What is a Future and how do I use it?
Solution 1
In case you are familiar with Task<T>
or Promise<T>
and the async
/ await
pattern, then you can skip right to the "How to use a Future with the widgets in Flutter" section.
What is a Future and how do I use it?
Well, the documentation says:
An object representing a delayed computation.
That is correct. It's also a little abstract and dry. Normally, a function returns a result. Sequentially. The function is called, runs and returns it's result. Until then, the caller waits. Some functions, especially when they access resources like hardware or network, take a little time to do so. Imagine an avatar picture being loaded from a web server, a user's data being loaded from a database or just the texts of the app in multiple languages being loaded from device memory. That might be slow.
Most applications by default have a single flow of control. When this flow is blocked, for example by waiting for a computation or resource access that takes time, the application just freezes. You may remember this as standard if you are old enough, but in today's world that would be seen as a bug. Even if something takes time, we get a little animation. A spinner, an hourglass, maybe a progress bar. But how can an application run and show an animation and yet still wait for the result? The answer is: asynchronous operations. Operations that still run while your code waits for something. Now how does the compiler know, whether it should actually stop everything and wait for a result or continue with all the background work and wait only in this instance? Well, it cannot figure that out on it's own. We have to tell it.
This is achieved through a pattern known as async and await. It's not specific to flutter or dart, it exists under the same name in many other languages. You can find the documentation for Dart here.
Since a method that takes some time cannot return immediately, it will return the promise of delivering a value when it's done.
That is called a Future
. So the promise to load a number from the database would return a Future<int>
while the promise to return a list of movies from an internet search might return a Future<List<Movie>>
. A Future<T>
is something that in the future will give you a T
.
Lets try a different explanation:
A future represents the result of an asynchronous operation, and can have two states: uncompleted or completed.
Most likely, as you aren't doing this just for fun, you actually need the results of that Future<T>
to progress in your application. You need to display the number from the database or the list of movies found. So you want to wait, until the result is there. This is where await
comes in:
Future<List<Movie>> result = loadMoviesFromSearch(input);
// right here, you need the result. So you wait for it:
List<Movie> movies = await result;
But wait, haven't we come full circle? Aren't we waiting on the result again? Yes, indeed we are. Programs would be utterly chaotic if they did not have some resemblence of sequential flow. But the point is that using the keyword await
we have told the compiler, that at this point, while we want to wait for the result, we do not want our application to just freeze. We want all the other running operations like for example animations to continue.
However, you can only use the await
keyword in functions that themselves are marked as async
and return a Future<T>
. Because when you await
something, then the function that is awaiting can no longer return their result immediately. You can only return what you have, if you have to wait for it, you have to return a promise to deliver it later.
Future<Pizza> getPizza() async {
Future<PizzaBox> delivery = orderPizza();
var pizzaBox = await delivery;
var pizza = pizzaBox.unwrap();
return pizza;
}
Our getPizza function has to wait for the pizza, so instead of returning Pizza
immediately, it has to return the promise that a pizza will be there in the future. Now you can, in turn, await
the getPizza function somewhere.
How to use a Future with the widgets in Flutter?
All the widgets in flutter expect real values. Not some promise of a value to come at a later time. When a button needs a text, it cannot use a promise that text will come later. It needs to display the button now, so it needs the text now.
But sometimes, all you have is a Future<T>
. That is where FutureBuilder
comes in. You can use it when you have a future, to display one thing while you are waiting for it (for example a progress indicator) and another thing when it's done (for example the result).
Let's take a look at our pizza example. You want to order pizza, you want a progress indicator while you wait for it, you want to see the result once it's delivered, and maybe show an error message when there is an error:
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
/// ordering a pizza takes 5 seconds and then gives you a pizza salami with extra cheese
Future<String> orderPizza() {
return Future<String>.delayed(const Duration(seconds: 5), () async => 'Pizza Salami, Extra Cheese');
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
theme: ThemeData.dark(),
home: Scaffold(
body: Center(
child: PizzaOrder(),
),
),
);
}
}
class PizzaOrder extends StatefulWidget {
@override
_PizzaOrderState createState() => _PizzaOrderState();
}
class _PizzaOrderState extends State<PizzaOrder> {
Future<String>? delivery;
@override
Widget build(BuildContext context) {
return Column(
crossAxisAlignment: CrossAxisAlignment.center,
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
ElevatedButton(
onPressed: delivery != null ? null : () => setState(() { delivery = orderPizza(); }),
child: const Text('Order Pizza Now')
),
delivery == null
? const Text('No delivery scheduled')
: FutureBuilder(
future: delivery,
builder: (context, snapshot) {
if(snapshot.hasData) {
return Text('Delivery done: ${snapshot.data}');
} else if(snapshot.hasError) {
return Text('Delivery error: ${snapshot.error.toString()}');
} else {
return const CircularProgressIndicator();
}
})
]);
}
}
This is how you use a FutureBuilder to display the result of your future once you have it.
Solution 2
Future<T>
returning the potential value which will be done by async
work
Eg:
Future<int> getValue() async {
return Future.value(5);
}
Above code is returning Future.value(5)
which is of int
type, but while receiving the value from method we can't use type Future<int>
i.e
Future<int> value = await getValue(); // Not Allowed
// Error
A value of type 'Future<int>' can't be assigned to a variable of type 'int'
To solve above getValue() should be received under int
type
int value = await getValue(); // right way as it returning the potential value.
Solution 3
Here's a list of analogies to Dart's Future
from other languages:
- JS:
Promise
- Java:
Future
- Python:
Future
- C#:
Task
Just like in other languages Future is a special type of object which allows to use async/await syntax sugar, write asynchronous code in synchronous/linear way. You return Future from an async method rather than accept a callback as a parameter and avoid the callback hell - both Futures and callbacks solve same problems (firing some code at a latter time) but in different way.
Solution 4
I hope this key point will be informative, I show it in two different Async methods:
Note the following method where showLoading()
, getAllCarsFromApi()
and hideLoading()
are inner Async
methods.
If I put the await
keyword before showLoading()
, the Operation waits until it's done then goes to the next line but I intentionally removed the await
because I need my Loading dialog be displayed simultaneously with getAllCarsFromApi()
is being processed, so it means showLoading()
and getAllCarsFromApi()
methods are processed on different Threads. Finally hideLoading()
hides the loading dialog.
Future<List<Car>> getData() async{
showLoading();
final List<Car> cars = await getAllCarsFromApi();
hideLoading();
return cars;
}
Now look at this another Async
method, here the getCarByIdFromApi()
method needs an id
which is calculated from the getCarIdFromDatabase()
, so there must be an await
keyword before the first method to make the Operation wait until id
is calculated and passed to the second method. So here two methods are processed one after another and in a single Thread.
Future<Car> getCar() async{
int id = await getCarIdFromDatabase();
final Car car = await getCarByIdFromApi(id);
return car;
}
nvoigt
Updated on December 22, 2022Comments
-
nvoigt 11 months
I get the following error:
A value of type 'Future<int>' can't be assigned to a variable of type 'int'
It might be another type instead of
int
, but basically the pattern isA value of type 'Future<T>' can't be assigned to a variable of type 'T'
So...
- What exactly is a
Future
? - How do I get the actual value I want to get?
- What widget do I use to display my value when all I have is a
Future<T>
?
- What exactly is a
-
Christopher Moore over 3 yearsYou might consider adding the equivalent
.then
examples forasync
/await
and make it clear thatawait
is just syntactic sugar for.then
. -
dev-aentgs over 3 years@ChristopherMoore check the comment below this .then and await.
-
Christopher Moore over 3 years@dev-aentgs That's inaccurate.
await
is just syntactic sugar for.then
. It exists to make async code appear more like sync code so that it is easier to understand. -
dev-aentgs over 3 years@ChristopherMoore It is more than syntactic sugar another reference
-
Christopher Moore over 3 years@dev-aentgs nested
.then
will have the exact same behavior asawait
.await
was created to make clear and more understandable code. -
Christopher Moore over 3 years@dev-aentgs And if you really need a reference, here is one from what is IMO a more reliable source.
-
nvoigt over 3 yearsI think you are mixing two things:
await
and.then
are not interchangeable. They do not do the same things. However, a program can achieve the same things with a ton of nested.then
chains (and at least oneFuture.wait
) that it can achieve much cleaner and clearer with async/await. I'm not sure if I would call it syntactic sugar though. Where does it stop? Isn't everything outside of mov/cmp/jmp syntactic sugar? -
Christopher Moore over 3 yearsThey certainly are not directly interchangeable, but
Future.wait
would not be necessary if.then
was implemented correctly. As to whether it's really "syntactic sugar", I guess that can be your own opinion, but it appears to be a consensus theawait
is just syntactic sugar in many of the languages that support it. -
Christopher Moore over 3 years@nvoigt Though since you do say "a program can achieve the same things [...] with
.then
[...] that it can achieve much cleaner and clearer with async/await" isn't that what syntactic sugar is? -
Christopher Moore over 3 yearsRegardless, I think it might be a good idea to provide this information within the answer and show alternatives with
.then
. -
jamesdlin over 3 years
await
is syntactic sugar for.then
.await
is transformed into corresponding.then
(and possibly.catchError
and.whenCompleted
) callbacks. Anything that can be done withawait
can be done by manually registering appropriate callbacks on theFuture
. As for where to draw the line: the typical definition of "syntactic sugar" is something that can be removed from the language without removing functionality. -
Rageh Azzazy almost 3 yearsI still can't use
int value = await getValue();
inside the widget build method,, it says 'The await expression can only be used in an async function.' because the Build method is not async Beginner question.. but I don't know where to put it -
Jitesh Mohite almost 3 yearsyes, you can not use directly, you should use FutureBuilder or create new method and used setState() to updte data