How do you implement cancellation tokens in MVVM to abort async tasks when needed?

Asynchronous programming is a powerful technique to make your MVVM applications more responsive and efficient, but it also comes with some challenges. One of them is how to cancel an async task when it is no longer needed, for example, when the user navigates away from a view or changes a filter. In this article, you will learn how to use cancellation tokens in MVVM to abort async tasks when needed, and avoid wasting resources or causing errors.

What are cancellation tokens?

Cancellation tokens are objects that signal when an async operation should be canceled. They are part of the System.Threading.Tasks namespace, and they work with the async and await keywords. You can create a cancellation token source (CTS) and pass its token property to any async method that supports cancellation. Then, you can call the CTS's cancel method to request the cancellation of the async operation.

How to use cancellation tokens in MVVM?

In MVVM, you typically have a view model that exposes properties and commands that are bound to the view. If you have an async command that performs a long-running or expensive operation, such as loading data from a web service, you can use a cancellation token to cancel it if the user changes their mind or the view model is disposed. To do this, you need to store the CTS in a private field in your view model, and create a new one every time you execute the async command. Then, you need to pass the CTS's token to the async method, and handle the OperationCanceledException if it is thrown. Finally, you need to dispose the CTS in the view model's cleanup method.

How to handle multiple async tasks?

Sometimes, you may have more than one async task running in your view model, and you may want to cancel them individually or all at once. For example, you may have a search command that triggers a new async task every time the user types a keyword, and you want to cancel the previous tasks when a new one starts. To achieve this, you can use a dictionary or a list to store the CTSs for each task, and use a unique key or an index to identify them. Then, you can cancel the specific CTS or iterate over the collection and cancel all of them when needed.

How to report cancellation progress?

Another useful feature of cancellation tokens is that they allow you to report the progress of the cancellation to the user. For example, you may want to show a spinner or a message while the async task is being canceled. To do this, you can use the CTS's IsCancellationRequested property to check if the cancellation has been requested, and the CTS's Token's Register method to register a callback that will be invoked when the cancellation is completed. Then, you can use a property or an event in your view model to notify the view of the cancellation status.

How to test cancellation tokens in MVVM?

Testing cancellation tokens in MVVM is important to ensure that your async operations are canceled correctly and gracefully. You can use a unit testing framework, such as NUnit or MSTest, and a mocking library, such as Moq or NSubstitute, to create fake views and services that interact with your view models. Then, you can use the CTS's CancelAfter method to simulate a cancellation request after a specified delay, and verify that the async methods throw the OperationCanceledException and that the view model's properties and events reflect the cancellation state.

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