一、前言：

• 冷启动：
  在启动应用时，系统中没有该应用的进程，这时系统会创建一个新的进程分配给该应用；

• 热启动：
  在启动应用时，系统中已有该应用的进程（例：按back键、home键，应用虽然会退出，但是该应用的进程还是保留在后台）

二、贴代码：

1、通用工具类判断是否在前台

object CommonUtils {
    /**
     * 程序是否在前台运行
     *
     * @return
     */
    fun isAppOnForeground(): Boolean {
        // Returns a list of application processes that are running on the
        val activityManager =
            ChargingElfApplication.instance.getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager
        val packageName: String =
            ChargingElfApplication.instance.getPackageName()
        val appProcesses = activityManager
            .runningAppProcesses ?: return false
        for (appProcess in appProcesses) {
            // The name of the process that this object is associated with.
            if (appProcess.processName == packageName && appProcess.importance == RunningAppProcessInfo.IMPORTANCE_FOREGROUND) {
                return true
            }
        }
        return false
    }


    /**
     * 判断APP是否在后台运行的方式
     *  true是在后台运行
     */
    fun isAppOnBackstage(context: Context): Boolean {
        val activityManager =
            context.getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager
        val tasks = activityManager.getRunningTasks(1)
        if (!tasks.isEmpty()) {
            val topActivity = tasks.get(0).topActivity
            if (!topActivity?.packageName.equals(context.packageName)) {
                return true
            }
        }
        return false
    }

}

2、在BaseActivity中统一判断是否是热启动

public abstract class BaseActivity extends AppCompatActivity {
    /**
     * 默认为true,此页面存活；false进入后台
     */
    boolean isLive = true;
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }



    @Override
    protected void onStop() {
        super.onStop();
        boolean appOnForeground = CommonUtils.INSTANCE.isAppOnForeground();
        if (!appOnForeground) {
            //app 进入后台
            isLive = false;
        }
    }

    @Override
    protected void onResume() {
        super.onResume();
        if (!isLive) {
            isLive = true;
            //app 从后台唤醒，进入前台（注意：会出现先展示当前页面，有跳转到热启动页面）
            Log.d("LUO", "=====onResume: app 从后台唤醒，进入前台======"); 
            Intent intent = new Intent(this, HotStartActivity.class);
            startActivity(intent);
        }
    }
}

3、通过万能工具类判断前后台

//就是这个万能工具 类
implementation 'com.blankj:utilcodex:1.30.6'

  AppUtils.registerAppStatusChangedListener(object : Utils.OnAppStatusChangedListener{
            override fun onBackground(activity: Activity?) {
                //停止服务
                stopOnlineTiming()
                Log.e("统计时长","---onBackground---")
            }

            override fun onForeground(activity: Activity?) {
                //判断服务是否启动
                startOnlineTiming()
                Log.e("统计时长","---onForeground---")
            }
        })