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.
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.
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.
Consider an example:
import 'dart:async';
class Employee {
int id;
String firstName;
String lastName;
Employee(this.id, this.firstName, this.lastName);
}
void main() async {
print("getting employee...");
var x = await getEmployee(33);
print("Got back ${x.firstName} ${x.lastName} with id# ${x.id}");
}
Future<Employee> getEmployee(int id) async {
//Simluate what a real service call delay may look like by delaying 2 seconds
await Future<Employee>.delayed(const Duration(seconds: 2));
//and then return an employee - lets pretend we grabbed this out of a database
var e = new Employee(id, "Joe", "Coder");
return e;
}