package com.vogella.android.lifecycle.activity;

import android.app.Activity;
import android.app.Notification;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.os.Build;
import android.os.Bundle;

public class TracerActivity extends Activity {

    public static final String TRACER = "tracer";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        createChannel();
        notify("onCreate");
    }

    @Override
    protected void onPause() {
        super.onPause();
        notify("onPause");
    }

    @Override
    protected void onResume() {
        super.onResume();
        notify("onResume");
    }

    @Override
    protected void onStop() {
        super.onStop();
        notify("onStop");
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        notify("onDestroy");
    }

    @Override
    protected void onRestoreInstanceState(Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
        notify("onRestoreInstanceState");
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        notify("onSaveInstanceState");
    }

    private void notify(String methodName) {
        String name = this.getClass().getName();
        String[] strings = name.split("\\.");
        Notification.Builder notificationBuilder;

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            notificationBuilder = new Notification.Builder(this, TRACER);
        } else {
            //noinspection deprecation
            notificationBuilder = new Notification.Builder(this);
        }

        Notification notification = notificationBuilder
                .setContentTitle(methodName + " " + strings[strings.length - 1])
                .setAutoCancel(true)
                .setSmallIcon(R.mipmap.ic_launcher)
                .setContentText(name).build();
        NotificationManager notificationManager =
                (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        notificationManager.notify((int) System.currentTimeMillis(), notification);
    }

    private void createChannel() {
        NotificationManager mNotificationManager = getSystemService(NotificationManager.class);
        // The id of the channel.
        String id = TRACER;
        // The user-visible name of the channel.
        CharSequence name = "Activity livecycle tracer";
        // The user-visible description of the channel.
        String description = "Allows to trace the activity lifecycle";
        int importance = NotificationManager.IMPORTANCE_HIGH;
        NotificationChannel mChannel = new NotificationChannel(id, name, importance);
        // Configure the notification channel.
        mChannel.setDescription(description);

        mNotificationManager.createNotificationChannel(mChannel);
    }
}

package com.vogella.android.lifecycle.activity;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;

public class SecondActivity extends TracerActivity {

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

package com.vogella.android.lifecycle.activity;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;

public class MainActivity extends TracerActivity {

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

    public void onClick(View view) {
        Intent intent = new Intent(this, SecondActivity.class);
        startActivity(intent);
    }

}

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="app_name">Lifecycle</string>
    <string name="action_settings">Settings</string>
    <string name="hello_world">Hello world!</string>
    <string-array name="operating_systems">
        <item >Ubuntu</item>
        <item >Android</item>
        <item >iOS</item>
    </string-array>

</resources>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity" >

    <Spinner
        android:id="@+id/spinner"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginRight="58dp"
        android:entries="@array/operating_systems" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onClick"
        android:layout_gravity="bottom"
        android:text="Start new Activity" />

</LinearLayout>

// configure the spinner in code
Spinner spinner = (Spinner) findViewById(R.id.spinner);
String[] values = getResources().getStringArray(R.array.operating_systems);
ArrayAdapter<String> adapter = 
    new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, values);
spinner.setAdapter(adapter);

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity" >

    <Spinner
        android:id="@+id/spinner"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginRight="58dp"
        />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onClick"
        android:layout_gravity="bottom"
        android:text="Start new Activity" />

</LinearLayout>

allprojects {
    repositories {
        jcenter()
        google()

    }
}

dependencies {
   implementation "android.arch.lifecycle:runtime:1.1.1"
   implementation "android.arch.lifecycle:extensions:1.1.1"
   annotationProcessor "android.arch.lifecycle:compiler:1.1.1"
   implementation 'com.android.support:appcompat-v7:26.1.0'
    // more stuff
}

package com.vogella.android.viewmodel;


import java.util.Date;

public class Task {

    public final long id;
    public String summary;
    public String description;
    public boolean done;
//    public Date dueDate;


    public Task(long id, String summary, String description, boolean done) {
        this.id = id;
        this.summary = summary;
        this.description = description;
        this.done = done;
//        this.dueDate = dueDate;
    }

    public static TaskBuilder builder(){
        return new TaskBuilder();
    }

    public static class TaskBuilder {
        private long id;
        private String summary = "";
        private String description = "";
        private boolean done = false;
        private Date dueDate;

        public TaskBuilder setId(long id) {
            this.id = id;
            return this;
        }

        public TaskBuilder setSummary(String summary) {
            this.summary = summary;
            return this;
        }

        public TaskBuilder setDescription(String description) {
            this.description = description;
            return this;
        }

        public TaskBuilder setDone(boolean done) {
            this.done = done;
            return this;
        }

        public TaskBuilder setDueDate(Date dueDate) {
            this.dueDate = new Date(dueDate.getTime());
            return this;
        }

        public Task build() {
            return new Task(id, summary, description, done);
        }
    }

    @Override
    public String toString() {
        return "Task{" +
                "id=" + id +
                ", summary='" + summary + '\'' +
                ", description='" + description + '\'' +
                ", done=" + done +
                '}';
    }
}

package com.vogella.android.viewmodel;

import android.arch.lifecycle.LiveData;
import android.arch.lifecycle.MutableLiveData;
import android.arch.lifecycle.ViewModel;

import java.util.ArrayList;
import java.util.List;

public class MyViewModel extends ViewModel {
    private MutableLiveData<List<Task>> tasks;

    public LiveData<List<Task>> getTasks() {
        if (tasks == null) {
            tasks = new MutableLiveData<List<Task>>();
            loadTasks();
        }
        return tasks;
    }

    private void loadTasks() {
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        List<Task> list = new ArrayList<>();
        Task task = Task.builder().setId(1).setSummary("Testing ViewModel").build();
        list.add(task);
        tasks.setValue(list);
        // do async operation to fetch users
    }
}

package com.vogella.android.viewmodel;

import android.arch.lifecycle.Observer;
import android.arch.lifecycle.ViewModelProviders;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.TextView;

import java.util.List;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final  MyViewModel model = ViewModelProviders.of(this).get(MyViewModel.class);
        model.getTasks().observe(MainActivity.this, new Observer<List<Task>>() {
            @Override
            public void onChanged(List<Task> users) {
                // update UI
                TextView textView = findViewById(R.id.result);
                textView.setText(model.getTasks().getValue().get(0).toString());
            }
        });
    }

}