Скачать или смотреть How to Retrieve Multiple API Values in Angular with TypeScript and RxJS fromArray() concatMap()

Learn how to efficiently collect and display multiple API data entries in your Angular application using TypeScript and RxJS.
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This video is based on the question https://stackoverflow.com/q/69208228/ asked by the user 'Henning Möllendorf' ( https://stackoverflow.com/u/10895452/ ) and on the answer https://stackoverflow.com/a/69208567/ provided by the user 'shutsman' ( https://stackoverflow.com/u/9200941/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

Have you ever faced a situation where you're attempting to fetch multiple data entries from an API using Angular and TypeScript, only to find that you can only display one of them? This common issue can be particularly frustrating, especially when the data you need is available but isn't visible in your application.

In this guide, we'll explore the problem of fetching multiple values from an API using RxJS and how to properly display all of them in your Angular application. We will analyze the code you might already have and provide a clear solution that resolves the issue not just with theory but also with practical implementation.

Understanding the Problem

In the problem scenario, the developer is trying to collect data for four different processes from an API. However, despite the effort, only one of the values is shown in the UI. The reason lies in the way the data is being collected and stored.

Current Implementation

Here's a simplified version of what’s happening in the original code:

An array (dataArray) holds the four different types of data needed: ['cron', 'import', 'export', 'delta'].

The HTTP requests for these types are made in a loop.

Each response is supposed to be logged and saved in a dataResults object.

Despite correct programming logic, the final rendering only displays data for one key (delta).

Issue Breakdown

The problem arises when the asynchronous nature of HTTP requests is not handled properly.

Multiple requests are made but only the last one’s output gets reflected in the UI, causing the other entries to be overlooked.

Solution Overview

To resolve this, we will utilize RxJS operators such as from(), concatMap(), and scan(). These will enable sequential HTTP requests and aggregating the results effectively. Let’s break it down step-by-step.

Step 1: Setup the dataArray

First, we need to prepare an array that holds all keys for which data will be fetched.

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Step 2: Use from() and concatMap()

The from() operator will convert the array of keys into an observable that emits each type one at a time, while concatMap() will handle the HTTP requests sequentially:

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

Step 3: Update HTML to Display All Results

With the dataResults object populated with all API entries, we ensure that our HTML iterates over this data correctly. This can be done using Angular directives to display each key's timestamp.

For example:

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Step 4: Modify getTimestamp Function (If Necessary)

In case the structure of dataResults has changed, make sure your getTimestamp function accommodates the new data model:

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

Conclusion

By employing the sequential fetching logic using RxJS operators, we can successfully collect and display multiple API responses in our Angular application. This method not only enhances understanding but also ensures more robust and cleaner code. So, if you find yourself in a situation where only one API result is available to the UI, try this approach to enjoy viewing all data efficiently!

By applying the outlined solution, you can ensure that all required values are displayed in your application without losing any important information. Happy coding!