捆绑的saveInstanceState在onCreate（）上为null，尽管saveInstanceState已经设置了数据

Question

I have a problem when getting state of activity. I have 2 activities: MainActivity and Activity2. In MainActivity I have put an editText and a button name GO. In Activity2, I have a button name BackMainActivity . What I want is that: I put a text, for example:"abc" into EditText then click button GO. App will navigate to Activity2. After that, I click button BackMainActivity, app will navigate to MainActivity and data in EditText is restore as "abc". I already used onSaveInstanceState and onRestoreInstanceState. App run through onSaveInstanceState before going to Activity2. But if I go back to MainActivity, onCreate(), savedInstanceState is null. Can you show me the reason ? I want to store data of mainActivity in bundle. So how can I do ? Thank you very much !

MainActivity

package com.example.demosavedataactivity;
import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends Activity {

    EditText t;
    Button b;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        t = (EditText) findViewById(R.id.editText1);
        b = (Button) findViewById(R.id.button1);
        b.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                Intent intent = new Intent(MainActivity.this, Activity2.class);
                startActivity(intent);
            }
        });

    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        // TODO Auto-generated method stub
        String a =  t.getText().toString();
        outState.putString("text",a);

        super.onSaveInstanceState(outState);
    }

    @Override
    protected void onRestoreInstanceState(Bundle savedInstanceState) {
        // TODO Auto-generated method stub

        t.setText(savedInstanceState.getString("text"));
        super.onRestoreInstanceState(savedInstanceState);
    }

    @Override
    protected void onDestroy() {
        // TODO Auto-generated method stub
        Log.w("<<<<<<<<<<<<<<<<<<<", "onDestroy");
        super.onDestroy();
    }

    @Override
    protected void onPause() {
        // TODO Auto-generated method stub
        Log.w("<<<<<<<<<<<<<<<<<<<", "onPause");
        super.onPause();
    }

    @Override
    protected void onRestart() {
        // TODO Auto-generated method stub
        Log.w("<<<<<<<<<<<<<<<<<<<", "onRestart");
        super.onRestart();
    }

    @Override
    protected void onResume() {
        // TODO Auto-generated method stub
        Log.w("<<<<<<<<<<<<<<<<<<<", "onResume");
        super.onResume();
    }

    @Override
    protected void onStart() {
        // TODO Auto-generated method stub
        Log.w("<<<<<<<<<<<<<<<<<<<", "onStart");
        super.onStart();
    }

    @Override
    protected void onStop() {
        // TODO Auto-generated method stub
        Log.w("<<<<<<<<<<<<<<<<<<<", "onStop");
        super.onStop();
    }
}

Activity2

package com.example.demosavedataactivity;    
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;

public class Activity2 extends Activity {

    Button back;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity2);
        back = (Button) findViewById(R.id.button1);
        back.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                Intent intent = new Intent(Activity2.this, MainActivity.class);
                startActivity(intent);
            }
        });
    }

}

activity2.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="CombackA1" />

</LinearLayout>

activity_main.xml

<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=".MainActivity" >

    <EditText
        android:id="@+id/editText1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:layout_marginLeft="14dp"
        android:layout_marginTop="18dp"
        android:ems="10" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/editText1"
        android:layout_below="@+id/editText1"
        android:layout_marginLeft="50dp"
        android:layout_marginTop="63dp"
        android:text="Button" />

</RelativeLayout>

Answer 1

Just do something like:

1: Fetch data from onSavedInstanceState in your oncreate() and put it in your edittext . Something like:

 @Override
 protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    t = (EditText) findViewById(R.id.editText1);
    b = (Button) findViewById(R.id.button1);

    /*Just fetch data from savedInstanceState */
    if(savedInstanceState != null){
         t.setText(savedInstanceState.getString("text"));
    }
    b.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            Intent intent = new Intent(MainActivity.this, Activity2.class);              
            startActivity(intent);
        }
    });
  }

And in your Activity2 set a FLAG_ACTIVITY_REORDER_TO_FRONT flag with your intent.like:

 Intent intent = new Intent(Activity2.this, MainActivity.class);
 intent.setFlags(Intent.FLAG_ACTIVITY_REORDER_TO_FRONT);
 startActivity(intent);

For a better idea just check this conversation .

Answer 2

In the onClick function of your Activity2 you don't go back to the previous MainActivity but you open a new MainActivity.

Just finish Activity2 by calling finish() then your first MainActivity will come up again. It should be working with the hardware back button of your device.