Declarative Persistence in Android

activity_main.java

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.fbalashov.persistableannotations.MainActivity">

    <TextView
        android:id="@+id/text_view"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="18sp"/>

    <Button
        android:id="@+id/increment_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/increment_button"
        android:layout_below="@id/text_view"/>
</RelativeLayout>

BaseActivity.java

package com.fbalashov.persistableannotations;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;

/**
 * @author Fuad.Balashov on 6/22/2016.
 */
public class BaseActivity extends AppCompatActivity {

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
  }
  
}

MainActivity.java

package com.fbalashov.persistableannotations;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
  private int persistedCount = 0;

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    final TextView textView = (TextView) findViewById(R.id.text_view);
    Button button = (Button) findViewById(R.id.increment_button);
    button.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        persistedCount++;
        textView.setText("Button clicked " + persistedCount + " times");
      }
    });
    textView.setText("Button clicked " + persistedCount + " times");
  }
}

MainActivityV2.java

package com.fbalashov.persistableannotations;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends BaseActivity {
  @Persistable private int persistedCount = 0;

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    final TextView textView = (TextView) findViewById(R.id.text_view);
    Button button = (Button) findViewById(R.id.increment_button);
    button.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        persistedCount++;
        textView.setText("Button clicked " + persistedCount + " times");
      }
    });
    textView.setText("Button clicked " + persistedCount + " times");
  }
}

onConfigurationChange.java

  @Override
  public Object onRetainCustomNonConfigurationInstance() {
    Map<String, Object> objectsToPersist = new HashMap<>();
    // get all fields with the Persistable annotation
    Field[] fields = this.getClass().getDeclaredFields();
    for (Field field : fields) {
      if (field.isAnnotationPresent(Persistable.class)) {
        // for each field, make the field accessible (in case it's private) and then write its value into the map
        field.setAccessible(true);
        try {
          objectsToPersist.put(field.getName(), field.get(this));
        } catch (IllegalAccessException e) {
          e.printStackTrace();
        }
      }
    }
    return objectsToPersist;
  }

onConfigurationChangeV2.java

  @Override
  public final Object onRetainCustomNonConfigurationInstance() {
    Map<String, Object> objectsToPersist = new HashMap<>();
    // get all fields with the Persistable annotation
    List<Field> annotatedFields = getAllFieldsWithAnnotation(new ArrayList<Field>(), Persistable.class, this.getClass(), BaseActivity.class);
    for (Field field : annotatedFields) {
      // for each field, make the field accessible (in case it's private) and then write its value into the map
      field.setAccessible(true);
      try {
        objectsToPersist.put(field.getName(), field.get(this));
      } catch (IllegalAccessException e) {
        e.printStackTrace();
      }
    }
    return objectsToPersist;
  }

  /**
   * Finds all the fields with the given annotation (public and private0 in the given class. Recursively repeats
   * this for super classes until reaching the given endClass.
   * @param annotatedFields an empty list that will be populated with any fields that match the annotationClazz
   * @param annotationClass the annotation to look for
   * @param startClazz the starting subclass to look for fields in
   * @param endClazz the super class at which to stop looking for fields
   * @return all fields with the given annotation
   */
  private List<Field> getAllFieldsWithAnnotation(List<Field> annotatedFields, Class<Persistable> annotationClass, Class startClazz, Class<BaseActivity> endClazz) {
    Field[] fields = startClazz.getDeclaredFields();
    for (Field field : fields) {
      if(field.isAnnotationPresent(annotationClass)) {
        annotatedFields.add(field);
      }
    }
    if (endClazz.equals(startClazz)) {
      return annotatedFields;
    }
    return getAllFieldsWithAnnotation(annotatedFields, annotationClass, startClazz.getSuperclass(), endClazz);
  }

onCreateV2.java

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // Right after super.onCreate we get the retained instance state and reinitiate any persisted fields.
    if (getLastCustomNonConfigurationInstance() instanceof Map) {    
      List<Field> annotatedFields = getAllFieldsWithAnnotation(new ArrayList<Field>(), Persistable.class, this.getClass(), BaseActivity.class);
      for (Field field : annotatedFields) {
        // for each field, make the field accessible (in case it's private) and then set it's value based on the value in the map
        field.setAccessible(true);
        try {
          field.set(this, ((Map<String, Object>) getLastCustomNonConfigurationInstance()).get(field.getName()));
        } catch (IllegalAccessException e) {
          e.printStackTrace();
        }
      }
    }
  }

onCreateV3.java

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // Right after super.onCreate we get the retained instance state and reinitiate any persisted fields.
    if (getLastCustomNonConfigurationInstance() instanceof Map) {
      reinitializePersistedFields(((Map<String, Object>) getLastCustomNonConfigurationInstance()));
    }

  }

  /**
   * Reinitializes any fields that have the Persistable annotation after state change
   */
  private void reinitializePersistedFields(Map<String, Object> persistedObjects) {
    // get all fields with the Persistable annotation
    List<Field> annotatedFields = getAllFieldsWithAnnotation(new ArrayList<Field>(), Persistable.class, this.getClass(), BaseActivity.class);
    for (Field field : annotatedFields) {
      // for each field, make the field accessible (in case it's private) and then set it's value based on the value in the map
      field.setAccessible(true);
      try {
        field.set(this, persistedObjects.get(field.getName()));
      } catch (IllegalAccessException e) {
        e.printStackTrace();
      }
    }
  }

Persistable.java

  /**
   * Any field marked with this annotation will be persisted on state changes.
   */
  @Retention(RetentionPolicy.RUNTIME)
  @Target(ElementType.FIELD)
  protected @interface Persistable{}