Dart series part: asynchronous programming in dart

brief introduction

be familiar with javascript Our friends should know , stay ES6 Introduced in await and async The grammar of , It is convenient for asynchronous programming , To get rid of hell .dart As a new language , There is no reason not to inherit this excellent quality . Very natural ,dart There are also await and async Language , Let's see .

Why asynchronous programming

So why use asynchronous programming ？ Can't it be solved only by synchronization ？

In fact, in most cases, synchronization is enough , But in the following cases , The synchronized scenario is still flawed .

1. It takes a long time to download data from the network .
2. How long it takes to read the database .
3. Reading data from a file .

In conclusion , If some operations take a lot of time , Then you can use asynchronous programming .

How do you use it?

async Is the descriptor of the method , If you want to use await, We must cooperate with async Use it together ：

Future<void> checkVersion() async {
  var version = await lookUpVersion();
  // Do something with version
}

Be careful ,await Usually followed by Future object .

Let's look at an example of using asynchronous programming incorrectly ：

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

Future<String> fetchUserOrder() =>
    Future.delayed(
      const Duration(seconds: 2),
      () => 'Order one!',
    );

void main() {
  print(createOrderMessage());
}

The above code is intended to print out the data taken from the database , But the result is not as expected , And the reason is fetchUserOrder Method is an asynchronous method , So it won't return immediately , This causes the result to fail to print .

Use the above code async rewrite ：

Future<String> createOrderMessage() async {
  var order = await fetchUserOrder();
  return 'Your order is: $order';
}

Future<String> fetchUserOrder() =>
    Future.delayed(
      const Duration(seconds: 2),
      () => 'Large Latte',
    );

Future<void> main() async {
  print('Fetching user order...');
  print(await createOrderMessage());
}

Future

We are using async and await Used in the process of Future. stay java in Future Represents the execution result of the thread . stay dart in Future Represents the result of an asynchronous execution .

Future There are two states ：uncompleted perhaps completed.

When you first execute an asynchronous function , An incomplete is returned Future. This unfinished Future It will wait for the completion or failure of asynchronous execution .

Whether the asynchronous program succeeds or fails , Finally, a completion status will be returned .

async Back to Future Can be generic , Represents the specific type returned when , such as Future Indicates that a string is returned , and Future Indicates that no value is returned .

Here are two examples of different returns ：

Future<String> fetchUserOrder() {
  return Future.delayed(const Duration(seconds: 2), () => 'Large Latte');
}

Future<void> fetchUserOrder() {
  return Future.delayed(const Duration(seconds: 2), () => print('Large Latte'));
}

The following is an example of an exception ：

Future<void> fetchUserOrder() {
  return Future.delayed(const Duration(seconds: 2),
      () => throw Exception('Logout failed: user ID is invalid'));
}

Asynchronous exception handling

stay async The function of , Yes await Exception thrown in asynchronous method of , It can be used directly try catch To catch exceptions ：

try {
  print('Awaiting user order...');
  var order = await fetchUserOrder();
} catch (err) {
  print('Caught error: $err');
}

Calling asynchronous functions in synchronous functions

It's described above fetchUserOrder() Back to a Future, Represents an asynchronous process .

So if it's a synchronous method , such as main() Function , How to call asynchronous methods , And get the return value ？

await Surely not , because await Only in async Call in the method . You can use then sentence ：

fetchUserOrder().then(order=>'do something');

then The statement waits for the asynchronous execution to return the result , And then process the results , It's actually equivalent to javascript Callback in .

summary

That's all dart in async and await Usage of .

This article has been included in http://www.flydean.com/12-dart-async/ The most popular interpretation , The deepest dry goods , The most concise tutorial , There are so many tricks you don't know about waiting for you to discover ！