Скачать или смотреть Solving groupBy Emission Issues with RxJS Timer

Discover how to effectively use the `groupBy` operator in RxJS with timers, ensuring your observables emit values as expected.
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This video is based on the question https://stackoverflow.com/q/62308736/ asked by the user 'Christophe Geers' ( https://stackoverflow.com/u/893099/ ) and on the answer https://stackoverflow.com/a/62309677/ provided by the user 'bryan60' ( https://stackoverflow.com/u/4855306/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Navigating the RxJS groupBy Operator with Timers

If you’re using RxJS, particularly with version ^6.5.5, you may find yourself encountering issues when trying to group emissions using the groupBy operator with a timer. In this post, we’ll break down a common scenario where the groupBy operator fails to emit values as intended when combined with timer. By understanding the problem and implementing a solid solution, you’ll be well on your way to mastering RxJS.

The Problem: Emissions Stop with timer

To illustrate this problem, let’s consider a simplified version of a function designed to retrieve files:

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Initially, when using the of operator, we structure our RxJS pipeline as follows:

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In this case, two values are correctly emitted to the subscriber:

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However, when we introduce the timer operator to periodically fetch files, the scenario changes:

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In this implementation, you might notice that no values are emitted despite the timer polling every second. What’s going on?

Understanding the Behavior

Key Differences

Completion State:

The of operator completes after emitting all values, which allows the toArray() operator to produce a final array and emit the values.

The timer operator, on the other hand, never completes since it is designed to emit continuously based on the specified interval.

Impact on groupBy:

Since groupBy relies on an ongoing stream of values, it creates grouped observables that also expect future emissions. Because timer never completes, the grouped observables remain open, and toArray() never has a chance to emit.

The Solution: Restructure Your Code

To achieve the desired behavior of generating new groups with each poll while using timer, you need to restructure your code slightly. This can be done by switching to the switchMap operator for clean handling of emissions:

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What We’ve Changed:

Using from: We wrap retrieveFiles() with from(), allowing it to be treated as an observable.

Using switchMap: This operator cancels any ongoing requests when a new timer tick happens, preventing potential overhead and ensuring that each emitted value generates new groups.

Rethinking groupBy

It's worth considering whether using groupBy for this purpose is necessary. In many cases, especially if you are just exploring RxJS functionality, a simpler synchronous array grouping might suffice. groupBy is typically more effective when used to handle continuous streams of values that need to be organized into separate observable streams.

Conclusion

In summary, while it may be tempting to use groupBy alongside timer to handle periodic polling of data, it’s crucial to understand the underlying behavior of these operators. By making small adjustments to your RxJS pipeline, you can ensure it operates as intended and emits the values you expect. Remember to choose the right operators based on your use case for optimal performance and clarity in your code!