Clean Architecture & Design Patterns in Practice · 课时

实现用例（交互器）

开发用例，编排实体以实现特定的应用功能，并体现应用专属的业务规则。

实现用例（交互器） 是 CoddyKit 上的免费 Clean Architecture & Design Patterns in Practice 课时。这是第 2 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Clean Architecture & Design Patterns in Practice 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Clean Architecture & Design Patterns in Practice 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

What are Use Cases (Interactors)?

In Clean Architecture, Use Cases (sometimes called Interactors) are at the heart of your application's logic. They represent the specific tasks or features your application can perform.

Think of them as the orchestrators of your core business entities. They define how your application uses those entities to achieve a goal, like 'create a new user' or 'process an order'.

Role of Use Cases in Clean Architecture

Use Cases sit in the 'Use Cases' layer, just outside the 'Entities' layer. This means:

They depend on Entities.
They do NOT depend on external layers like the UI, databases, or web frameworks.

Their main job is to coordinate the flow of data to and from the Entities and direct the Entities to perform their specific business rules.

Embodying Application Rules

While Entities hold the enterprise-wide business rules (rules true for the entire business, regardless of application), Use Cases embody application-specific business rules.

For example, an 'Order' Entity might have a rule that an order total cannot be negative. A 'Process Order' Use Case might have an application rule that a user must be logged in to place an order, or that certain promotions apply at checkout.

Defining Use Case Boundaries

To keep Use Cases isolated, they communicate with the outside world through interfaces:

Input Port: An interface that defines what data a Use Case expects to receive.
Output Port: An interface that defines what data a Use Case will produce.

These ports ensure the Use Case doesn't know about specific UI components or database implementations.

Anatomy of a Use Case

A typical Use Case is a class with a single public method (e.g., execute or handle). This method usually takes an 'Input Data Transfer Object' (DTO) and returns an 'Output DTO'.

Inside, it interacts with entities and often uses 'Gateway' or 'Repository' interfaces to talk to data storage or external services, without knowing their concrete implementations.

Example: Create User Use Case

Let's consider a CreateUserUseCase. First, we define the simple data structures for input and output. These are plain Java objects (POJOs).

public class CreateUserInput {
  public String name;
  public String email;
}

public class CreateUserOutput {
  public String userId;
  public String message;
  public boolean success;
}

Implementing the Logic

Now, let's see how the CreateUserUseCase orchestrates a User entity and interacts with a UserRepository (an interface) to perform its task.

Try running this example:

public class User {
  private String id;
  private String name;
  private String email;

  public User(String id, String name, String email) {
    this.id = id;
    this.name = name;
    this.email = email;
  }

  public String getId() { return id; }
  public String getName() { return name; }
  public String getEmail() { return email; }
  
  public boolean isValid() { return name != null && !name.isEmpty() && email != null && email.contains("@"); }
}

interface IUserRepository {
  void save(User user);
  String generateId();
}

class InMemoryUserRepository implements IUserRepository {
  @Override
  public void save(User user) {
    System.out.println("Saving user: " + user.getName() + " with ID: " + user.getId());
  }

  @Override
  public String generateId() {
    return "user-" + System.currentTimeMillis();
  }
}

public class CreateUserInput {
  public String name;
  public String email;
}

public class CreateUserOutput {
  public String userId;
  public String message;
  public boolean success;
}

class CreateUserUseCase {
  private final IUserRepository userRepository;

  public CreateUserUseCase(IUserRepository userRepository) {
    this.userRepository = userRepository;
  }

  public CreateUserOutput execute(CreateUserInput input) {
    CreateUserOutput output = new CreateUserOutput();

    if (input.name == null || input.name.isEmpty()) {
      output.success = false;
      output.message = "Name cannot be empty.";
      return output;
    }
    if (input.email == null || !input.email.contains("@")) {
      output.success = false;
      output.message = "Invalid email address.";
      return output;
    }

    String newId = userRepository.generateId();
    User newUser = new User(newId, input.name, input.email);

    if (!newUser.isValid()) {
      output.success = false;
      output.message = "User data is invalid.";
      return output;
    }
    
    userRepository.save(newUser);
    
    output.success = true;
    output.userId = newId;
    output.message = "User created successfully.";
    return output;
  }
}

public class Main {
  public static void main(String[] args) {
    IUserRepository repo = new InMemoryUserRepository();
    CreateUserUseCase useCase = new CreateUserUseCase(repo);

    CreateUserInput input = new CreateUserInput();
    input.name = "Alice";
    input.email = "alice@example.com";

    CreateUserOutput output = useCase.execute(input);

    if (output.success) {
      System.out.println("Result: " + output.message + " ID: " + output.userId);
    } else {
      System.out.println("Error: " + output.message);
    }

    // Test with invalid input
    CreateUserInput invalidInput = new CreateUserInput();
    invalidInput.name = "";
    invalidInput.email = "invalid";
    CreateUserOutput errorOutput = useCase.execute(invalidInput);
    System.out.println("Error test: " + errorOutput.message);
  }
}

Benefits of Use Cases

Using Use Cases brings significant advantages to your software design:

Isolation: Application-specific logic is isolated from UI, database, and frameworks.
Testability: Use Cases can be tested easily in isolation, without needing a UI or database connection.
Independence: Changes in the UI or database technology won't directly impact your core application logic.
Clarity: Each Use Case clearly defines a single feature or task of the application.

Use Case Interaction Flow

When a user interacts with your application, here's a simplified flow involving a Use Case:

UI/Controller: Receives a request (e.g., button click).
Input Port: The controller translates the request into a specific input format for the Use Case.
Use Case: Executes its logic, orchestrating entities and interacting with repositories/gateways.
Output Port: The Use Case provides its result in a generic output format.
Presenter/UI: Transforms the Use Case output into something displayable to the user.

This flow ensures a clean separation of concerns.

Quick Check: Use Case's Role

Based on what you've learned, what is the primary role of a Use Case (Interactor) in Clean Architecture?

Recap: Orchestrating Actions

You've now learned about Use Cases, the application-specific orchestrators in Clean Architecture. They define your app's features, coordinate entities, and encapsulate business rules specific to the application.

By using Input and Output Ports, Use Cases remain independent of external layers, making your core logic highly testable, maintainable, and flexible to change. This is a crucial step towards building robust and adaptable software systems!

常见问题解答

「实现用例（交互器）」课时是免费的吗？

是的 — 「实现用例（交互器）」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Clean Architecture & Design Patterns in Practice 课程的其余内容，请升级到 CoddyKit PRO。 Clean Architecture & Design Patterns in Practice 课程共包含 4 节课。

「实现用例（交互器）」这节课中我会学到什么？

开发用例，编排实体以实现特定的应用功能，并体现应用专属的业务规则。你通过在浏览器中直接运行的动手代码来练习 Clean Architecture & Design Patterns in Practice，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Clean Architecture & Design Patterns in Practice 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Clean Architecture & Design Patterns in Practice 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 2 节课，共 4 节。

「实现用例（交互器）」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Clean Architecture & Design Patterns in Practice 课中编写并运行代码吗？

能。每节 Clean Architecture & Design Patterns in Practice 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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