方法无法识别 kotlin 中的变量

Question

Something really strange is happening.

If I execute fetchListOfNews() I get an error.

class SearchNewsViewModel(repository: Repository) : NewsViewModel(repository) {

    var keyword: String = "word"

    override suspend fun fetchListOfNews(): List<News> = repository.getNews(keyword)

    class Factory(private val repository: Repository) : ViewModelProvider.Factory {
        @Suppress("UNCHECKED_CAST")
        override fun <T : ViewModel> create(modelClass: Class<T>): T =
            SearchNewsViewModel(repository) as T
    }
}

override suspend fun getNews(keyword: String): List<News> {
        return listOfNews.filter { news -> news.title.contains(keyword) }
    }

But when I execute this code I don't get any error:

class SearchNewsViewModel(repository: Repository) : NewsViewModel(repository) {

    var keyword: String = "word"

    override suspend fun fetchListOfNews(): List<News> = repository.getNews("word")

    class Factory(private val repository: Repository) : ViewModelProvider.Factory {
        @Suppress("UNCHECKED_CAST")
        override fun <T : ViewModel> create(modelClass: Class<T>): T =
            SearchNewsViewModel(repository) as T
    }
}

I only changed the variable keyword to its value.

Why does it happen? How can I solve it?

Answer 1

I found the solution. I call fetchListOfNews() in the init block of NewsViewModel. The init block is executed before initializing keyword.

Answer 2

In the generated Java bytecode for your first snippet, keyword is not initialized until the constructor runs:

@NotNull
private String keyword;

public SearchNewsViewModel(@NotNull Repository repository) {
    Intrinsics.checkParameterIsNotNull(repository, "repository");
    super();
    this.repository = repository;
    this.keyword = "word";
}

@Nullable
public Object fetchListOfNews(@NotNull Continuation $completion) {
    return this.repository.getNews(this.keyword, $completion);
}

In the second snippet, everything is the same except that fetchListOfNews() no longer references keyword :

@Nullable
public Object fetchListOfNews(@NotNull Continuation $completion) {
    return this.repository.getNews("word", $completion);
}

So, if your first implementation's fetchListOfNews() is called without the constructor for the class having finished execution, then you would hit this NPE.

How is your class being instantiated? While not common, there are ways to create instances without calling the constructor.

Or perhaps fetchListOfNews() is called in the super() constructor?