Code

gistfile1.txt

//------LoginActivty.java--------

package com.example.android.foodrecipes;

import android.arch.lifecycle.Observer;
import android.arch.lifecycle.ViewModelProviders;
import android.content.Intent;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.example.android.foodrecipes.viewmodels.UserViewModel;

public class LoginActivity extends BaseActivity {

    private static final String TAG = "LoginActivity";

    private EditText editTextId, editTextPassword;
    private Button buttonLogin, buttonSignUp;
    private UserViewModel mUserViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        mUserViewModel = ViewModelProviders.of(this).get(UserViewModel.class);
        initUI();
        subscribeObservers();

    }

    private void initUI() {

        editTextId = findViewById(R.id.editTextId);
        editTextPassword = findViewById(R.id.editTextPassword);

        buttonLogin = findViewById(R.id.buttonLogin);
        buttonSignUp = findViewById(R.id.buttonSignup1);

        buttonSignUp.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(LoginActivity.this, SignupActivity.class);
                startActivity(intent);
            }
        });

        buttonLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.d(TAG, "onClick: ");

                if(!TextUtils.isEmpty(editTextId.getText()) &&
                        !TextUtils.isEmpty(editTextPassword.getText())) {

                    String id = editTextId.getText().toString().trim();
                    String password = editTextPassword.getText().toString().trim();

                    mUserViewModel.loginUser(id, password);

                }
            }
        });
    }

    private void subscribeObservers() {

        mUserViewModel.didLoginSuccess().observe(this, new Observer<Boolean>() {
            @Override
            public void onChanged(@Nullable Boolean aBoolean) {

                if(aBoolean) {
                    Toast.makeText(LoginActivity.this, "login success", Toast.LENGTH_SHORT).show();
                    goToRecipeListActivity();
                } else {
                    Toast.makeText(LoginActivity.this, "login fail", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }

    private void goToRecipeListActivity() {
        Intent intent = new Intent(LoginActivity.this, RecipeListActivity.class);
        startActivity(intent);
        finish();
    }
}

//------LoginActivty.java--------


// -------UserApiClient class----

package com.example.android.foodrecipes.requests;

import android.arch.lifecycle.MutableLiveData;
import android.util.Log;

import com.example.android.foodrecipes.model.User;
import com.example.android.foodrecipes.requests.responses.ServerPostRequestResponse;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;

public class UserApiClient {

    private static final String TAG = "UserApiClient";

    private static UserApiClient instance;
    private MutableLiveData<Boolean> didSignUpSuccess = new MutableLiveData<>();
    private MutableLiveData<Boolean> didLoginSuccess = new MutableLiveData<>();

    public static UserApiClient getInstance() {
        if(instance == null) {
            instance = new UserApiClient();
        }
        return instance;
    }

    public UserApiClient() {

    }

    public void signUpUser(String id, String password) {

        ServiceGenerator.getUserApi().createUser(id, password).
                enqueue(new Callback<ServerPostRequestResponse>() {
                    @Override
                    public void onResponse(Call<ServerPostRequestResponse> call, Response<ServerPostRequestResponse> response) {

                        if(response.isSuccessful()) {
                            Log.d(TAG, "onResponse: success " + response.body());

                            if(!response.body().isError()) {
                                String message = response.body().getMessage();
                                Log.d(TAG, "onResponse: message : " + message);
                                didSignUpSuccess.postValue(true);

                            } else {
                                Log.d(TAG, "onResponse: " + response.body().getMessage());
                                didSignUpSuccess.postValue(false);
                            }

                        } else {
                            Log.d(TAG, "onResponse: " + response.errorBody());
                        }
                    }

                    @Override
                    public void onFailure(Call<ServerPostRequestResponse> call, Throwable t) {
                        Log.d(TAG, "onFailure: " + t.getMessage());
                    }
                });

    }

    public void loginUser(String id, String password) {

        ServiceGenerator.getUserApi().loginUser(id, password)
                .enqueue(new Callback<ServerPostRequestResponse>() {
                    @Override
                    public void onResponse(Call<ServerPostRequestResponse> call, Response<ServerPostRequestResponse> response) {

                        if(response.isSuccessful()) {

                            if(!response.body().isError()) {
                                Log.d(TAG, "onResponse: " + response.body().getMessage());
                                didLoginSuccess.postValue(true);

                            } else {
                                Log.d(TAG, "onResponse: " + response.body().getMessage());
                                didLoginSuccess.postValue(false);
                            }
                        }
                    }

                    @Override
                    public void onFailure(Call<ServerPostRequestResponse> call, Throwable t) {
                        Log.d(TAG, "onFailure: " + t.getMessage());
                    }
                });
    }

    public MutableLiveData<Boolean> didSignUpSuccess() {
        return didSignUpSuccess;
    }

    public MutableLiveData<Boolean> didLoginSuccess() {
        return didLoginSuccess;
    }
}
// -------UserApiClient class----