如何在 dart 的外部范围内更改变量的值？

Question

 Product getProduct(QueryDocumentSnapshot doc) {
    Product product;
    _firestore
        .collection('products')
        .where('isAvailable', isEqualTo: true)
        .snapshots(includeMetadataChanges: true)
        .map(
          (event) => event.docs.map(
            (document) {
              if (document.id == doc.data()['id']) {
                product = Product.fromJson(document.data());
              }
            },
          ),
        );
    return product;
  }

In the code above, each time the function is called, I want from it to override the value of product in line 2 , the desired operation of overriding is being done in the 11th line, but it always returns Null , so how can I assign a value to it from the inner function?

Answer 1

Data is loaded from Firestore asynchronously. While this is going on, your main code continues to run (so that the user can continue using the app). It's easiest to see what this means by adding some logging code:

print('Before starting to load data');
_firestore
    .collection('products')
    .where('isAvailable', isEqualTo: true)
    .snapshots(includeMetadataChanges: true)
    .map(
        print('Got data');
    );
print('After starting to load data');

If you run this code, it prints:

Before starting to load data

After starting to load data

Got data

Got data

...

This is probably not what you expected, but explains perfectly why your calls to getProduct() don't get back any data: by the time your return product; runs, the product = Product.fromJson(document.data()); hasn't been called yet.

---

A common solution for this is to return a Future from the method, which then resolves to the value once it is loaded.

That'd look something like this (typos possible, as I haven't run this code):

  Future<Product> getProduct(QueryDocumentSnapshot doc) async {
    Product product;
    var snapshot = await _firestore
        .collection('products')
        .where('isAvailable', isEqualTo: true)
        .get();
    snapshot.docs.map((document) {
      if (document.id == doc.data()['id']) {
        product = Product.fromJson(document.data());
      }
    })
    return product;
  }

Note that I also use get() instead of snapshots(...) above, as you're only looking to return a value once, instead of continuing to listen to updates.