java根据Android中其他微调器的选择填充微调器

Question

我正在制作一个使用Firebase作为后端的考勤应用程序。我有一个微调器Semester，应该基于它填充另一个微调器类。 我能够从Firebase获取数据并填充学期微调器，但基于它的选择，类微调器不起作用

以下是我的尝试：

package info.安卓hive.firebase;

import 安卓.os.Bundle;
import 安卓.support.design.widget.FloatingActionButton;
import 安卓.support.design.widget.Snackbar;
import 安卓.support.v7.app.AppCompatActivity;
import 安卓.support.v7.widget.Toolbar;
import 安卓.view.View;


import 安卓.provider.ContactsContract;
import 安卓.support.annotation.NonNull;
import 安卓.support.annotation.Nullable;
import 安卓.support.v7.app.AppCompatActivity;
import 安卓.util.Log;
import 安卓.view.ViewParent;
import 安卓.widget.AdapterView;
import 安卓.widget.AdapterView.OnItemSelectedListener;
import 安卓.widget.ArrayAdapter;
import 安卓.widget.ListView;
import 安卓.widget.Spinner;
import 安卓.widget.Toast;


import com.google.安卓.gms.tasks.OnCompleteListener;
import com.google.安卓.gms.tasks.Task;
import com.google.firebase.firestore.DocumentSnapshot;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.QueryDocumentSnapshot;
import com.google.firebase.firestore.QuerySnapshot;

import java.util.ArrayList;
import java.util.List;
/*

import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
*/


public class TakeAttendanceMenu extends AppCompatActivity implements         OnItemSelectedListener{

String baseDb = "Courses";
String selectedSem;
String semClassDb;

Spinner semSpinner,classSpinner,subjectSpinner;
private static final String TAG = "TakeAttendanceMenu";

FirebaseFirestore db = FirebaseFirestore.getInstance();
List<String> semSpinnerList =  new ArrayList<String>();
List<String> ClassSpinnerList = new ArrayList<String>();



public void getAllSemesters()
{
    db.collection(baseDb)
            .get()
            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                @Override
                public void onComplete(@NonNull Task<QuerySnapshot> task) {
                    if(task.isSuccessful()){
                        for(DocumentSnapshot document: task.getResult()){
                            Log.d(TAG,document.getId() + "=>" +     document.getData());
                            semSpinnerList.add(document.getId());
                        }
                    }
                    Toast.makeText(getApplicationContext(), "Please chal ja bc", Toast.LENGTH_SHORT).show();
                }
});
}

public void getSemesterClasses()
{
    semClassDb = baseDb + "/" + selectedSem;
    db.collection(semClassDb).get().addOnCompleteListener(new     OnCompleteListener<QuerySnapshot>() {
        @Override
        public void onComplete(@NonNull Task<QuerySnapshot> task) {
            if(task.isSuccessful()){
                for(DocumentSnapshot documentSnapshot: task.getResult()){
                    ClassSpinnerList.add(documentSnapshot.getId());
                }
            }
        }
    });
}





@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_take_attendance_menu);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    semSpinner = (Spinner)findViewById(R.id.semester_spinner);
    classSpinner = (Spinner)findViewById(R.id.class_spinner);
    subjectSpinner = (Spinner)findViewById(R.id.subject_spinner);

    setSupportActionBar(toolbar);

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    }
    );
    getAllSemesters(); // calling this method to instantiate the first spinner(semester spinner) with the values of firestore

    ArrayAdapter<String> semAdapter= new ArrayAdapter<String>(
            this, 安卓.R.layout.simple_spinner_item, semSpinnerList);
    semAdapter.setDropDownViewResource(安卓.R.layout.simple_spinner_dropdown_item);
    semSpinner.setAdapter(semAdapter);

    /*Adapter for the Semester -> Class*/
    ArrayAdapter<String> classAdapter = new ArrayAdapter<String>(this,安卓.R.layout.simple_spinner_dropdown_item,ClassSpinnerList);
    classAdapter.setDropDownViewResource(安卓.R.layout.simple_spinner_dropdown_item);
    classSpinner.setAdapter(classAdapter);
    semSpinner.setOnItemSelectedListener(this);
}
@Override
public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
    Toast.makeText(getApplicationContext(), "Item Selected "+position, Toast.LENGTH_SHORT).show();
}

@Override
public void onNothingSelected(AdapterView<?> parent) {
    Toast.makeText(getApplicationContext(), "Please chal ja bc", Toast.LENGTH_SHORT).show();
}
}

Answer 1

代码中的问题是，您正在使用尚未可用的结果创建和设置适配器。换句话说，在设置适配器时，从数据库获取的列表还不可用。为了解决这个问题，我建议您在这两种情况下都在回调中创建和设置适配器。请将相应方法中的每一行代码移到for循环的后面

Answer 2

看起来你错过了这一行：

classspinner.setOnItemSelectedListener(this);

写在下面：

/*Adapter for the Semester -> Class*/
ArrayAdapter<String> classAdapter = new ArrayAdapter<String>(this,android.R.layout.simple_spinner_dropdown_item,ClassSpinnerList);
classAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
classSpinner.setAdapter(classAdapter);
semSpinner.setOnItemSelectedListener(this);
..... (Write Here)

下一步：

@Override
public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {

switch (view.getId()) {

    case R.id.first_spinner:
        Toast.makeText(getApplicationContext(), "Item Selected "+position, Toast.LENGTH_SHORT).show();
        break;

    case R.id.second_spinner:
        Toast.makeText(getApplicationContext(), "Item Selected "+position, Toast.LENGTH_SHORT).show();
        break;

    default:
        break;
}
}

希望它能起作用