Скачать или смотреть How to Display List of Markers on a Map in React Native

Learn how to render markers from a database on your React Native map using the `react-native-maps` library with an easy, step-by-step guide.
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This video is based on the question https://stackoverflow.com/q/70228248/ asked by the user 'Emanuele Ago' ( https://stackoverflow.com/u/17586837/ ) and on the answer https://stackoverflow.com/a/70228396/ provided by the user 'Gilles Medeiros Henriques' ( https://stackoverflow.com/u/9299992/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Display List of Markers on a Map in React Native

As a beginner to React Native, you might face numerous challenges. One common problem developers encounter is displaying a list of markers on a map where the data is stored in a database, particularly SQLite. If you are using the react-native-maps library, you may find yourself puzzled when the markers do not appear as expected despite your code seemingly being correct. This guide aims to clarify this problem and provide a comprehensive solution.

Understanding the Problem

You are trying to show markers on a map using the MapView component. You have an array (arrayData) that you intend to populate with data fetched from a SQLite database. However, when rendering the MapView, the array always shows only the initial marker instead of all the markers that should be fetched from your database.

This issue occurs because the data-fetching process is asynchronous. When your map tries to render, the fetch operation doesn’t complete in time, which is why you're only seeing the initial marker.

Step-by-Step Solution

To resolve the problem, you need to make a few changes to your code to properly handle asynchronous data fetching in React Native. Here’s how to do it:

1. Use useState Hook to Manage Your Data

Instead of defining the array as a constant, use the useState hook to create a state variable that will hold the data. This allows you to re-render the component when the array is updated.

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2. Create an Asynchronous Function to Fetch Data

Make your data-fetching function asynchronous. This will allow you to pause execution until the data is fully retrieved before proceeding to set the state.

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3. Use the useEffect Hook to Fetch Data on Component Mount

Use the useEffect hook to call your data-fetching function when the component mounts. This ensures that your component waits for the data before rendering the markers.

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4. Enhance Your Database Query

Ensure your database query within the funzione method correctly waits for the data retrieval to complete. Here’s a refined example:

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Rendering Markers on the Map

With the state properly set and data fetched, you can now render the markers in the MapView component as follows:

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Conclusion

By following these steps, you should be able to successfully render multiple markers on your map based on dynamic data fetched from a SQLite database.

If you encounter any issues, feel free to reach out for further assistance. Happy coding!