2次旋转后的应用状态

Question

i have been through this documentation regarding the topic save a state before the foreground activity will be destroyed...

and everything works really good now (after a device rotation), but when i rotate my device again after a rotation, i will loose my data again :(

here is my code

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    final MainActivity activity = this;
    activity.setTitle("Cow Counter");

    TextView QntyResultField = findViewById(R.id.textView);
    QntyResultField.setText(Integer.toString(cowQnty));
}

// invoked when the activity may be temporarily destroyed, save the instance state here
@Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putInt("qnty", cowQnty);
}

// How we retrieve the data after app crash...
@Override
public void onRestoreInstanceState(Bundle savedInstanceState) {
    super.onRestoreInstanceState(savedInstanceState);
    //cowQnty = savedInstanceState.getInt("qnty");

    TextView QntyResultField = findViewById(R.id.textView);
    QntyResultField.setText("Cows: "+Integer.toString(savedInstanceState.getInt("qnty")));
}

I think the solution will be maybe to implement a check if an instance state was already restored before...

i have tried then this here:

if(savedInstanceState.getInt("qnty") != 0){
    TextView QntyResultField = findViewById(R.id.textView);
    QntyResultField.setText("Cows: "+Integer.toString(savedInstanceState.getInt("qnty")));
}

buit then my inital part in my onCreate() method will write a zero in my result field

TextView QntyResultField = findViewById(R.id.textView);
QntyResultField.setText(Integer.toString(cowQnty));

Could anyone tell me if I am close to the solution?

Answer 1

You use a variable called cowQnty to store the value that is then saved in the bundle for your onSaveInstanceState as outState.putInt("qnty", cowQnty); , then when you restore it in onRestoreInstanceState you only set the TextView 's value to the retrieved value and do not update the value for cowQnty .

How do you expect then to save an empty field again? There are two solutions to this;

Firstly, if cowQnty is not a sizeable amount and you do not mind using a tad of RAM, make cowQnty a static field and it will persist the data without needing to save it in a Bundle at all.

Secondly, just set cowQnty 's value once again when you restore your state (why did you comment it out??), like so:

@Override
public void onRestoreInstanceState(Bundle savedInstanceState) {
    super.onRestoreInstanceState(savedInstanceState);
    cowQnty = savedInstanceState.getInt("qnty");

    TextView QntyResultField = findViewById(R.id.textView);
    QntyResultField.setText("Cows: "+Integer.toString(savedInstanceState.getInt("qnty")));
}