Dart Event Loop

Summary: in this tutorial, you’ll learn about the Dart event loop and how Dart uses the event loop to run asynchronous tasks.

Introduction to the Dart event loop

Dart is a single-threaded language. It means that Dart uses a single thread to execute the code. Dart’s single thread runs in what is known as an isolate.

The following program displays three numbers from 1 to 3 to the console:

void main() {
  print(1);
  print(2);
  print(3);
}Code language: PHP (php)

Output:

1
2
3

In this program, the main() function runs line by line from top to bottom until it completes. And this program runs synchronously because it never displays the number 2 before 1 for example.

In practice, there’re many tasks that can run asynchronously. The order does not matter. For example, reading from a file, downloading an image from a remote server, and connecting to a database.

If you run these tasks synchronously, the program needs to wait for them to complete, which causes the program to freeze for a while. Therefore, if you have any task that takes time to complete and may block the main execution of the program, you should run it asynchronously.

To schedule tasks asynchronously, Dart uses an event loop:

The event loop has two queues:

• A microtasks queue.
• An event queue.

Dart mostly uses the microtasks queue internally. Also, Dart uses an event queue for asynchronous tasks.

In this model, Dart always runs synchronous tasks in the main isolate immediately. It is not possible to interrupt them.

If Dart finds long-running tasks that can be postponed, it puts them in the event queue.

The event loop is always running. It continuously checks the synchronous tasks, microtask queue, and event queue:

• If all the tasks in the main isolate are completed, the event loop moves the tasks (if available) from the microtask queue to the main isolate for execution until the microtask queue is empty.
• If both synchronous tasks and the microtask queue are empty, the event loop moves the tasks from the event queue to the main thread for execution.
• The event loop continues until all the queues are empty.

Dart is single-threaded but it doesn’t mean that Dart can’t have tasks running on another thread.

For example, when you read a file, the work doesn’t happen on the main isolate. The operating system (OS) is reading the file inside its own process. Once the OS completes reading the file, it passes the data back to Dart. And Dart schedules a task to handle the data in the event queue. A lot of I/O tasks from the dart:io library work in this way.

Summary

• Dart is a single-threaded language.
• Dart uses an event loop to schedule asynchronous tasks.
• Dart always executes synchronous code immediately. It only executes the tasks in the microtask queue once the synchronous tasks are empty. And it only executes the tasks in the event queue once both synchronous tasks and the microtask queue are empty.