Variable changes when it hasn't been changed

Question

I have a List variable to store the last saved value of some data. When this variable is different from the current data, it updates it to match. But, when I check if they are the same, they always are! When the data variable is updated, I check both and the last saved variable has changed as well. Does Dart link the variables somehow when I assign their values?

I tries printing both variables before I do the check, to make sure that's not changing them, but they are still the same at the instant the data variable is changed.

print("checking if data updated");
print(lastSavedData);
print(datas);
if (lastSavedData == datas) {
  print("no new data");
} else {
  print("new data is there");
  saveData();
  lastSavedData = datas;
}

I/flutter ( 8751): checking if data updated
I/flutter ( 8751): []
I/flutter ( 8751): []
I/flutter ( 8751): no new data
I/flutter ( 8751): checking if data updated
I/flutter ( 8751): [{name: Fu, notes: , goodness: 0}]
I/flutter ( 8751): [{name: Fu, notes: , goodness: 0}]
I/flutter ( 8751): no new data

You'll have to provide more code snippet especially showing where these variables are being set. — Sukhi, Sep 03 '19 at 03:35
I added the only place it is ever changing the lastSavedData variable. — Joshua Anderson, Sep 03 '19 at 03:37
You're probably updating the `datas` variable somewhere, it's holding a reference to the variable when you assign it to it. — Train, Sep 03 '19 at 03:41
The datas variable is what should be updated... so of course I am. It's the lastSavedData variable that should not change unless there is new data. — Joshua Anderson, Sep 03 '19 at 03:42
Oh, so it does have a reference to it? How would I make it just hold the data and not a reference? — Joshua Anderson, Sep 03 '19 at 03:42
That's not how it works, you need to copy over a shallow copy. When you assign it, it assigns a deep copy. — Train, Sep 03 '19 at 03:42
You can try `.clone()` or you can create a new array and add to that one. https://stackoverflow.com/questions/50460367/how-to-copy-listt-in-dart-without-old-reference — Train, Sep 03 '19 at 03:48
How should I copy a shallow copy? I can only find people complaining about doing it online. — Joshua Anderson, Sep 03 '19 at 03:48
Please post your entire dart file in the above question description. — Mohit Shetty, Sep 03 '19 at 04:02

score 0 · Accepted Answer · answered Sep 03 '19 at 03:55

0

It was storing a link to the variable, not the data. Just get the data from the variable in a way that suits your code, and assign that to the variable instead of the reference.

answered Sep 03 '19 at 03:55

Joshua Anderson

107
9

Variable changes when it hasn't been changed

1 Answers1