onSaveInstanceState() and onRestoreInstanceState()

As mentioned in the article "Activity will be re-started when screen orientation changed", the activity will be destroyed and re-created once screen orientation changed.



We can call onSaveInstanceState() to retrieve per-instance state from an activity before being killed so that the state can be restored in onCreate(Bundle) or onRestoreInstanceState(Bundle) (the Bundle populated by this method will be passed to both).



example:

package com.exercise.AndroidSavedInstanceState;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class AndroidSavedInstanceStateActivity extends Activity {

EditText edit;
Button update;
TextView text;

/** Called when the activity is first created. */
   @Override
   public void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.main);
      
       edit = (EditText)findViewById(R.id.edit);
       update = (Button)findViewById(R.id.update);
       text = (TextView)findViewById(R.id.text);
      
       update.setOnClickListener(new Button.OnClickListener(){

  @Override
  public void onClick(View arg0) {
   // TODO Auto-generated method stub
   text.setText(edit.getText().toString());
  }});
   }

@Override
protected void onSaveInstanceState(Bundle outState) {
 // TODO Auto-generated method stub
 super.onSaveInstanceState(outState);
 outState.putString("TEXT", (String)text.getText());
}

@Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
 // TODO Auto-generated method stub
 super.onRestoreInstanceState(savedInstanceState);
 text.setText(savedInstanceState.getString("TEXT"));
}
  
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:orientation="vertical"
   android:layout_width="fill_parent"
   android:layout_height="fill_parent"
   >
<TextView 
   android:layout_width="fill_parent"
   android:layout_height="wrap_content"
   android:text="@string/hello"
   />
<EditText
   android:id="@+id/edit"
   android:layout_width="fill_parent"
   android:layout_height="wrap_content"
   />
<Button
   android:id="@+id/update"
   android:layout_width="fill_parent"
   android:layout_height="wrap_content"
   android:text="Update"
   />
<TextView
   android:id="@+id/text"
   android:layout_width="fill_parent"
   android:layout_height="wrap_content"
   />
</LinearLayout>

Without onSaveInstanceState() and onRestoreInstanceState(), TextView text will be cleared everytime the screen orientation changed.

Notice that if the activity finished completely, the state will not be saved.

Next:
- More on onSaveInstanceState() and onRestoreInstanceState()