Android Callback方法未填充列表

Question

I'm working on an Android Project right now and I'm trying to parse from an URL. In my "ApiClient" I have no problem to parse. Here is my "ApiClient" class:

public class ApiClient implements Callback<Map<String, Channel>> {

    static final String BASE_URL = "someURL";

    public void start() {

        Gson gson = new GsonBuilder()
                .setLenient()
                .create();

        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(GsonConverterFactory.create(gson))
                .build();

        RestInterface restInterface = retrofit.create(RestInterface.class);

        Call<Map<String, Channel>> call = restInterface.getChannels();
        call.enqueue(this);

    }

    @Override
    public void onResponse(retrofit2.Call<Map<String, Channel>> call, Response<Map<String, Channel>> response) {
        System.out.println(response.code());
        if(response.isSuccessful()) {
            Map<String, Channel> body = response.body();
            List<Channel> channels = new ArrayList<>(body.values());
        }
...
    }

I'm trying to get the response into a List from using callback in my "Radio" class. This the place where I'm having the problem. I tried this three too but it didn't solved my problem:

private List<Channel> listChannels = new ArrayList<Channel>();
private List<Channel> listChannels = new ArrayList<>();
private List<Channel> listChannels = new List<>();

Here is my "Radio" class:

public class Radio {
    private static final String STORAGE = "radio";
    private List<Channel> listChannels;

    public static Radio get() {
        return new Radio();
    }

    private SharedPreferences storage;

    private Radio() {
        storage = App.getInstance().getSharedPreferences(STORAGE, Context.MODE_PRIVATE);
    }

    public List<Channel> getData() {
        RestInterface restInterface = SingletonClass.getService();
        restInterface.getChannels().enqueue(new Callback<Map<String, Channel>>() {
            @Override
            public void onResponse(Call<Map<String, Channel>> call, Response<Map<String, Channel>> response) {
                if(response.isSuccessful()){
                    Map<String, Channel> body = response.body();
                    List<Channel> channels = new ArrayList<>(body.values());
                    loadChannels(channels);
                }
            }

            @Override
            public void onFailure(Call<Map<String, Channel>> call, Throwable t) {

            }
        });
        System.out.println(listChannels.get(1).getArtist());
        return listChannels;
    }

    public boolean isRated(int itemId) {
        return storage.getBoolean(String.valueOf(itemId), false);
    }

    public void setRated(int itemId, boolean isRated) {
        storage.edit().putBoolean(String.valueOf(itemId), isRated).apply();
    }

    private void loadChannels(List<Channel> channels){
        listChannels.clear();
        listChannels.addAll(channels);
    }


}

Here is my interface class:

public interface RestInterface {

    @GET("someURL")
    retrofit2.Call<Map<String, Channel>> getChannels();
}

and my SingletonClass:

public class SingletonClass{

    private static final Retrofit RETROFIT = new Retrofit.Builder()
            .baseUrl(someURL)
            .addConverterFactory(GsonConverterFactory.create())
            .build();

    private static final RestInterface SERVICE = RETROFIT.create(RestInterface.class);


    public static RestInterface getService(){
        return SERVICE;
    }

}

I don't know what should I do to fill the List in my Radio class now. I'm totally open to suggestions. Thanks for the help.

Answer 1

Are you getting an empty list? You're asynchronously setting in the channel data in getData(), so if you're trying to get the data by reading it in the next line, it may not be loaded yet.

This means that when you call System.out.println(listChannels.get(1).getArtist()), you won't see the result of loadChannels, because that call happens right after you call enqueue() while loadChannels() is running on a separate thread. If you moved that into onResponse() you might have more luck.

In Android, a fairly easy way to do things like this and interact with the UI is by using AsyncTask , which for you would look something like this:

private class loadChannelTask extends AsyncTask<Void, Void, List<Channel>> {
    protected List<Channel> doInBackground() {
        //get response
        //pass to load channels
    }

    protected void onPostExecute() {
         System.out.println(listChannels.get(1).getArtist()); //presumably the artist name
    }
}