Скачать или смотреть How to Resolve the json.Decode error: FormatException: Unexpected character in Flutter Applications

Encountering a `FormatException` when decoding JSON in your Flutter app? This guide will guide you through fixing JSON encoding issues between Dart servers and Flutter clients.
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Understanding the json.Decode error: FormatException in Flutter

Developing applications often comes with its own set of challenges. One common error that developers encounter while working with Flutter and Dart is the FormatException related to JSON decoding. If you have a similar experience, especially when trying to fetch a list of companies from MongoDB, you’re in the right place. Let’s dive into the problem and how we can resolve it effectively.

The Problem

We want to retrieve a list of companies from a MongoDB database to use in a Flutter application. Here’s the key part of the Dart server code responsible for fetching data:

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When data is fetched from the server, the output looks like this:

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This data, however, is not formatted as valid JSON. When trying to decode this data on the client side, an error occurs:

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

Identifying the Issue

The main reason for this error lies in how the JSON is being generated on the server. The response you’re currently sending is not valid JSON. Instead of proper JSON formatting, you are using list.toString(), which does not produce a valid JSON structure.

What is Valid JSON?

Valid JSON requires:

String keys and string values must be enclosed in double quotes "

Data must be structured in objects {} or arrays [] properly

Example of Valid JSON

A valid JSON format for the fetched data should look like this:

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

The Solution

The solution is to encode the fetched data correctly on the server side before returning it to the client. Here’s how you can achieve this:

Step-by-Step Solution

Import the necessary package: Ensure you have imported dart:convert for JSON encoding.

Use the jsonEncode function: Instead of using list.toString(), you should convert the list to valid JSON using jsonEncode.

Here’s the corrected server code:

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

By utilizing jsonEncode(list), you ensure that the output adheres to JSON standards.

Final Thoughts

By following the steps outlined, you should be able to successfully fetch and decode the list of companies without encountering the FormatException. This adjustment not only solves your error but also enhances the data communication between your Dart server and Flutter client.

Armed with this knowledge, you should now feel more equipped to tackle data fetching in your application. Happy coding!