide built-in override mechanisms (e.g., _overrideNSPopUpButtonMethod_mouseDown:)

Best Practices

Always use dispatch_once in +load for thread safety
Include the class_addMethod check for inheritance safety
Document which methods are swizzled and why
Test thoroughly to ensure original behavior is preserved