2024-01-03 10:32:04.384 4305-4305/com.sevenlearn.mvvmproject E/AndroidRuntime: FATAL EXCEPTION: main
   Process: com.sevenlearn.mvvmproject, PID: 4305
   java.lang.RuntimeException: Unable to start activity ComponentInfo{com.sevenlearn.mvvmproject/com.sevenlearn.mvvmproject.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'io.reactivex.Single com.sevenlearn.mvvmproject.ApiService.getStudents()' on a null object reference
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2666)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2727)
       at android.app.ActivityThread.-wrap12(ActivityThread.java)
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1478)
       at android.os.Handler.dispatchMessage(Handler.java:102)
       at android.os.Looper.loop(Looper.java:154)
       at android.app.ActivityThread.main(ActivityThread.java:6121)
       at java.lang.reflect.Method.invoke(Native Method)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779)
    Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'io.reactivex.Single com.sevenlearn.mvvmproject.ApiService.getStudents()' on a null object reference
       at com.sevenlearn.mvvmproject.StudentRepository.refreshStudents(StudentRepository.java:20)
       at com.sevenlearn.mvvmproject.MainVewModel.<init>(MainVewModel.java:23)
       at com.sevenlearn.mvvmproject.MainViewModelFactory.create(MainViewModelFactory.java:22)
       at androidx.lifecycle.ViewModelProvider$Factory.create(ViewModelProvider.kt:83)
       at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.kt:187)
       at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.kt:153)
       at com.sevenlearn.mvvmproject.MainActivity.onCreate(MainActivity.java:15)
       at android.app.Activity.performCreate(Activity.java:6739)
       at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2619)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2727) 
       at android.app.ActivityThread.-wrap12(ActivityThread.java) 
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1478) 
       at android.os.Handler.dispatchMessage(Handler.java:102) 
       at android.os.Looper.loop(Looper.java:154) 
       at android.app.ActivityThread.main(ActivityThread.java:6121) 
       at java.lang.reflect.Method.invoke(Native Method) 
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889) 
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779) 
2024-01-03 10:32:06.261 4305-4322/com.sevenlearn.mvvmproject D/LeakCanary: Setting up flushing for Thread[LeakCanary-Heap-Dump,5,main]

---------------------

package com.sevenlearn.mvvmproject;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.ViewModelProvider;

import android.os.Bundle;
import android.util.Log;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        MainVewModel mainVewModel= new ViewModelProvider(this,new MainViewModelFactory(new StudentRepository(ApiServiceProvider.getApiService(),AppDatabase.getInstance(getApplicationContext()).studentDao()))).get(MainVewModel.class);
        mainVewModel.getStudents().observe(this,students->{
            Log.i("mainactivity", "onCreate: ");

        });
        mainVewModel.getError().observe(this,error->{
            Log.e("mainactivity", "Error: "+error  );
        });
    }
}

----------------------

package com.sevenlearn.mvvmproject;

import androidx.annotation.NonNull;
import androidx.lifecycle.ViewModel;
import androidx.lifecycle.ViewModelProvider;

import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class MainViewModelFactory implements ViewModelProvider.Factory {

    private StudentRepository repository;

    public MainViewModelFactory(StudentRepository repository){
        this.repository = repository;

    }

    @NonNull
    @Override
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        return (T) new MainVewModel(repository);
    }
}

--------------------

package com.sevenlearn.mvvmproject;

import androidx.lifecycle.LiveData;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.ViewModel;

import java.util.List;

import io.reactivex.CompletableObserver;
import io.reactivex.android.schedulers.AndroidSchedulers;
import io.reactivex.disposables.Disposable;
import io.reactivex.schedulers.Schedulers;

public class MainVewModel extends ViewModel {
    private ApiService apiService;
    private MutableLiveData<String> error=new MutableLiveData<>();
    private StudentRepository repository;
    private Disposable disposable;

    public MainVewModel(StudentRepository repository){

        this.repository = repository;
        repository.refreshStudents()
                .subscribeOn(Schedulers.io())
                .subscribe(new CompletableObserver() {
                    @Override
                    public void onSubscribe(Disposable d) {
                        disposable=d;

                    }

                    @Override
                    public void onComplete() {

                    }

                    @Override
                    public void onError(Throwable e) {
                        error.setValue("خطای نامشخص");

                    }
                });

    }



    public LiveData<List<Student>> getStudents() {
        return repository.getStudent();
    }

    public LiveData<String> getError() {
        return error;
    }

    @Override
    protected void onCleared() {
        disposable.dispose();
        super.onCleared();
    }
}
---------------------

package com.sevenlearn.mvvmproject;

import androidx.lifecycle.LiveData;

import java.util.List;

import io.reactivex.Completable;
import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;

public class StudentRepository {
    private ApiService apiService;
    private StudentDao studentDao;

    public StudentRepository(ApiService apiService, StudentDao studentDao) {
    }

    public Completable refreshStudents(){
       return apiService.getStudents().doOnSuccess(students -> studentDao.addStudents(students)).ignoreElement();

    }

    public LiveData<List<Student>> getStudent(){
        return studentDao.getStudents();

    }
}