Скачать или смотреть Resolving the Expected a value of type 'Iterable dynamic ' Error in Flutter

Learn how to address the `Expected a value of type 'Iterable dynamic '` error when working with JSON in Flutter. Explore solutions to extract data and fix type mismatches for better performance.
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This video is based on the question https://stackoverflow.com/q/69816632/ asked by the user 'MCB' ( https://stackoverflow.com/u/15909371/ ) and on the answer https://stackoverflow.com/a/69821918/ provided by the user 'ACR' ( https://stackoverflow.com/u/10091883/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving the Expected a value of type 'Iterable<dynamic>' Error in Flutter: A Complete Guide

Working with JSON data in Flutter applications is a common task, but running into errors can be frustrating. One such error you might encounter is:

Error: Expected a value of type 'Iterable dynamic ', but got one of type '_JsonMap'

This error typically arises when you attempt to create a list from a JSON file, but the structure of your data doesn't match your expectations. Let's dive into why this error occurs and how you can effectively resolve it.

Understanding the Problem

Source of the Error

In the provided code, the Flutter application tries to decode a JSON response and populate a list of objects (inspections). Here’s a snippet where the error occurs:

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

The code expects inspectionsJson to be a list or an iterable collection. However, the json.decode function returns a _JsonMap, which is essentially a map containing key-value pairs parsed from the JSON.

The Solution

To fix this error, follow these steps:

Step 1: Correctly Access JSON Data

Instead of attempting to loop through inspectionsJson directly, you need to access the specific array of inspections in the JSON structure.

Here’s an updated version of your function:

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

Step 2: Adjust Your Inspection Class Data Types

Another potential issue in your current implementation involves mismatched data types between your JSON file and the Inspection class.

Review the JSON:

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

In this JSON, the state, interval, and date fields are strings. But looking at your Inspection class:

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

Recommended Changes:

Change int types to String in your Inspection class for interval and state.

Store the date as a String, and only convert it to a DateTime object when you're ready to use it.

Example of Updated Inspection Class

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

Conclusion

By correctly accessing the inspection array in your JSON and ensuring that your data types align with the data being returned from the JSON file, you can effectively resolve the Expected a value of type 'Iterable<dynamic>' error.

Key Takeaways:

Always inspect the structure of your JSON data.

Match your Dart class properties to those types in JSON to prevent conversion errors.

Focus on extracting specific lists from JSON maps to avoid type mismatches.

With these adjustments, your Flutter application should be well on its way to handling JSON data smoothly.