Скачать или смотреть How to Ensure New Elements in ObservableCollection Persist Across Pages in Xamarin.Forms

Discover how to effectively manage `ObservableCollection` in Xamarin.Forms to retain new elements added from different pages.
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This video is based on the question https://stackoverflow.com/q/69263275/ asked by the user 'Karlos18' ( https://stackoverflow.com/u/16962541/ ) and on the answer https://stackoverflow.com/a/69264389/ provided by the user 'deczaloth' ( https://stackoverflow.com/u/7432242/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Ensure New Elements in ObservableCollection Persist Across Pages in Xamarin.Forms

When developing applications using Xamarin.Forms, particularly those that utilize ObservableCollection, you might encounter an issue where new elements added from one page do not appear when you navigate back to another page. This common problem can lead to confusion, especially if you're new to C# . In this blog, we will explore how to persist new elements added to an ObservableCollection across different pages effectively.

Understanding ObservableCollection

ObservableCollection is a special collection type in C# that provides notifications when items get added, removed, or the whole list is refreshed. It is often used in GUI applications because it allows the UI to automatically update when the collection changes. However, if new instances of a view model are created, the changes made to those instances will not be reflected elsewhere in your application.

The Problem Statement

In a specific scenario, you have two pages:

The first page displays the items from an ObservableCollection.

The second page allows you to add new items to that collection.

However, even though you can see new items added during debugging, these changes do not persist when you return to the first page. This problem arises because you are creating separate instances of your view model every time you navigate to the second page.

Solution Overview

To solve this issue, we need to ensure that both pages are referencing the same instance of the view model. Here are the steps to accomplish this:

Step 1: Create a Static Instance of the ViewModel

In your ListViewFlashcard.xaml.cs (the code-behind file for the first page), we need to create a static instance of the ListViewFlascardViewModel:

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Step 2: Update the Binding Context in XAML

Next, we must update the XAML for ListViewFlashcard.xaml. Instead of setting the BindingContext in the XAML, we rely on the static instance we just created in the code-behind:

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

Step 3: Modify the Second Page's ViewModel

In your NoweFiszkiViewModel, you need to ensure you're adding new elements to the static instance instead of creating a new instance each time. Here's how to modify your NoweFiszkiViewModel.cs:

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

Conclusion

By following these steps, you ensure that both pages reference the same instance of ListViewFlascardViewModel. Therefore, any changes made to the ObservableCollection in the second page will reflect correctly when you navigate back to the first page.

This solution not only resolves the current issue but also enhances your understanding of how ViewModels and data binding work in Xamarin.Forms. Happy coding, and may your applications thrive with seamless data management!