frontend/src/services/backgroundService.ts

/**
 * 后台保活服务
 * 使用 expo-background-fetch 和 expo-task-manager 实现 App 后台保活
 * 
 * 功能：
 * - 定期后台任务保持 App 在内存中
 * - 配合 SSE 服务保持长连接
 * - 收到消息时触发震动提示
 */

import { AppState, AppStateStatus, Platform } from 'react-native';
import * as BackgroundFetch from 'expo-background-fetch';
import * as TaskManager from 'expo-task-manager';
import { sseService } from './sseService';
import {
  triggerVibration,
  vibrateOnMessage,
  setVibrationConfig,
  getVibrationConfig,
  setVibrationEnabled,
} from './messageVibrationService';
export { triggerVibration, vibrateOnMessage, setVibrationConfig, getVibrationConfig, setVibrationEnabled };

// 后台任务名称
const BACKGROUND_FETCH_TASK = 'background-fetch-keepalive';
const REALTIME_KEEPALIVE_TASK = 'realtime-keepalive';

// 后台任务间隔（Android 最小 15 分钟，iOS 最小 15 分钟）
const BACKGROUND_INTERVAL = 15; // 15 分钟

// 后台服务状态
let isInitialized = false;
let appStateSubscription: any = null;

// 定义后台任务
TaskManager.defineTask(BACKGROUND_FETCH_TASK, async () => {
  try {
    // 检查 SSE 连接状态
    if (!sseService.isConnected()) {
      await sseService.connect();
    }
    
    // 返回收到新数据
    return BackgroundFetch.BackgroundFetchResult.NewData;
  } catch (error) {
    console.error('[BackgroundService] 后台任务执行失败:', error);
    return BackgroundFetch.BackgroundFetchResult.Failed;
  }
});

// SSE 保活任务
TaskManager.defineTask(REALTIME_KEEPALIVE_TASK, async () => {
  try {
    if (!sseService.isConnected()) {
      await sseService.connect();
    }
    return BackgroundFetch.BackgroundFetchResult.NewData;
  } catch (error) {
    console.error('[BackgroundService] SSE 保活失败:', error);
    return BackgroundFetch.BackgroundFetchResult.Failed;
  }
});

/**
 * 注册后台任务
 */
async function registerBackgroundTasks(): Promise<void> {
  if (Platform.OS === 'web') {
    return;
  }
  try {
    // 检查是否已注册
    const isRegistered = await TaskManager.isTaskRegisteredAsync(BACKGROUND_FETCH_TASK);
    if (!isRegistered) {
      await BackgroundFetch.registerTaskAsync(BACKGROUND_FETCH_TASK, {
        minimumInterval: BACKGROUND_INTERVAL * 60, // 转换为秒
        stopOnTerminate: false, // App 终止后继续运行
        startOnBoot: true, // 设备启动后自动运行
      });
    }

    // 注册 SSE 保活任务
    const isSseKeepaliveRegistered = await TaskManager.isTaskRegisteredAsync(REALTIME_KEEPALIVE_TASK);
    if (!isSseKeepaliveRegistered) {
      await BackgroundFetch.registerTaskAsync(REALTIME_KEEPALIVE_TASK, {
        minimumInterval: 60, // 1 分钟检查一次
        stopOnTerminate: false,
        startOnBoot: true,
      });
    }
  } catch (error) {
    console.error('[BackgroundService] 注册后台任务失败:', error);
  }
}

/**
 * 取消后台任务
 */
async function unregisterBackgroundTasks(): Promise<void> {
  if (Platform.OS === 'web') {
    return;
  }
  try {
    await BackgroundFetch.unregisterTaskAsync(BACKGROUND_FETCH_TASK);
    await BackgroundFetch.unregisterTaskAsync(REALTIME_KEEPALIVE_TASK);
  } catch (error) {
    console.error('[BackgroundService] 取消后台任务失败:', error);
  }
}

/**
 * 设置 App 状态监听
 */
function setupAppStateListener(): void {
  if (appStateSubscription) {
    return;
  }

  appStateSubscription = AppState.addEventListener('change', (nextAppState: AppStateStatus) => {
    void nextAppState;
    if (nextAppState === 'active') {
      // App 回到前台，确保连接
      if (!sseService.isConnected()) {
        sseService.connect();
      }
    }
  });
}

/**
 * 初始化后台保活服务
 */
export async function initBackgroundService(): Promise<boolean> {
  if (isInitialized) {
    return true;
  }

  if (Platform.OS === 'web') {
    setupAppStateListener();
    isInitialized = true;
    return true;
  }

  try {
    // 检查后台任务状态
    const status = await BackgroundFetch.getStatusAsync();
    if (status !== BackgroundFetch.BackgroundFetchStatus.Available) {
      console.warn('[BackgroundService] 后台任务不可用，状态:', status);
      // 即使后台任务不可用，也继续初始化其他功能
    }

    // 注册后台任务
    await registerBackgroundTasks();

    // 设置 App 状态监听
    setupAppStateListener();

    isInitialized = true;
    return true;
  } catch (error) {
    console.error('[BackgroundService] 初始化失败:', error);
    return false;
  }
}

/**
 * 停止后台保活服务
 */
export async function stopBackgroundService(): Promise<void> {
  try {
    await unregisterBackgroundTasks();
    
    if (appStateSubscription) {
      appStateSubscription.remove();
      appStateSubscription = null;
    }

    isInitialized = false;
  } catch (error) {
    console.error('[BackgroundService] 停止服务失败:', error);
  }
}

/**
 * 检查后台服务状态
 */
export async function checkBackgroundStatus(): Promise<{
  isAvailable: boolean;
  isInitialized: boolean;
  registeredTasks: string[];
}> {
  if (Platform.OS === 'web') {
    return {
      isAvailable: false,
      isInitialized,
      registeredTasks: [],
    };
  }
  const status = await BackgroundFetch.getStatusAsync();
  const registeredTasks = await TaskManager.getRegisteredTasksAsync();

  return {
    isAvailable: status === BackgroundFetch.BackgroundFetchStatus.Available,
    isInitialized,
    registeredTasks: registeredTasks.map(task => task.taskName),
  };
}

// 导出服务实例
export const backgroundService = {
  init: initBackgroundService,
  stop: stopBackgroundService,
  vibrate: triggerVibration,
  vibrateOnMessage,
  setVibrationConfig,
  getVibrationConfig,
  setVibrationEnabled,
  checkStatus: checkBackgroundStatus,
};

export default backgroundService;