Implementing Software Design Principles in a no-code tool, such as MIT App Inventor

UI Design in MIT App Inventor

Blocks to code in MIT App Inventor

Parameterizing the UI widgets to not repeat the code

Below are examples of Software Design Principles commonly applied when designing code in object-oriented languages like Java.

• Single Responsibility Principle (SRP): A class should have only one reason to change, meaning it should have a single, well-defined purpose.
• Open-Closed Principle (OCP): Software entities (like classes and modules) should be open for extension but closed for modification.
• Liskov Substitution Principle (LSP): Objects of a superclass should be replaceable with objects of a subclass without affecting the program's correctness.
• Interface Segregation Principle (ISP): Clients should not be forced to depend on interfaces they do not use; it's better to have many specific interfaces than one general-purpose interface.
• Dependency Inversion Principle (DIP): High-level modules should not depend on low-level modules; both should depend on abstractions, and abstractions should not depend on details, but details should depend on abstractions.
• DRY (Don't Repeat Yourself): Avoid duplicating code to reduce complexity and improve maintainability.
• Keep It Simple: Design systems to be as simple as possible, avoiding unnecessary complexity.
• YAGNI (You Aren't Gonna Need It): Do not implement functionality until it is actually required, preventing the accumulation of unnecessary code.
• Abstraction: Hide complex details and provide a clear, understandable interface to users.
• Modularity: Break down a software system into smaller, independent, and manageable components.
• Testability and Maintainability: Design code that is easy to test, understand, and maintain by fixing bugs or adding new features.
• Reusability: Design software that can be used in other projects with minimal modification.

Above principles are straightforward to implement in object-oriented languages like Java. However, in a no-code environment such as MIT App Inventor, things can quickly become complex. While drag-and-drop, Lego-like code blocks make it easy to get started, the number of blocks can grow rapidly and become overwhelming as the application scales.

This is where good code design and organization practices prove invaluable. They help developers manage complexity and build maintainable, real-world applications. In the following sections, I will demonstrate how the above design principles can be applied effectively within MIT App Inventor and other similar no-code systems.

Single Responsibility Principle (SRP):

Unlike traditional code-based environments, in MIT App Inventor, the Screen serves as the primary unit of code—similar to a class in Java. Each Screen has its own properties, contains child UI components arranged in a tree-like hierarchy, and is controlled by Lego-style blocks that manage both UI events and backend services such as CloudDB.

Because there is no built-in mechanism for reusing UI elements across different Screens, it is best to design each Screen with a single responsibility. This approach keeps the widget hierarchy and associated blocks simple, organized, and easier to maintain.

When a Screen must serve multiple purposes with only minor variations, you can adopt a delegation model. By introducing a flag to indicate the current mode, the Screen’s behavior can be adjusted without duplicating the entire structure. However, avoid applying this technique to Screens with vastly different roles, as overuse can increase complexity and make the app harder to maintain.

Open-Closed Principle (OCP):

In object-oriented programming (OOP) languages like Java, extension is typically achieved through inheritance. In no-code environments, achieving the same effect is more challenging.

A recommended practice is to design your blocks and UI hierarchy so that new functionality can be added as an extension, rather than modifying existing features. Avoid altering existing behavior, as this can introduce errors and compromise stability. By extending instead of modifying, you ensure your app remains reliable while still allowing it to evolve over time.

Additionally, the Open-Closed Principle can be applied when creating custom components, enabling them to be extended without changing their original implementation.

Liskov Substitution Principle (LSP):

Since MIT App Inventor does not directly support inheritance, the concepts of subclass and superclass can only be implemented through custom components. In this approach, a subclass can be used wherever its superclass would normally appear. For example, a specialized Button component can be created and substituted for a default Button, adhering to this principle.

Interface Segregation Principle (ISP):

Rather than creating a single screen or block setup that handles many unrelated features, break your app into smaller, purpose-driven modules. Develop utility procedures (functions) that perform one well-defined task instead of large, catch-all procedures. Similarly, design each screen to serve one primary purpose rather than bundling multiple features together.

Dependency Inversion Principle (DIP):

Rather than tying your logic directly to a specific implementation, encapsulate it in a procedure that represents the abstract behavior. Use flags or configuration variables to switch between implementations when minor variations are needed. Additionally, create helper screens or components that act as reusable “providers” for shared functionality.

DRY (Don't Repeat Yourself):

If a sequence of blocks is used multiple times, extract it into a procedure. Use constants or configuration values stored in global variables instead of repeating them across screens. Avoid creating multiple screens that are nearly identical but differ only in minor ways; instead, repurpose a single screen and control variations with mode flags. For logic shared across multiple screens—such as file picking or error handling—consider creating a dedicated screen or custom component to centralize that functionality.

Keep It Simple:

Avoid creating too many screens; instead, reuse screens by using flags or parameters to modify behavior. Keep your blocks organized—don’t create spaghetti code, and avoid cramming multiple complex algorithms into a single block; separate them into smaller, manageable procedures. Similarly, don’t overload screens with too many buttons or labels; use clear labels and group controls logically for a clean and user-friendly interface.

YAGNI (You Aren't Gonna Need It):

Avoid adding screens prematurely based on assumptions or anticipated features. Keep the UI minimal and allow it to evolve according to actual needs and requirements. Similarly, don’t over-generalize procedures; while it may be tempting to create a “super procedure” that handles every possible case, breaking logic into focused, well-defined procedures is more maintainable and easier to debug.

Abstraction:

Use procedures to encapsulate logic instead of repeating long sequences of blocks. Abstract UI actions into events and triggers, and move all complex logic into dedicated procedures. Create reusable helper procedures to handle recurring or complex tasks. Additionally, keep data storage blocks separate from UI update blocks to maintain clarity and modularity.

Modularity:

Use multiple screens for different features, ensuring each screen represents a single main functionality or module. Define procedures as mini-modules that perform one specific task. Avoid mixing UI event handling (buttons, sliders) with data handling (TinyDB, lists). If a group of UI elements and blocks serves the same purpose in multiple places, create a custom component (using extensions or templates) for reuse. Finally, don’t overload a single screen with too many features; keep each screen focused and manageable.

Testability:

Make screens as testable as possible by keeping them independent. Make data storage mockable by using intermediary procedures, which can also be leveraged for testing. Additionally, add debug labels or temporary notifiers to display intermediate results and facilitate easier debugging.

Maintainability:

Use self-explanatory names for procedures, variables, and components. If you find yourself copy-pasting the same blocks multiple times, encapsulate them into a procedure. Add block comments to clarify any tricky logic. Since MIT App Inventor does not support direct Git integration, export your .aia files frequently and maintain versioned backups (v1, v2, v3). Consider using cloud storage to store these backups and snapshots of key features.

Reusability:

Encapsulate commonly used operations into procedures instead of repeating blocks. Parameterize procedures rather than hardcoding values, so they can be reused in different contexts. Organize shared data access by creating wrapper procedures for TinyDB, CloudDB, file operations, and similar tasks.

Since MIT App Inventor does not support “fragments” like Android, reuse screens for similar purposes with minor variations by using flags or parameters. Abstract repeated UI patterns: if multiple screens share a UI pattern (e.g., a “list of items + delete button”), copy the UI once and adjust the data source or title dynamically.

Take advantage of extensions and community modules to implement reusable functionality, and export blocks or use the backpack feature to transfer procedures and components to other projects.