PatternTest.java

package com.hanserwei.patterns.observer;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertThrows;
import static org.junit.jupiter.api.Assertions.assertTrue;

import java.util.List;
import org.junit.jupiter.api.Test;

/** 验证本模式的协作契约与边界行为. */
public final class PatternTest {
  /** 去重订阅和取消订阅影响后续通知. */
  @Test
  void managesSubscriptions() {
    PublicationHub hub = new PublicationHub();
    InboxListener inbox = new InboxListener();
    hub.subscribe(inbox);
    hub.subscribe(inbox);
    hub.publish("A");
    hub.unsubscribe(inbox);
    hub.publish("B");
    assertEquals(List.of("A"), inbox.titles());
  }

  /** 回调内取消其他订阅，不改变当前事件的订阅快照. */
  @Test
  void dispatchesSnapshot() {
    PublicationHub hub = new PublicationHub();
    InboxListener inbox = new InboxListener();
    hub.subscribe(new RemovingListener(hub, inbox));
    hub.subscribe(inbox);
    hub.publish("A");
    hub.publish("B");
    assertEquals(List.of("A"), inbox.titles());
  }

  /** 明确采用失败即中断语义，后续观察者不会收到本次事件. */
  @Test
  void propagatesListenerFailure() {
    PublicationHub hub = new PublicationHub();
    InboxListener inbox = new InboxListener();
    hub.subscribe(new FailingListener());
    hub.subscribe(inbox);
    assertThrows(IllegalStateException.class, () -> hub.publish("A"));
    assertTrue(inbox.titles().isEmpty());
  }

  /** 模拟在回调中变更订阅的观察者. */
  private static final class RemovingListener implements PublicationListener {
    /** 被操作的事件主题. */
    private final PublicationHub hub;

    /** 将要取消的观察者. */
    private final PublicationListener target;

    /** 保存主题和取消目标. */
    private RemovingListener(PublicationHub hub, PublicationListener target) {
      this.hub = hub;
      this.target = target;
    }

    /** 当前事件中取消目标，只影响后续事件. */
    @Override
    public void onPublished(String title) {
      hub.unsubscribe(target);
    }
  }

  /** 模拟业务回调失败的观察者. */
  private static final class FailingListener implements PublicationListener {
    /** 抛出异常以验证同步传播策略. */
    @Override
    public void onPublished(String title) {
      throw new IllegalStateException("Listener failed");
    }
  }
}