🚀 Summary

This PR enhances the existing override* button APIs (overrideDone, overrideNext, overrideSkip, overrideBack) by passing them an onPressed callback. Consumers can now supply fully custom widgets and wire up the built-in navigation logic, without having to manually hook into IntroductionScreenState.

🧩 Motivation

Currently, overrideDone (and its siblings) accept a Widget?, which makes it impossible to integrate the built-in navigation behavior (e.g. advancing to the next page, skipping). Users who want a custom button must:

1. Provide their own widget.
2. Manually call navigation methods on the GlobalKey<IntroductionScreenState>.

This is cumbersome and error-prone. By passing the onPressed handler directly to the override builder, we:

• Promote DRY: reuse existing navigation callbacks.
• Simplify usage: consumer only needs to define appearance.
• Ensure consistent behavior across all buttons.

🛠️ Implementation

• Signature change

  // Before
  final Widget? overrideDone;
  
  // After
  final Widget Function(BuildContext context, VoidCallback? onPressed)? overrideDone;

• Apply the same pattern to:

  • overrideNext
  • overrideSkip
  • overrideBack

• Internally, when rendering an override button:

  if (overrideDone != null) {
    child = overrideDone!(context, _handleDone);
  } else {
    child = done;
  }

• Where _handleDone is the existing navigation callback that invokes onDone and advances the screen.

🔍 Usage Example

IntroductionScreen(
  pages: pages,
  overrideDone: (context, onPressed) {
    return ElevatedButton.icon(
      icon: const Icon(Icons.check_circle),
      label: const Text("Let’s Go!"),
      style: ElevatedButton.styleFrom(
        primary: Colors.green,
        shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(12)),
      ),
      onPressed: onPressed, // Automatically calls `onDone`
    );
  },
  onDone: () {
    // Custom completion logic
  },
);

All other override parameters work identically:

overrideNext: (ctx, onPressed) => MyCustomNextButton(onPressed: onPressed),
overrideSkip: (ctx, onPressed) => TextButton(onPressed: onPressed, child: Text("Skip")),
overrideBack: (ctx, onPressed) => IconButton(icon: Icon(Icons.arrow_back), onPressed: onPressed),