[TomEdwardsEnscape]

This PR addresses as longstanding irritation with the boilerplate code required to execute continuation tasks on Avalonia's UI thread.

What is the current behaviour?

To ensure that a continuation task executes on the UI thread, you must execute TaskScheduler.FromCurrentSynchronizationContext on the UI thread, and pass the returned object to ContinueWith.

This isn't difficult, but is annoying and unintuitive.

Now that Avalonia permits multiple threads to have dispatchers, the situation is more confusing than ever because you can no longer store the scheduler object in a static property and access this everywhere. You instead need each object tied to a dispatcher to convert Dispatcher.Current to a scheduler by itself.

If you didn't create the scheduler when your object was constructed then to you probably have to post the TaskScheduler.FromCurrentSynchronizationContext call to the dispatcher and wait for it to execute there!

What is the updated/expected behavior with this PR?

Avalonia.Dispatcher can now be converted to TaskScheduler, letting us write simple and intuitive code like this:

myTask.ContinueWith(HandleResultMethod, Dispatcher.ToTaskScheduler());

This will work no matter what thread it's executed on.

Because the dispatcher is being converted to a different type, there is no change to its API. This avoids confusion between Avalonia's invocation methods and those of TaskScheduler.

How was the solution implemented (if it's not obvious)?

Unfortunately Microsoft don't make this job easy. TaskScheduler is an abstract class that you can implement yourself, but if you try then you will find that you can't execute continuation tasks. The method you need to call to do this is internal and exposed only via the object returned by TaskScheduler.FromCurrentSynchronizationContext...which ONLY uses the current thread's context.

So we have to change SynchronizationContext to be the target dispatcher, call the method, then reset SynchronizationContext back to its original value. Luckily this is a per-thread field and there are no side-effects to this switching.

Breaking changes

None.

Obsoletions / Deprecations

None.

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0064212-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[kekekeks]

Won't it be cheaper/easier to copy-paste https://github.com/dotnet/runtime/blob/51cfb0e382532e43500216c755d8bd8e1a8f371d/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/TaskScheduler.cs#L536-L602, wire it directly to Dispatcher::Post (or other API) rather than synchronization context and keep a lazy-crated reference in Dispatcher's field?

[TomEdwardsEnscape]

Won't it be cheaper/easier to copy-paste https://github.com/dotnet/runtime/blob/51cfb0e382532e43500216c755d8bd8e1a8f371d/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/TaskScheduler.cs#L536-L602, wire it directly to Dispatcher::Post (or other API) rather than synchronization context and keep a lazy-crated reference in Dispatcher's field?

This is the class which calls the internal method I mention in the PR description.

[kekekeks]

Why do we need to call the internal method? It's called by TaskScheduler.TryExecuteTask: https://github.com/dotnet/runtime/blob/5dd24c0265d6682025aa4a2678776b7dc1200b15/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/TaskScheduler.cs#L398-L406

[MrJul]

Notes from the API review meeting:

While the feature itself is good, we'd prefer explicit methods over implicit operators, especially since we need overloads.

• Add Dispatcher.ToTaskScheduler() that captures the current synchronization context's priority if the dispatchers match.
• Add Dispatcher.ToTaskScheduler(DispatcherPriority) to specify the priority.

[TomEdwardsEnscape]

API changes made!

Why do we need to call the internal method? It's called by TaskScheduler.TryExecuteTask: https://github.com/dotnet/runtime/blob/5dd24c0265d6682025aa4a2678776b7dc1200b15/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/TaskScheduler.cs#L398-L406

task.ExecutingTaskScheduler != this, so the method throws before executing the task.

[MrJul]

The PR needs to be rebased onto the main branch for the tests to pass, and your branch is protected.

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0065451-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM!