The docs clearly state that "This method is called after onStart() when the activity is being re-initialized from a previously saved state, given here in savedInstanceState". Several StackOverflow questions concur.
However I am finding this is not entirely accurate and can be misleading. onRestoreInstanceState is only called when switching orientation between horizontal-vertical. It is not called whenever onStart is called.
I modified this simple lab exercise to print out the log traces, and to save mCount into the bundle and display it.
onRestoreInstanceState is not called when you press back button to go back to main Activity nor when you press Home button and relaunch.
Even though I see that onSaveInstanceState and onStop was called each time (from the logs), I do not see a corresponding onRestoreInstanceState after onStart. This is unexpected. The docs should clearly state that only if onDestroy is called, then after onStart, will onRestoreInstanceState be called. Are there are any other cases (other than switching orientation) when it is reliably called?
Knowing these use cases is useful for white-box testing.
Update: android docs have since been clarified - see my answer below.
MainActivity.java
public class MainActivity extends AppCompatActivity {
private int mCount = 0;
private TextView mShowCount;
public static final String EXTRA_MESSAGE =
"com.example.hellotoast.extra.MESSAGE";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mShowCount = (TextView) findViewById(R.id.show_count);
Log.d("MainActivity", "Hello World");
}
public void showToast(View view) {
Toast toast = Toast.makeText(this, R.string.toast_message,
Toast.LENGTH_SHORT);
toast.show();
Intent intent = new Intent(this, HelloCount.class);
intent.putExtra(EXTRA_MESSAGE, mCount);
startActivity(intent);
}
@Override
public void onSaveInstanceState(Bundle outState) {
outState.putInt(EXTRA_MESSAGE, mCount);
super.onSaveInstanceState(outState);
Log.d("MainActivity", "onSaveInstanceState. saved mCount = " + mCount);
}
@Override
public void onRestoreInstanceState(Bundle b) {
super.onRestoreInstanceState(b);
Log.d("MainActivity", "onRestoreInstanceState");
if(b != null) {
mCount = b.getInt(EXTRA_MESSAGE);
Log.d("MainActivity", "onRestoreInstanceState mCount = "+mCount);
mShowCount.setText(Integer.toString(mCount));
}
}
@Override
protected void onStart() {
super.onStart();
Log.d("MainActivity", "onStart");
}
@Override
protected void onStop() {
super.onStop();
Log.d("MainActivity", "onStop");
}
@Override
protected void onDestroy() {
super.onDestroy();
Log.d("MainActivity", "onDestroy");
}
public void countUp(View view) {
mCount++;
if (mShowCount != null) {
mShowCount.setText(Integer.toString(mCount));
}
}
}
HelloCount.java
import static com.example.hellotoast.MainActivity.EXTRA_MESSAGE;
public class HelloCount extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.d("HelloCount", "onCreate");
setContentView(R.layout.activity_hello_count);
TextView tv = findViewById(R.id.helloCount);
int i = getIntent().getIntExtra(EXTRA_MESSAGE,0);
tv.setText(String.valueOf(i));
}
@Override
protected void onStart() {
super.onStart();
Log.d("HelloCount", "onStart");
}
@Override
protected void onStop() {
super.onStop();
Log.d("HelloCount", "onStop");
}
@Override
protected void onDestroy() {
super.onDestroy();
Log.d("HelloCount", "onDestroy");
}
}
activity_main.xml
<RelativeLayout
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="com.example.hellotoast.MainActivity"
android:orientation="vertical">
<Button
android:id="@+id/button_toast"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:layout_marginEnd="8dp"
android:background="@color/colorPrimary"
android:onClick="showToast"
android:text="@string/button_label_toast"
android:textColor="@android:color/white" />
<TextView
android:id="@+id/show_count"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:background="#FFFF00"
android:gravity="center_vertical"
android:text="@string/count_initial_value"
android:textAlignment="center"
android:textColor="@color/colorPrimary"
android:textSize="120sp"
android:textStyle="bold"
android:layout_centerHorizontal="true"
android:layout_below="@+id/button_toast"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
/>
<Button
android:id="@+id/button_count"
android:layout_below="@+id/show_count"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginStart="8dp"
android:layout_marginEnd="8dp"
android:layout_marginBottom="8dp"
android:background="@color/colorPrimary"
android:onClick="countUp"
android:text="@string/button_label_count"
android:textColor="@android:color/white" />
</RelativeLayout>
activity_second.xml
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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">
<TextView
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="8dp"
android:layout_marginTop="156dp"
android:layout_marginEnd="8dp"
android:text="Hello"
android:textColor="#8BC34A"
android:textColorHighlight="#00882A2A"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/helloCount"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:layout_marginEnd="8dp"
android:layout_marginBottom="8dp"
android:text="TextView"
android:textAppearance="@style/TextAppearance.AppCompat.Medium"
android:textStyle="bold"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/textView"
app:layout_constraintVertical_bias="0.099" />
</android.support.constraint.ConstraintLayout>
