Скачать или смотреть Understanding the Just and first() in Combine: Key Differences and Solutions

Explore the differences between `Just` and `first()` in Combine using Swift. Learn why your stream might not be finishing and how to correctly implement these methods in your code.
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Understanding the Just and first() in Combine: Key Differences and Solutions

If you're diving into the world of Combine with Swift, you may have stumbled upon some perplexing behaviors, particularly regarding the difference between using Just and first(). These terms often come up when discussing publishers and streams in Combine, but understanding how they work can significantly impact your code's behavior.

In this post, we'll answer a common question: Why does a stream with flatMap not finish as expected, even when Just is returned? We'll explore this issue step by step and provide clarity on how to ensure your streams behave as intended.

The Problem: Navigating Stream Completion

You may have written two functions, getPublisherWithFirst and getPublisherWithFlatMap, that both process an upstream publisher. Here’s a quick overview of what each function does:

getPublisherWithFirst

This function:

Uses a PassthroughSubject as the upstream.

Applies compactMap to filter values.

Employs first() to complete after emitting the first value.

This function behaves as expected, completing when the first value is received.

getPublisherWithFlatMap

In comparison, this function:

Also relies on a PassthroughSubject.

Uses flatMap to handle the emitted values.

Returns a Just value if certain conditions are met; otherwise, it returns Empty.

However, the confusion arises when getPublisherWithFlatMap doesn't complete as anticipated, even if a valid Just publisher is returned.

The Cause: Understanding PassthroughSubject and Completion

1. PassthroughSubject Illuminated

A PassthroughSubject is special in that it doesn't complete on its own unless explicitly instructed to do so. If it remains active, any downstream subscribers will continue to listen, causing the stream to wait indefinitely.

2. The Role of first()

Using first() is crucial. It allows the stream to complete as soon as it receives a single value from the upstream publisher, regardless of whether the upstream itself has completed. This is why the getPublisherWithFirst function works seamlessly.

3. Understanding flatMap Behavior

With flatMap, the sceneries change:

flatMap executes its closure for every value emitted from its upstream.

The flatMap only completes when the entire upstream has completed, which in this case is the PassthroughSubject.

Even if Just emits a value, the upstream subject's state remains open, leading to a situation where the stream appears to hang indefinitely.

The Solution: Adjusting Your Implementation

To ensure that getPublisherWithFlatMap behaves like getPublisherWithFirst, you need to introduce first() after the flatMap operator. Here's how you can implement this:

Revised getPublisherWithFlatMap Function

[[See Video to Reveal this Text or Code Snippet]]

Key Takeaways

Use first(): This operator lets your stream complete after the initial value, regardless of upstream state.

Understand Upstream: Streams that don’t complete won’t allow downstream values to complete, causing confusion in behaviors.

Know your Publishers: Just completes after delivering its value, but PassthroughSubject requires explicit completion.

By incorporating these changes into your Combine code, you'll be better equipped to handle the intricacies of stream completion. With practice, Combine will become a powerful tool in your Swift programming arsenal.

Happy coding!