Plotly.NET/src/Plotly.NET.ImageExport/AsyncHelper.fs at dev · plotly/Plotly.NET · GitHub

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module Plotly.NET.ImageExport.AsyncHelper

open System.Threading

open System.Threading.Tasks

This is a workaround to avoid deadlocks

https://medium.com/rubrikkgroup/understanding-async-avoiding-deadlocks-e41f8f2c6f5d

TL;DR in many cases, for example, GUI apps, SynchronizationContext

is overriden to *post* the executing code on the initial (UI) thread. For example,

consider this code

public async Task OnClick1()

{

var chart = ...;

var base64 = ImageExport.toBase64PNGStringAsync()(chart).Result;

myButton.Text = base64;

}

Here we have an async method. Normally you should use await and not use .Result, but

assume for some reason the sync version is used. What happens under the hood is,

public async Task OnClick1()

{

var chart = ...;

var task = ImageExport.toBase64PNGStringAsync()(chart);

task.ContinueWith(() =>

UIThread.Schedule(() =>

myButton.Text = Result;

)

);

task.Wait();

}

(this is pseudo-code)

So basically, we set the task to wait until it finishes. However, part of it being

finished is to actually execute the code with button.Text = .... The waiting happens

on the UI thread, exactly on the same thread as where we're waiting for it to do

another job!

That's not the only place we potentially deadlock by using fake synchronous functions.

The reason why it happens, is because frameworks (or actually anyone) override

SynchronizationContext. In GUI and game development it's very useful to keep UI logic

on one thread. But our rendering does not ever callback to it, we're independent of

where the logic actually happens.

That's why what we do is we set the synchronization context to null, do the job, and

then restore it. It is a workaround, because it doesn't have to work everywhere and

independently. But it will work for most cases.

When will it also break? For example, if we decide to take in some callback as a para-

meter, which potentially accesses the UI thread (or whatever). In Unity, for instance,

you can only access Unity API from the main thread. So our fake synchronous function

will crash in the end, because due to the overriden (by us) sync context, the callback

will be executed in some random thread (as opposed to being posted back to the UI one).

However, our solution should work in most cases.

Credit to [@DaZombieKiller](https://github.com/DaZombieKiller) for helping.

let runSync job input =

let current = SynchronizationContext.Current

SynchronizationContext.SetSynchronizationContext null

try

job input

finally

SynchronizationContext.SetSynchronizationContext current

let taskSync (task: Task<'a>) = task |> runSync (fun t -> t.Result)

let taskSyncUnit (task: Task) = task |> runSync (fun t -> t.Wait())

Plotly.NET/src/Plotly.NET.ImageExport/AsyncHelper.fs at dev · plotly/Plotly.NET · GitHub

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