How do I get the value of this variable outside the .then((){}); statement in flutter?

Question

I have the following lines of code

bool check;

Firestore.instance.collection('tests').where('name',isEqualTo: snapshot['name'].replaceAll('.mp4','.txt')).getDocuments().then((docs){
  if(docs.documents[0].exists) check = true;
  else check = false;
});
debugPrint(check.toString());

This produces an error saying that a bool can't have a value of null, so how do I retrieve the value of check from inside the .then statement so that I can use it outside of the .then statement?

Solution

If you use the .then() clause you need to do all your work inside it and you can't use its values outside because you don't know when you are receiving the answer.

You could await the answer there and mark the method that encloses your code with async.

bool check;
final docs = Firestore.instance.collection('tests').where('name',isEqualTo: snapshot['name'].replaceAll('.mp4','.txt')).getDocuments();
if(docs.documents[0].exists)
  check = true;
else
  check = false;
debugPrint(check.toString());

or

Future<void> doSomething() async {
  bool check;
  final docs = Firestore.instance.collection('tests').where('name',isEqualTo: 
  snapshot['name'].replaceAll('.mp4','.txt')).getDocuments();
  if(docs.documents[0].exists)
    check = true;
  else
    check = false;
  debugPrint(check.toString());
}

Comments

[pskink]

why would await block UI thread? in dart there is only one, UI thread so any use of await would block it? it simply suspends the execution, and not blocks the whole thread

[chemamolins]

I have rephrased my answer. It is not possible to infer from your question where your code is located. If you can't use an await then you must use the then() but then you can't reliable access the variable inside from outside. await can be used without blocking the ui if the method call is marked with async and is used accordingly.

[chemamolins]

Could you provide more context to the question?

[pskink]

await and then are really the same things - and no, you cannot use await in a method that is not async

[chemamolins]

Telling that await and then are the same thing is a bit misleading. They are two different ways of handling the same asynchronous response. The end result is the same but through different paths.

[pskink]

i agree - different paths - but in any way, neither await nor then blocks UI thread as you wrote in your answer

[chemamolins]

Right. Async call don't block the ui per se. What I meant is that, depending on how and where you call your async methods with await, you could block the ui.

[pskink]

fortunately await will never block ui thread, see dartlang.org/articles/language/await-async or similar articles

[chemamolins]

I understand what you mean really mean but that is only true if you use await/async correctly. In the gist gist.github.com/jmolins/cd426981fa3f675aed55b035cb63d4b4 you can see that the ui is easily blocked with an incorrect call.

[Richard Heap]

No, it does not block the UI. Try tapping the FAB multiple times. Each time you see the ink ripple, and after 4 seconds the counter increments N times. The UI is perfectly functional and responsive; you've just asked it to change the counter after 4 seconds - which it does.

[chemamolins]

Yes. You';re right. I see what you mean. The UI is not blocked. But depending on how you use the await, the UI may not be updated as you expect. At least in this case. Thanks for the patience.

[pskink]

as Richard said the UI is not blocked: think of var x = await future; print(x); as: future.then((x) => print(x)); - this is really the same code