Can you name some main design patterns and explain their application?

Design patterns and their applications:

• Singleton — guarantees that a class has only one instance and provides a global point of access to it. Used, for example, for settings managers or database connections.

• Factory Method — defines an interface for creating an object but allows subclasses to alter the type of objects that will be created. Helps in creating objects without tight coupling to specific classes.

• Observer — allows objects to subscribe to events of another object and receive notifications about changes. Often used for updating UI when data changes.

• Builder — separates the construction of a complex object from its representation, allowing the same construction process to create different representations.

• Decorator — dynamically adds responsibilities to objects by wrapping them in other objects.

In Flutter, patterns related to state management are often used, such as:

• Provider — for dependency injection and state management.
• BLoC (Business Logic Component) — separates business logic from UI using data streams.

Example of a simple Singleton pattern in Dart:

class Singleton {
  Singleton._privateConstructor();

  static final Singleton _instance = Singleton._privateConstructor();

  factory Singleton() {
    return _instance;
  }
}

This guarantees that the Singleton class will have only one instance.