How to Merge a Hot Source with a Cold Source in C# using Reactive Extensions

Описание: Learn how to effectively merge a hot source that continuously emits values with a cold source that completes, using Reactive Extensions in C# .
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This video is based on the question https://stackoverflow.com/q/70546962/ asked by the user 'Apostolis Bekiaris' ( https://stackoverflow.com/u/161739/ ) and on the answer https://stackoverflow.com/a/70547132/ provided by the user 'Enigmativity' ( https://stackoverflow.com/u/259769/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Merging a Hot Source with a Cold Source using Reactive Extensions

Working with observables in C# can be both powerful and challenging, especially when it comes to merging different types of data sources. One common scenario developers face is merging a hot source that emits values continuously with a cold source that completes after emitting a finite set of values.

In this post, we will tackle the problem of merging these two sources, exploring why it’s important, and how to implement an effective solution using Reactive Extensions (Rx.NET).

Understanding the Problem

Let's break down the issue:

Source 1: A hot observable that emits A, B, C, D and does not complete. This means it continuously produces values over time.

Source 2: A cold observable that emits values 1, 2 and then completes. This type of source is cold because it produces the same sequence of values every time it is subscribed to but does so only once.

The goal is to merge these two sources to produce a sequence like A1, B2, C1, D2 and so on.

The Challenge

The challenge arises from the fact that the cold source (source2) is expensive to generate. In a previous attempt, the use of Zip and Repeat seemed promising, but it ended up creating a lock due to the expensive nature of the cold source, causing performance issues.

The Solution

To effectively merge these two sources without running into locking issues, we can make use of the ToArray method to compute the values of the cold source only once. Here is how this can be achieved step-by-step:

Step 1: Convert Source2 into an Array

The first step is to convert the cold observable (source2) into an array so that we can repeat it as needed without recalculating each time. Here’s the code snippet:

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

Step 2: Use Zip to Merge the Sources

Now that we have our cold source ready as an array, we can use the Zip method along with Repeat() to merge it with the hot observable (source1). The Zip method pairs values from both sources, producing a tuple:

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

source1: The hot observable.

array.Repeat(): This allows source2 values to repeat indefinitely, effectively matching the output of source1.

Resulting Output

The output of the above implementation will yield pairs of emitted values in the form you specified:

A1, B2, C1, D2, etc.

Conclusion

By leveraging the power of Rx.NET, we can effectively merge a hot source with a cold source, ensuring both performance and clarity in our reactive programming experience. Remember, the principles of observables—hot vs cold—are crucial in understanding how data flows in your applications, and how you can manipulate it for optimal results.

Next time you find yourself needing to merge different observables, try this approach. It could save you from performance pitfalls while achieving your desired result.

Happy coding with Reactive Extensions!