异步编程：使用 Future 和 async-await

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 and await keywords.
• How using async and await affects execution order.
• How to handle errors from an asynchronous call using try-catch expressions in async 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.

Such asynchronous computations usually provide their result as a Future or, if the result has multiple parts, as a Stream.
These computations introduce asynchrony into a program. To accommodate that initial asynchrony, other plain Dart functions also need to become asynchronous.

To interact with these asynchronous results, you can use the async and await keywords. Most asynchronous functions are
just async Dart functions that depend, possibly deep down, on an inherently asynchronous computation.

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 call fetchUserOrder() and wait for it to finish.
• Because createOrderMessage() does not wait for fetchUserOrder() to finish, createOrderMessage()
• fails to get the string value that fetchUserOrder() 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<String>’”.

In the next sections you’ll learn about futures and about working with futures (using 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 in async 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.

String createOrderMessage() {
  var order = fetchUserOrder();
  return 'Your order is: $order';
}

Future<String> fetchUserOrder() =>
    // Imagine that this function is
    // more complex and slow.
    Future.delayed(
      const Duration(seconds: 2),
      () => 'Large Latte',
    );

void main() {
  print('Fetching user order...');
  print(createOrderMessage());
}

Fetching user order...
Your order is: Instance of '_Future<String>'

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(
      const 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 from String to Future<String>.
• The async keyword appears before the function bodies for createOrderMessage() and main().
• The await keyword appears before calling the asynchronous functions fetchUserOrder() and createOrderMessage().

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:

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"
• 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"
• Gets the number of logins by calling the provided function fetchLoginAmount().

Handling errors

To handle errors in an async function, use try-catch:

try {
  print('Awaiting user order...');
  var order = await fetchUserOrder();
} 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:

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"
• 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():

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 function fetchUsername().
• greetUser() creates a greeting for the user by calling addHello(), passing it the username, and returning the result.
  Example: If fetchUsername() returns 'Jenny', then greetUser() 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 calling logoutUser().

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.
  • Concurrency in Dart Understand and learn how to implement concurrency in Dart.
  • 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.

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