Flutter Code Challenges & Tips

1. Create a custom Flutter widget that implements a toggle button. The button should change its appearance based on its on/off state and should maintain the state between rebuilds.

2. Write a function that takes a list of integers and returns a new list containing only the prime numbers from the original list.

3. Build a Flutter app that fetches data from a given API endpoint and displays the data in a ListView, implementing error handling for the network request.

4. Implement a function in Dart that checks if a given string is a palindrome. Consider edge cases like case sensitivity and special characters.

5. Create a Flutter widget that implements a simple animation where a square box moves across the screen. The animation should reverse direction on reaching the edges of the screen.

6. Write a Dart function that takes two sorted lists and merges them into a single sorted list without using any built-in list methods to do the sort.

7. Develop a Flutter app that allows users to draw freehand on a canvas. Implement features such as changing colors and clearing the drawing.

8. Write a function that converts a JSON string into a Dart object, and also converts that object back to a JSON string.

9. Create a custom Flutter form widget that validates email and password inputs, providing feedback based on the validity of the entries.

10. Implement a function to calculate the Fibonacci sequence both recursively and iteratively. Provide a parameter to specify how many numbers to generate.