Saving and restoring text when rotating

问题

I have seen a few examples on how to achieve this but still I can't get it working.

I followed this example, but the TextView is empty in OnCreate and when I rotate the screen. Here is how I implemented it:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(R.layout.activity_image_viewer);

    displayFileName = (TextView) findViewById(R.id.displayFileName);

    //Getting the fileName
    Bundle extras = getIntent().getExtras();
    uri = extras.getString("uriAsString");
    Uri myUri = Uri.parse(uri);
    filename = uri.substring(uri.lastIndexOf("/") + 1);
    nameWitoutExtention = filename.substring(0, filename.lastIndexOf('.'));

    //Checking if Text was stored
    if (savedInstanceState != null) {
        CharSequence savedText = savedInstanceState.getCharSequence(KEY_TEXT_VALUE);
        displayFileName.setText(savedText);
    }

}

@Override
protected void onSaveInstanceState (Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putCharSequence(KEY_TEXT_VALUE, displayFileName.getText());
}

What I'm trying to achieve is to set nameWitoutExtention to displayFileName and when the device is rotated to let the text remain.

I got a null pointer on the text and that is why I'm trying this, any help on what I'm doing wrong?

---

EDIT:

This is how my implementation looks like at the moment:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(R.layout.activity_image_viewer);

    displayFileName = (TextView) findViewById(R.id.displayFileName);

    if(savedInstanceState != null){
        CharSequence savedText = savedInstanceState.getCharSequence(KEY_TEXT_VALUE);

        //When displaying this toast the correct filename gets displayed
        //Toast.makeText(getApplicationContext(), savedText, Toast.LENGTH_LONG).show();

        displayFileName.setText(savedText+"");
    }
    else{
        //Getting the fileName
        Bundle extrsas = getIntent().getExtras();
        uri = extrsas.getString("selectedStudentVid");
        Uri mUri = Uri.parse(uri);
        filename = uri.substring(uri.lastIndexOf("/") + 1);
        nameWitoutExtention = filename.substring(0, filename.lastIndexOf('.'));
        displayFileName.setText(nameWitoutExtention);
    }

@Override
protected void onSaveInstanceState (Bundle outState) {
    super.onSaveInstanceState(outState);      
    outState.putCharSequence(KEY_TEXT_VALUE, displayFileName.getText());
}

I have also tried onRestoreInstanceState() as suggested by @PorasBhardwaj

The exception I'm getting is:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference

回答1:

Try doing it this way:

//Checking if Text was stored
 if(savedInstanceState != null){
       CharSequence savedText = 
      savedInstanceState.getCharSequence(KEY_TEXT_VALUE);
      displayFileName.setText(savedText + "");
 }
else{
 //Getting the fileName
Bundle extras = getIntent().getExtras();
uri = extras.getString("uriAsString");
Uri myUri = Uri.parse(uri);
filename = uri.substring(uri.lastIndexOf("/") + 1);
nameWitoutExtention = filename.substring(0, filename.lastIndexOf('.'));
}

What you are doing wrong is. Whenever the orientation changes it gets the value again from extras which is null after the rotation change and it passes the same value to it. That's why you are getting the exception. try this hope it will help you.

回答2:

Try using onRestoreInstanceState() callback method to get saved data.

@Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
 // TODO Auto-generated method stub
 super.onRestoreInstanceState(savedInstanceState);
 CharSequence savedText = savedInstanceState.getCharSequence(KEY_TEXT_VALUE);
 displayFileName.setText(savedText);
}

As per official documentation.

Most implementations will simply use onCreate(Bundle) to restore their state, but it is sometimes convenient to do it here after all of the initialization has been done or to allow subclasses to decide whether to use your default implementation.

So for best practice, keep your view hierarchy in onCreate and restore the previous state in onRestoreInstanceState.

来源：https://stackoverflow.com/questions/47345425/saving-and-restoring-text-when-rotating