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Confused about the Dart event loop and asynchronous programming? This post explains the differences and inner workings of Futures to help clarify your understanding.
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Understanding the Dart Event Loop: A Clear Guide to Asynchronous Programming

If you're diving into the world of Dart and asynchronous programming, the concept of the event loop can be quite puzzling. Recently, while exploring this topic, a question emerged: Why do we see different outputs from seemingly similar code, and how do we properly handle Futures in Dart? In this guide, we’ll break it down step-by-step to clarify any confusion you might have and help you grasp the essentials of the event loop in Dart.

The Example That Sparked Confusion

Consider the following Dart code example showcasing the use of Futures:

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When executing this code, the output is as follows:

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Breakdown of the Problem

You may have noticed two different syntactic forms in the code which led to some confusion:

Shorthand Syntax:

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Full Syntax:

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Key Questions:

What is the difference between these two syntaxes?

Why does the output look the way it does?

Understanding the Differences

1. Shorthand vs Full Syntax

The shorthand syntax is simply a concise way of writing a function that returns a Future, whereas the full syntax requires an explicit return statement. Here’s how they compare:

Shorthand

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This form directly returns the new Future, which allows the event loop to execute it as expected.

Full Syntax (without return)

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Here, since there is no return statement, the new Future isn't returned to the scheduler. This means the flow isn't preserved as expected in your code.

2. Impact on Execution Flow

By missing the return in the full syntax, the spawned Future doesn't communicate back to the event loop, resulting in the sequence of outputs you observe. To fix this, simply include the return keyword in the full syntax:

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What Happens After Fixing the Syntax?

With this adjustment, the output changes to the following:

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This output now represents a more intuitive understanding of how Futures work in Dart’s event loop.

Conclusion

Understanding the nuances of Dart's event loop and how we handle asynchronous programming with Futures is essential for effective Dart development. By being mindful of your syntax—particularly with return statements—you can ensure that your code behaves as expected and avoid confusion in the future.

If you're just getting started with Dart or want to deepen your understanding of asynchronous programming, I encourage you to keep practicing and experimenting with these concepts. Happy coding!