Flutter how to use Future return value as if varia

问题:

I want to get Future return value and use it like variable. I have this Future function

  Future<User> _fetchUserInfo(String id) async {
    User fetchedUser;
    await Firestore.instance
        .collection('user')
        .document(id)
        .get()
        .then((snapshot) {
      final User user = User(snapshot);
      fetchedUser = user;
    });
    return fetchedUser;
  }

And I want get value like so

final user = _fetchUserInfo(id);

However when I tried to use like this

new Text(user.userName);

Dart doesn't recognize as User class. It says dynamic.
How Can I get return value and use it?
Am I doing wrong way first of all? Any help is appreciated!

回答1:

You can simplify the code:

Future<User> _fetchUserInfo(String id) async {
    User fetchedUser;
    var snapshot = await Firestore.instance
        .collection('user')
        .document(id)
        .get();
    return User(snapshot);
  }

you also need async/await to get the value

void foo() async {
  final user = await _fetchUserInfo(id);
}

回答2:

use like this

var username;
dbHelper.getUsers().then((user) {
  username = user.getName;
});

---

this function returns future value

dbHelper.getUsers()

it is written like this

getUsers() async { .... }

回答3:

To add a little more detail on the original question from Daibaku which might help clarify using Futures in Flutter/dart, user was:

final user = _fetchUserInfo(id);

Without letting compiler infer type, that would be:

final Future<User> user = _fetchUserInfo(id);

Get & Use a Future Value

Daibaku asked how to get the return value of user & use it.

To use user in a Widget, your widget must watch/observe for changes, and rebuild itself when it does.

You can do this manually with a lot of boilerplate code, or you could use FutureBuilder, which has done this for you:

  FutureBuilder(future: user, builder: (context, snapshot) 
  {
    if (snapshot.hasData) return Text(snapshot.data.userName);
    return Container(width: 0.0, height: 0.0,);
  }

More explicitly, you could write the above as:

  FutureBuilder<User>(future: user, builder: (context, AsyncSnapshot<User> userSnapshot)
  {
    if (userSnapshot.hasData) return Text(userSnapshot.data.userName);
    return Container(width: 0.0, height: 0.0,);
  }

FutureBuilder in the above example, synchronously (i.e. immediately) returns a zero width/height Container Widget, but also attaches an observer to your future user, to know when Future data arrives.

When user is eventually updated with "data", FutureBuilder's observer is notified and it calls setState(), which starts a rebuild of itself. Now that snapshot.hasData is true, Text(snapshot.data.userName) is returned instead of the empty Container.

---

If you weren't using user within the synchronous world of Widget Element tree building and just wanted to print out the value for debugging you could attach an observer using then:

user.then((u) => print(u.userName));

回答4:

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

class ThreadExample implements Callable<String>{

    @Override
    public String call() throws Exception {
        // TODO Auto-generated method stub
        return "Ashish";
    }

}
public class FutureThreadExample {

    public static void main(String a[]) throws InterruptedException, ExecutionException {
        ExecutorService executorService=Executors.newFixedThreadPool(1);
        List <Future<String>>objList=new ArrayList<Future<String>>();
       for(int i=0;i<10;i++) {
            Future<String> obj=executorService.submit(new ThreadExample());
            objList.add(obj);
       }
        for( Future<String> fut:objList) {
            System.out.println(fut.get());
        }
        executorService.shutdown();
    }


}