• Controller - How to represent a screen
• Navigation Pattern - How to change screens
• Data Passing - How to exchange data between screens

• Controller - Activity
• Navigation Pattern - Code
• Data Passing - Intent

• Activity
• Fragment

• Code
• Navigation Host

• Intent
• Bundle
• Interface
• ViewModel
• More...

• Intent - small data
• Bundle - can handle larger data, image etc.
• ViewModel - complicate situations, share with fragments

• Android Architecture Components

  Until recently, Google did not recommend a specific approach ...
  That changed in 2017 ...
  

• What is Android Jetpack

  ... this chapter is to introduce the key architectural guidelines together with the 
  ViewModel, LiveData, Lifecycle components while also introducing 
  Data Binding and the use of Repositories.
  

• The “Old” Architecture

  Android project was created consisting of a single activity which contained all of the code
  for presenting and managing the user interface together with the back-end logic of the app.
  Up until the introduction of Jetpack, ... apps consisting of multiple activities (one for each 
  screen within the app) with each activity class to some degree mixing user interface and back-end code.
  

• Modern Android Architecture

  At the most basic level, Google now advocates single activity apps where different screens
  are loaded as content within the same activity.
  

• The ViewModel Component

  The purpose of ViewModel is to separate the user interface-related data model and 
  logic of an app from the code responsible for actually displaying and managing the
  user interface and interacting with the operating system.
  
  Recall Model-View-Controller (MVC)
  

• The LiveData Component

  Consider an app that displays realtime data ....
  LiveData is a data holder that allows a value to become observable ...
  
  Recall Polling vs. Interrupt
  

• ViewModel Saved State

  Android allows the user to place an active app into the background 
  and return to it later ...
  

• LiveData and Data Binding

  allows data in a ViewModel to be mapped directly to specific views within the XML user
  interface layout file. 	
  
  automatically update the view whenever there is any change in data.
  

• Repository Modules

  If a ViewModel obtains data from one or more external sources (such as databases or web services)
  it is important to separate the code involved in handling those data sources from the ViewModel class.
  
  Modern Android App Architecture with Jetpack Google’s architecture guidelines recommend placing this 
  code in a separate Repository module.	
  

Prior to the introduction of Android Jetpack, the implementation of navigation within an app
was largely a manual coding process with no easy way to view and organize potentially complex
navigation paths. This situation has improved considerably,??? ...

A fragment is a self-contained, modular section of an application’s user interface and corresponding
behavior that can be embedded within an activity.	

Fragments may only be used as part of an activity and cannot be instantiated as standalone application elements.
Fragment is a 'lightweight' activity

Recall Process vs. Thread

• Activities are navigation controllers
• Fragments are content controllers

• Statically -- simply embed the fragment in the activity's xml layout file
• Dynamically -- in Java using the FragmentManager

• Simlest fragment demo
• Two fragements, added statically
• Create Fragement Template -> Right click Java Folder -> New -> Fragment -> Fragment (Blank)
• FragmentOne.java -- generated by the template

  import android.app.Fragment;
  import android.os.Bundle;
  import android.view.LayoutInflater;
  import android.view.View;
  import android.view.ViewGroup;
  public class FragmentOne extends Fragment {
      @Override
      public View onCreateView(LayoutInflater inflater, ViewGroup container,
                               Bundle savedInstanceState) {
          return inflater.inflate(R.layout.fragment_one, container, false);
      }
  }
  

• FragmentTwo.java
• fragment_one.xml

  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:orientation="vertical"
      android:background="#2c2e43" >
  
      <TextView
          android:id="@+id/textViewOne"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:textColor="#FFFFFF"
          android:gravity="center_horizontal"
          android:text="First Fragment" />
  
  </LinearLayout>
  

• fragment_two.xml
• activity_main.xml

  <androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto"
      xmlns:tools="http://schemas.android.com/tools"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      tools:context=".MainActivity">
  
  
      <fragment
          android:id="@+id/fragment1"
          android:name="com.example.a13_frag.FragmentOne"
          android:layout_width="195dp"
          android:layout_height="0dp"
          android:layout_marginStart="16dp"
          android:layout_marginTop="16dp"
          android:layout_marginBottom="33dp"
          app:layout_constraintBottom_toTopOf="@+id/fragment2"
          app:layout_constraintStart_toStartOf="parent"
          app:layout_constraintTop_toTopOf="parent" />
  
      <fragment
          android:id="@+id/fragment2"
          android:name="com.example.a13_frag.FragmentTwo"
          android:layout_width="189dp"
          android:layout_height="0dp"
          android:layout_marginEnd="16dp"
          android:layout_marginBottom="16dp"
          app:layout_constraintBottom_toBottomOf="parent"
          app:layout_constraintEnd_toEndOf="parent"
          app:layout_constraintTop_toBottomOf="@+id/fragment1" />
  </androidx.constraintlayout.widget.ConstraintLayout>
  

• MainActivity.java -- no change

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <FrameLayout
        android:id="@+id/fragment1"
        android:layout_width="195dp"
        android:layout_height="0dp"
        android:layout_marginStart="16dp"
        android:layout_marginTop="16dp"
        android:layout_marginBottom="33dp"
        app:layout_constraintBottom_toTopOf="@+id/fragment2"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <FrameLayout
        android:id="@+id/fragment2"
        android:layout_width="189dp"
        android:layout_height="0dp"
        android:layout_marginEnd="16dp"
        android:layout_marginBottom="16dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/fragment1" />
</androidx.constraintlayout.widget.ConstraintLayout>

import androidx.fragment.app.Fragment;	

public class MainActivity extends AppCompatActivity {

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

        //FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
        //ft.replace(R.id.fragment1, new FragmentOne());
        //ft.commit();
        loadFragment(this, R.id.fragment1,new FragmentOne(),"fragment1");
        loadFragment(this, R.id.fragment2,new FragmentTwo(),"fragment2");
    }

    // HERE
    public static void loadFragment(AppCompatActivity activity, int containerId, Fragment fragment, String tag) {
        activity.getSupportFragmentManager().beginTransaction().
                replace(containerId, fragment,tag).commitAllowingStateLoss();
    }
}

• activity_main.xml

  android:id="@+id/buttonLeft"
  android:text="First@Left"	
  ...
  android:id="@+id/buttonRight"
  android:text="First@Right"
  

• MainActivity.java

  public class MainActivity extends AppCompatActivity {
      FragmentOne frag01;
      FragmentTwo frag02;
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
          frag01 = new FragmentOne();
          frag02 = new FragmentTwo();
  
          loadFragment(this, R.id.fragment1,frag01,"fragment1");
          loadFragment(this, R.id.fragment2,frag02,"fragment2");
  
          Button leftButton = (Button) findViewById(R.id.buttonLeft);
          leftButton.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View v) {
                  FragmentManager fm = getSupportFragmentManager();
                  fm.beginTransaction().remove(frag01).commit();
                  fm.beginTransaction().remove(frag02).commit();
                  fm.executePendingTransactions();
                  loadFragment(MainActivity.this, R.id.fragment1,frag01,"fragment1");
                  loadFragment(MainActivity.this, R.id.fragment2,frag02,"fragment2");
  
              }
          });
  
          Button rightButton = (Button) findViewById(R.id.buttonRight);
          rightButton.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View v) {
                  FragmentManager fm = getSupportFragmentManager();
                  FragmentTransaction ft = fm.beginTransaction();
                  ft.remove(frag02);
                  ft.commit();
  
                  fm.beginTransaction().remove(frag01).commit();
                  fm.executePendingTransactions();
  
                  loadFragment(MainActivity.this, R.id.fragment1,frag02,"fragment2");
                  loadFragment(MainActivity.this, R.id.fragment2,frag01,"fragment1");
              }
          });
      }
  	...
  }	
  

• Note: have to commit and execute remove transaction before trying to add fragment to another container.
     

• Intent - for Activity
• Bundle - Activity can construct a fragment and set arguments
• Interface = Listener
• ViewModel

• Methods - Activity can call methods on a fragment instance
• Listener - Fragment can fire listener events on an activity via an interface

• MainActivity.java

  ...
      public static void loadFragment(AppCompatActivity activity, int containerId, Fragment fragment, 
  					String tag, int ID) {
          //HERE
          final int ARG = 426;
          Bundle bundle = new Bundle();
          bundle.putInt("IntArg", ARG);
          fragment.setArguments(bundle);
  
          activity.getSupportFragmentManager().beginTransaction().
                  replace(containerId, fragment,tag).commitAllowingStateLoss();
      }
  }
  

• FragmentOne.java

  public class FragmentOne extends Fragment {
      int intarg = -1;
      @Override
      public void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
  
          Bundle bundle = this.getArguments();
          if (bundle != null) {
              intarg = bundle.getInt("IntArg", 0);
          }
      }
  
      @Override
      public View onCreateView(LayoutInflater inflater, ViewGroup container,
                               Bundle savedInstanceState) {
          View v = inflater.inflate(R.layout.fragment_one, container, false);
  
          TextView tv = (TextView) v.findViewById(R.id.textViewOne);
          tv.setText("First Fragment -> " + intarg);
          return v;
      }
  }
  

• FragmentTwo.java

  almost the same as FragmentOne.java
  

• Multiple Fragments
• Data pass as 'part of initialization'
• Fragment once created, not much change
• Fragment does pass data
• A10-RV06 or WeatherApp-ver01 (one list, one detail) WON'T fit

• 1. Activity to Fragment
• 2. Fragment to Activity
• 3. Fragment to Fragment

• a. Intent - Fragment To Activity Via Intent
• b. Listener/Interface - Passing Data between fragments in Android using Interface and an complicated example
• c. Shared ViewModel Across Fragments (you might google)

The important thing to keep in mind is that fragments should not directly communicate with each other
and should generally only communicate with their parent activity. 


You don't want the Fragments talking directly to each other or to the Activity. 
That ties them to a particular Activity and makes reuse difficult.

The solution is to make an callback listener interface that the Activity will implement.
When the Fragment wants to send a message to another Fragment or its parent activity, 
it can do it through the interface.

It is ok for the Activity to communicate directly to its child fragment public methods.
Thus the Activity serves as the controller, passing messages from one fragment to another.

• CAN NOT use bundle (why? construct)
• Method (ad hoc, bad approach)
• Interface (Listener), such as here and here
• Shared ViewModel

At the most basic level, Google now advocates single activity apps where different screens
are loaded as content within the same activity.

• Intent only for fragment back to MainActivity - rare
• Bundle for fragment init - useful, but limited
• Method - adhoc, works but too 'dirty'
• Listener/Interface for fragement update - very useful
• ViewModel - 'modern way'

• So far, two activities via Intent - reasonable
• However, not very efficient - creating too many detailed intents

  ...	
          holder.itemView.setOnClickListener(new View.OnClickListener() {
                  @Override
                  public void onClick(View view) {
                      Intent lv_it = new Intent(cv_activity, MyListDetailsActivity.class);
                      lv_it.putExtra("OSName",
                              (String) mv_data02.get(holder.getAdapterPosition()).toString());
                      cv_activity.startActivity(lv_it);
                      cv_activity.overridePendingTransition(R.anim.anim_slide_in_left, R.anim.anim_slide_out_left);
                  }
              });
  ...
  

• More efficient - one MainActivity, one List Fragment, one Detail Fragment
• List touch - replace fragment with update
• Demand driven - Listener/Interface perhaps ViewModel