[feat] hasPreviousValue getter

[manuel-plavsic]

This PR adds the hasPreviousValue getter suggested in https://github.com/nank1ro/solidart/pull/27#issuecomment-1588990715.

It is a bit unclear to me whether or not I should make a reportObserved call before returning _hasPreviousValue (similarly to what the previousValue getter does).

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[manuel-plavsic]

Done :)

[nank1ro]

Thank you so much for your contributions, I really appreciate it! 👏

[manuel-plavsic]

Btw, I think that we still need a getter, otherwise one could manually change the value of hasPreviousValue, for example:

  @override
  void initState() {
    super.initState();
    counter = createSignal(0);
    counter.hasPreviousValue = true;
  }

I will make another PR soon if it is fine for you.

That is so good to hear, thanks! I really enjoy contributing to this amazing library! Btw, I sent you a DM on Twitter a few days ago ;)

[nank1ro]

You're right I hadn't thought about it that the value can be changed from outside.

Maybe you can put a getter in the ReadSignal that returns false. Then in the Signal you may have a private _hasPreviousSignal value and then override the getter returning this private variable.

I didn't get any DM on Twitter 😕 , this is my profile https://twitter.com/nank1ro

[manuel-plavsic]

I don't know what is going on at Twitter... I can see the message... 😂 I will open a GitHub discussion instead.

[manuel-plavsic]

I have a question regarding the next pull request: why should the getter in ReadSignal return false instead of "correctly" returning true or false. Also, I don't understand why previousValue is a getter that returns null. I see that it has something to do with it being a ReadSignal and not a WriteSignal, but the value held by the ReadSignal can also change.

Edit: it is actually immutable. I confused Computed with ReadSignal, since createComputed returns a ReadSignal.

[nank1ro]

Don't worry I'm going to do the PR myself cause I have to release a new dev.