Q1 ViewModel需要Context

使用AndroidViewModel，初始化与ViewModel有点不同

ViewModelProvider(this,ViewModelProvider.AndroidViewModelFactory.getInstance(application)).get(NoticeViewModel::class.java)

注意是ViewModelProvider.AndroidViewModelFactory而不是ViewModelProvider.NewInstanceFactory

Q2 LiveData与Room

要达到Room修改能实时刷新界面，必须得在同一个实例下进行操作

        val user1 = User(null,"大虎")
        val user2 = User(null,"二郎")
        val user3 = User(null,"三师兄")
        GlobalScope.launch(Dispatchers.IO){
            userDao.addUser(user1,user2,user3)
            val all = userDao.getAll()
            withContext(Dispatchers.Main){
                all.observe(this@MainActivity, Observer {
                    it.forEach { user-> Log.d(TAG, "LiveData+Room: id = ${user.id} name = ${user.name}") }
                })
                Log.d(TAG, "onCreate: 1111111111111111")
            }
            delay(2000)
            Log.d(TAG, "onCreate: 分割线--------------------------------")
            userDao.addUser(User(null,"我是验证LiveData的"))
            delay(2000)
            Log.d(TAG, "onCreate: 分割线--------------------------------")
            userDao1.delUser()
            delay(2000)
            Log.d(TAG, "onCreate: 分割线--------------------------------")
            userDao2.updateUser(User(1,"李四"))
          }

2020-07-08 16:28:57.236 13991-13991/com.liuzhi.myapp D/MainActivity: LiveData+Room: id = 1 name = 大虎
2020-07-08 16:28:57.236 13991-13991/com.liuzhi.myapp D/MainActivity: LiveData+Room: id = 2 name = 二郎
2020-07-08 16:28:57.236 13991-13991/com.liuzhi.myapp D/MainActivity: LiveData+Room: id = 3 name = 三师兄
2020-07-08 16:28:59.227 13991-14059/com.liuzhi.myapp D/MainActivity: onCreate: 分割线--------------------------------
2020-07-08 16:28:59.251 13991-13991/com.liuzhi.myapp D/MainActivity: LiveData+Room: id = 1 name = 大虎
2020-07-08 16:28:59.251 13991-13991/com.liuzhi.myapp D/MainActivity: LiveData+Room: id = 2 name = 二郎
2020-07-08 16:28:59.251 13991-13991/com.liuzhi.myapp D/MainActivity: LiveData+Room: id = 3 name = 三师兄
2020-07-08 16:28:59.251 13991-13991/com.liuzhi.myapp D/MainActivity: LiveData+Room: id = 4 name = 我是验证LiveData的
2020-07-08 16:29:01.231 13991-14164/com.liuzhi.myapp D/MainActivity: onCreate: 分割线--------------------------------
2020-07-08 16:29:03.258 13991-14164/com.liuzhi.myapp D/MainActivity: onCreate: 分割线--------------------------------

Q3 Room的数据库连接实例创建

自己在网上搜Kotlin实例创建方法，发现各种高深，完全看不懂，特别是by lazy这种(自己学艺不精),最后只能用Studio自带的Java to Kotlin 转换插件，发现代码差不多

companion object{
        private var databaseInstance : RoomDatabase? = null
        fun getDatabase(application: Application) : RoomDatabase?{
            if(databaseInstance == null){
                synchronized(RoomDatabase::class.java){
                    if(databaseInstance == null){
                        databaseInstance = Room.databaseBuilder(
                            application,
                            RoomDatabase::class.java,
                            "database.db"
                        ).build()
                    }
                }
            }
            return databaseInstance
        }