class WordViewModel(application: Application) : AndroidViewModel(application) {
    //方式一：
    private val _allWord: LiveData<List<Word>> by lazy {
        WordRepository.getInstance(application.applicationContext).allWord
    }
    //方式二：
//    private val _allWord: LiveData<List<Word>>
//        init {
//        //word仓库
//        val wordRepository = WordRepository.getInstance(application.applicationContext)
//        _allWord = wordRepository.allWord
//    }

    val allWord: LiveData<List<Word>> get() = _allWord //外部访问allWord
}