字符串名称不会改变 dart flutter

Question

im assigning the string name in this code to a name from a document but it dose not change from null i cant see why.

i will use this variable inside a listTile

String name='';
    FirebaseFirestore.instance
        .collection('Users')
        .doc('list_students')
        .collection('Students')
      ..where('Email', isEqualTo: userEmail)
          .get()
          .then((QuerySnapshot querySnapshot) {
        querySnapshot.docs.forEach((doc) {
          name = doc['Full Name'];
        });
      }); 

Answer 1

As commenters have said, most of your code is running asynchronously, because the data needs to be loaded from the cloud. This messes up the order in which your code runs, and is likely why the variable doesn't have a value.

Some logging should make this clearer:

String name='';
print("1. Before starting query");
FirebaseFirestore.instance
    .collection('Users')
    .doc('list_students')
    .collection('Students')
  ..where('Email', isEqualTo: userEmail)
      .get()
      .then((QuerySnapshot querySnapshot) {
    print("2. Got data");
    querySnapshot.docs.forEach((doc) {
      name = doc['Full Name'];
    });
  }); 
print("3. After starting query");

When you run this code, the output will be:

1. Before starting query

2. After starting query

3. Got data

This is probably not the order you expected, but is actually working as intended. And this means that if you'd print the name in the place of 3. you'd not see the value from the database.

---

The solution for this is always the same: the code that needs the data from the database needs to be inside the then callback, or it needs to be called from there, or it needs to be otherwise synchronized to run when the data has loaded.

For example, if you want to use the name in the UI of your app, you can store it in the state of your (stateful) widget by wrapping it like this:

setState(() {
  name = doc['Full Name'];
})

Now Flutter knows that it needs to update the UI after the data has loaded.

Since this is such a common occurrence, there is a dedicated widget to handling it known as a FutureBuilder . So I recommend reading up on that too.