This codelab teaches you how to write asynchronous code using
futures and the async
and await
keywords. Using embedded DartPad
editors, you can test your knowledge by running example code and completing
exercises.
To get the most out of this codelab, you should have the following:
- Knowledge of basic Dart syntax.
- Some experience writing asynchronous code in another language.
This codelab covers the following material:
- How and when to use the
async
andawait
keywords. - How using
async
andawait
affects execution order. - How to handle errors from an asynchronous call using
try-catch
expressions inasync
functions.
Estimated time to complete this codelab: 40-60 minutes.
Why asynchronous code matters
Asynchronous operations let your program complete work while waiting for another operation to finish. Here are some common asynchronous operations:
- Fetching data over a network.
- Writing to a database.
- Reading data from a file.
To perform asynchronous operations in Dart, you can use the Future
class
and the async
and await
keywords.
Example: Incorrectly using an asynchronous function
The following example shows the wrong way to use an asynchronous function
(fetchUserOrder()
). Later you’ll fix the example using async
and await
.
Before running this example, try to spot the issue – what do you think the
output will be?
Here’s why the example fails to print the value that fetchUserOrder()
eventually
produces:
-
fetchUserOrder()
is an asynchronous function that, after a delay, provides a string that describes the user’s order: a “Large Latte”. - To get the user’s order,
createOrderMessage()
should callfetchUserOrder()
and wait for it to finish. BecausecreateOrderMessage()
does not wait forfetchUserOrder()
to finish,createOrderMessage()
fails to get the string value thatfetchUserOrder()
eventually provides. - Instead,
createOrderMessage()
gets a representation of pending work to be done: an uncompleted future. You’ll learn more about futures in the next section. - Because
createOrderMessage()
fails to get the value describing the user’s order, the example fails to print “Large Latte” to the console, and instead prints “Your order is: Instance of ‘_Future'".
In the next sections you’ll learn the about futures, async
, and await
so that you’ll be able to write the code necessary to make fetchUserOrder()
print the desired value (“Large Latte”) to the console.
What is a future?
A future (lower case “f”) is an instance of the Future (capitalized “F”) class. A future represents the result of an asynchronous operation, and can have two states: uncompleted or completed.
Uncompleted
When you call an asynchronous function, it returns an uncompleted future. That future is waiting for the function’s asynchronous operation to finish or to throw an error.
Completed
If the asynchronous operation succeeds, the future completes with a value. Otherwise it completes with an error.
Completing with a value
A future of type Future<T>
completes with a value of type T
.
For example, a future with type Future<String>
produces a string value.
If a future doesn’t produce a usable value, then the future’s type is
Future<void>
.
Completing with an error
If the asynchronous operation performed by the function fails for any reason, the future completes with an error.
Example: Introducing futures
In the following example, fetchUserOrder()
returns a future that completes after
printing to the console. Because it doesn’t return a usable value,
fetchUserOrder()
has the type Future<void>
. Before you run the example,
try to predict which will print first: “Large Latte” or “Fetching user order…”.
In the preceding example, even though fetchUserOrder()
executes before
the print()
call on line 8, the console shows the output from line 8
(“Fetching user order…”) before the output from fetchUserOrder()
(“Large Latte”).
This is because fetchUserOrder()
delays before it prints “Large Latte”.
Example: Completing with an error
Run the following example to see how a future completes with an error. A bit later you’ll learn how to handle the error.
In this example, fetchUserOrder()
completes with an error indicating that the
user ID is invalid.
You’ve learned about futures and how they complete, but how do you use the
results of asynchronous functions? In the next section you’ll learn how to get
results with the async
and await
keywords.
Working with futures: async and await
The async
and await
keywords provide a declarative way to define
asynchronous functions and use their results. Remember these two basic guidelines
when using async
and await
:
- To define an async function, add
async
before the function body: - The
await
keyword works only inasync
functions.
Here’s an example that converts main()
from a synchronous to asynchronous
function.
First, add the async
keyword before the function body:
void main() async { ··· }
If the function has a declared return type, then update the type to be
Future<T>
, where T
is the type of the value that the function returns.
If the function doesn’t explicitly return a value, then the return type is
Future<void>
:
Future<void> main() async { ··· }
Now that you have an async
function, you can use the await
keyword to wait
for a future to complete:
print(await createOrderMessage());
As the following two examples show, the async
and await
keywords result in
asynchronous code that looks a lot like synchronous code.
The only differences are highlighted in the asynchronous example, which — if
your window is wide enough — is to the right of the synchronous example.
Example: synchronous functions
String createOrderMessage() {
var order = fetchUserOrder();
return 'Your order is: $order';
}
Future<String> fetchUserOrder() =>
// Imagine that this function is
// more complex and slow.
Future.delayed(
Duration(seconds: 2),
() => 'Large Latte',
);
void main() {
print('Fetching user order...');
print(createOrderMessage());
}
Fetching user order...
Your order is: Instance of _Future<String>
Example: asynchronous functions
Future<String> createOrderMessage() async { var order = await fetchUserOrder(); return 'Your order is: $order'; } Future<String> fetchUserOrder() => // Imagine that this function is // more complex and slow. Future.delayed( Duration(seconds: 2), () => 'Large Latte', ); Future<void> main() async { print('Fetching user order...'); print(await createOrderMessage()); }
Fetching user order...
Your order is: Large Latte
The asynchronous example is different in three ways:
- The return type for
createOrderMessage()
changes fromString
toFuture<String>
. - The
async
keyword appears before the function bodies forcreateOrderMessage()
andmain()
. - The
await
keyword appears before calling the asynchronous functionsfetchUserOrder()
andcreateOrderMessage()
.
Execution flow with async and await
An async
function runs synchronously until the first
await
keyword. This means that within an async
function body, all
synchronous code before the first await
keyword executes immediately.
Example: Execution within async functions
Run the following example to see how execution proceeds within an async
function body. What do you think the output will be?
After running the code in the preceding example, try reversing lines 2 and 3:
var order = await fetchUserOrder();
print('Awaiting user order...');
Notice that timing of the output shifts, now that print('Awaiting user order')
appears after the first await
keyword in printOrderMessage()
.
Exercise: Practice using async and await
The following exercise is a failing unit test that contains partially completed
code snippets. Your task is to complete the exercise by writing code to make the
tests pass.
You don’t need to implement main()
.
To simulate asynchronous operations, call the following functions, which are provided for you:
Function | Type signature | Description |
---|---|---|
fetchRole() | Future<String> fetchRole() |
Gets a short description of the user’s role. |
fetchLoginAmount() | Future<int> fetchLoginAmount() |
Gets the number of times a user has logged in. |
Part 1: reportUserRole()
Add code to the reportUserRole()
function so that it does the following:
- Returns a future that completes with the following
string:
"User role: <user role>"
- Note: You must use the actual value returned by
fetchRole()
; copying and pasting the example return value won’t make the test pass. - Example return value:
"User role: tester"
- Note: You must use the actual value returned by
- Gets the user role by calling the provided function
fetchRole()
.
Part 2: reportLogins()
Implement an async
function reportLogins()
so that it does the following:
- Returns the string
"Total number of logins: <# of logins>"
.- Note: You must use the actual value returned by
fetchLoginAmount()
; copying and pasting the example return value won’t make the test pass. - Example return value from
reportLogins()
:"Total number of logins: 57"
- Note: You must use the actual value returned by
- Gets the number of logins by calling the provided function
fetchLoginAmount()
.
Handling errors
To handle errors in an async
function, use try-catch:
try {
var order = await fetchUserOrder();
print('Awaiting user order...');
} catch (err) {
print('Caught error: $err');
}
Within an async
function, you can write try-catch clauses
the same way you would in synchronous code.
Example: async and await with try-catch
Run the following example to see how to handle an error from an asynchronous function. What do you think the output will be?
Exercise: Practice handling errors
The following exercise provides practice handling errors with asynchronous code, using the approach described in the previous section. To simulate asynchronous operations, your code will call the following function, which is provided for you:
Function | Type signature | Description |
---|---|---|
fetchNewUsername() | Future<String> fetchNewUsername() |
Returns the new username that you can use to replace an old one. |
Use async
and await
to implement an asynchronous changeUsername()
function
that does the following:
- Calls the provided asynchronous function
fetchNewUsername()
and returns its result.- Example return value from
changeUsername()
:"jane_smith_92"
- Example return value from
- Catches any error that occurs and returns the string value of the error.
- You can use the toString() method to stringify both Exceptions and Errors.
Exercise: Putting it all together
It’s time to practice what you’ve learned in one final exercise.
To simulate asynchronous operations, this exercise provides the asynchronous
functions fetchUsername()
and logoutUser()
:
Function | Type signature | Description |
---|---|---|
fetchUsername() | Future<String> fetchUsername() |
Returns the name associated with the current user. |
logoutUser() | Future<String> logoutUser() |
Performs logout of current user and returns the username that was logged out. |
Write the following:
Part 1: addHello()
- Write a function
addHello()
that takes a single String argument. -
addHello()
returns its String argument preceded by ‘Hello ‘.
Example:addHello('Jon')
returns'Hello Jon'
.
Part 2: greetUser()
- Write a function
greetUser()
that takes no arguments. - To get the username,
greetUser()
calls the provided asynchronous functionfetchUsername()
. -
greetUser()
creates a greeting for the user by callingaddHello()
, passing it the username, and returning the result.
Example: IffetchUsername()
returns'Jenny'
, thengreetUser()
returns'Hello Jenny'
.
Part 3: sayGoodbye()
- Write a function
sayGoodbye()
that does the following:- Takes no arguments.
- Catches any errors.
- Calls the provided asynchronous function
logoutUser()
.
- If
logoutUser()
fails,sayGoodbye()
returns any string you like. - If
logoutUser()
succeeds,sayGoodbye()
returns the string'<result> Thanks, see you next time'
, where<result>
is the String value returned by callinglogoutUser()
.
What’s next?
Congratulations, you’ve finished the codelab! If you’d like to learn more, here are some suggestions for where to go next:
- Play with DartPad.
- Try another codelab.
- Learn more about futures and asynchrony:
- Streams tutorial: Learn how to work with a sequence of asynchronous events.
- Dart videos from Google: Watch one or more of the videos about asynchronous coding. Or, if you prefer, read the articles that are based on these videos. (Start with isolates and event loops.)
- Get the Dart SDK.
If you’re interested in using embedded DartPads, like this codelab does, see best practices for using DartPad in tutorials.