Android 空 ArrayList 和 Spinner

Question

嘿，我正在创建与我的数据库 MySQL 连接的 Android 应用程序。我创建了一个带有类别的 ArrayList，并将其添加到 Adapter，然后添加到 Spinner。我看到 Spinner 项目，但在 onClick 之后没有任何反应，当我尝试检查 ArrayList 中的内容时，我看到它是空的。数据已下载。

点击 Spinner 后没有任何变化

这是我的代码：

public class InsertProductActivity extends AppCompatActivity {
    Spinner categories;
    String categoriesURL = "myURL";
    ArrayList<String> downloadedCategories = new ArrayList<>();
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_insert_product);
        categories = findViewById(R.id.categories);
        getCategories();
        ArrayAdapter<String> adapter = new ArrayAdapter<>(this,android.R.layout.simple_spinner_item,downloadedCategories);
        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        categories.setAdapter(adapter);
    }
    protected void getCategories(){
        RequestQueue requestQueue = Volley.newRequestQueue(getApplicationContext());
        JsonObjectRequest jsonObjectRequest = new JsonObjectRequest(Request.Method.POST, categoriesURL
                , new Response.Listener<JSONObject>() {
                    @Override
                    public void onResponse(JSONObject response) {
                        try {
                            JSONArray jsonArray = response.getJSONArray("categories");
                            for(int i=0;i<jsonArray.length();i++){
                                downloadedCategories.add(jsonArray.getJSONObject(i).getString("name"));
                            }
                        } catch (JSONException e) {
                            e.printStackTrace();
                        }
                    }
                }, new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError error) {

                    }
        });
        requestQueue.add(jsonObjectRequest);
    }
}

Answer 1

试试这个。

protected void getCategories(){
    RequestQueue requestQueue = Volley.newRequestQueue(getApplicationContext());
    JsonObjectRequest jsonObjectRequest = new JsonObjectRequest(Request.Method.POST, categoriesURL, new Response.Listener<JSONObject>() {
        @Override
        public void onResponse(JSONObject response) {
            try {
                JSONArray jsonArray = response.getJSONArray("categories");
                downloadedCategories = new ArrayList<>(); // try to add your instantiation of your arrayList here.
                for(int i=0;i<jsonArray.length();i++) {
                    downloadedCategories.add(jsonArray.getJSONObject(i).getString("name"));
                }
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
    }, new Response.ErrorListener() {
    @Override
    public void onErrorResponse(VolleyError error) {

    }
});
//this block of code will prompt after your downloadedCategories finish storing your objects to your arrayList because if you put this in onCreate this wil call before your getCategories process not finish yet. So much better its much safer here.
    ArrayAdapter<String> adapter = new ArrayAdapter<>(this,android.R.layout.simple_spinner_item,downloadedCategories);
    adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    categories.setAdapter(adapter);
    requestQueue.add(jsonObjectRequest);
}