Features Required

1. Pattern Input Handling

   • Support for users to input unlock patterns on a 3x3 grid.

   • Visual feedback for touch events, including highlighting selected dots and drawing lines between them.

   • Validation of input patterns according to Android's unlock rules:

     • No repeated dots.

     • Lines cannot skip dots unless the skipped dot has been used.

2. Pattern Storage and Verification

   • Securely store the user's chosen unlock pattern.

   • Allow users to set, confirm, and change their unlock patterns.

   • Verify input patterns against the stored pattern.

3. Security Features

   • Encrypt the stored pattern to prevent unauthorized access.

   • Implement a lockout mechanism after a certain number of failed attempts.

   • Provide feedback messages for incorrect patterns and lockout status.

4. User Interface

   • Intuitive and responsive UI for pattern input.

   • Indicate errors and lockout status to the user.

   • Support for different screen sizes and orientations.

5. Accessibility

   • Support for accessibility features such as TalkBack.

   • Ensure that the app is usable for users with disabilities.

6. Performance

   • Smooth and responsive touch handling.

   • Efficient use of resources to minimize battery consumption.

Design Patterns Used

1. Singleton Pattern

   • Purpose: Ensure a class has only one instance and provide a global point of access.

   • Usage: Managing a single instance of the security manager and pattern storage.

2. Factory Pattern

   • Purpose: Create objects without specifying the exact class of object that will be created.

   • Usage: Generating different types of pattern validators or encryption strategies.

3. Strategy Pattern

   • Purpose: Define a family of algorithms, encapsulate each one, and make them interchangeable.

   • Usage: Implementing different encryption algorithms or validation strategies.

4. Observer Pattern

   • Purpose: Allow an object to publish changes to its state. Other objects subscribe to be immediately notified of any changes.

   • Usage: Updating the UI based on changes in pattern input or lockout status.

5. Decorator Pattern

   • Purpose: Attach additional responsibilities to an object dynamically.

   • Usage: Adding encryption functionality to pattern storage without changing its interface.

6. Command Pattern

   • Purpose: Encapsulate a request as an object, thereby allowing for the parameterization of clients with queues, requests, and operations.

   • Usage: Handling user actions such as pattern drawing, undo, and reset.

Algorithms Involved

1. Pattern Validation Algorithm

   • Validates the unlock pattern according to Android's rules.

2. Encryption Algorithm

   • Securely encrypts and decrypts the stored pattern.

3. Lockout Algorithm

   • Locks the pattern input after a certain number of failed attempts for a specified duration.

4. Touch Event Handling Algorithm

   • Efficiently handles touch events to provide smooth pattern drawing.

Code (In Java)

Will be Covered in Our LLD Course (with Video Explanation) - http://interview.lldcoding.com/course/low-level-design-and-concurrency-machine-coding-course-360