Describe the difference between async and sync in Dart.

In Dart, async and sync are two important concepts that relate to how code execution is managed, especially in the context of asynchronous programming. They determine whether a block of code runs synchronously (in order) or asynchronously (potentially concurrently). Here's a description of the difference between async and sync in Dart with examples:

Synchronous (Sync) Code:

Synchronous code executes line by line, one statement at a time, in a sequential and blocking manner. It means that each statement has to complete before the next one begins. Synchronous code is easier to reason about because it follows a predictable flow.

Synchronous code is the default behavior in Dart and doesn't require any specific keywords like async and await.

Example of Synchronous Code:

void main() {
  print('Start');
  print('Middle');
  print('End');
}

In this example, the output will be in the same order as the code: "Start," "Middle," and "End."

Asynchronous (Async) Code:

Asynchronous code, on the other hand, allows tasks to run concurrently without blocking the main program's execution. It's essential for handling operations that might take a long time, such as network requests, file I/O, or waiting for user input.

To write asynchronous code in Dart, you mark a function as async and use the await keyword to pause execution until an asynchronous operation is completed. This enables you to write non-blocking code while maintaining a sequential flow.

Example of Asynchronous Code:

Future<void> fetchData() async {
  await Future.delayed(Duration(seconds: 2)); // Simulate a 2-second delay.
  print('Data fetched.');
}

void main() async {
  print('Start');
  await fetchData(); // Wait for fetchData to complete asynchronously.
  print('End');
}

In this example, the code inside fetchData is asynchronous and causes a delay of 2 seconds. The await keyword is used to pause the execution of main until fetchData is completed. As a result, the output will be: "Start," "Data fetched," and then "End."

To summarize, the main difference between async and sync in Dart is in how code execution is managed:

Sync (Synchronous): Executes sequentially, one statement at a time, in a blocking manner.
Async (Asynchronous): Allows concurrent execution, can pause and resume execution using await, and is essential for non-blocking tasks.
You choose between these two modes based on the requirements of your program. Synchronous code is suitable for simple, straightforward operations, while asynchronous code is necessary for tasks that involve waiting for external resources or user input without freezing the application.