Saving Activity state in Android

I've been playing around with the Android SDK, and I am a little unclear on saving an application's state. So given this minor re-tooling of the 'Hello, Android' example:

package com.android.hello;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class HelloAndroid extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        mTextView = new TextView(this);

        if (savedInstanceState == null) {
            mTextView.setText("Welcome to HelloAndroid!");
        } else {
            mTextView.setText("Welcome back.");
        }

        setContentView(mTextView);
    }

    private TextView mTextView = null;
}

I thought that might be all one needed to do for the simplest case, but it always gives me the first message, no matter how I navigate away from the app. I'm sure it's probably something simple like overriding onPause or something like that, but I've been poking away i

You need to override onSaveInstanceState(Bundle savedInstanceState) and write the application state values you want to change to the Bundle parameter like this:

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
  super.onSaveInstanceState(savedInstanceState);
  // Save UI state changes to the savedInstanceState.
  // This bundle will be passed to onCreate if the process is
  // killed and restarted.
  savedInstanceState.putBoolean("MyBoolean", true);
  savedInstanceState.putDouble("myDouble", 1.9);
  savedInstanceState.putInt("MyInt", 1);
  savedInstanceState.putString("MyString", "Welcome back to Android");
  // etc.
}

The Bundle is essentially a way of storing a NVP ("Name-Value Pair") map, and it will get passed in toonCreate and also onRestoreInstanceState where you'd extract the values like this:

@Override
public void onRestoreInstanceState(Bundle savedInstanceState) {
  super.onRestoreInstanceState(savedInstanceState);
  // Restore UI state from the savedInstanceState.
  // This bundle has also been passed to onCreate.
  boolean myBoolean = savedInstanceState.getBoolean("MyBoolean");
  double myDouble = savedInstanceState.getDouble("myDouble");
  int myInt = savedInstanceState.getInt("MyInt");
  String myString = savedInstanceState.getString("MyString");
}

You'd usually use this technique to store instance values for your application (selections, unsaved text, etc.).

Posted in: Android