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co_wait, C++/CX and the UI Thread

16th January 2017, three minutes to read

I was working on some supporting code for my Windows 10 Instapaper client, and I ended up going down a rabbit hole of trying to understand the way that co_await works with the XAML/WinRT dispatcher. I ended up writing some sample code & sharing on GitHub. I’ve included the readme below, but the most up to date details will be on the repo.

Readme

This project is an example of how to wrangle co_await with C++/CX into a situation where it can be used to enforce contuation of the work on the UI Thread.

Usage

You need to co_await onto the UI thread? (Or, any specific thread). To do that, you need to allocate the thread_context, and then await on that when you need to continue on the UI thread.

#include “AwaitHelperWinRT.h”
ref class MyControl sealed // Allocate me on the UI thread
{
  await_asynchelper::thread_context ui_thread;
  event Windows::UI::Xaml::RoutedEventHandler^ MyEvent;
  concurrency::task<void> DoWork() // Call me somewhere else thats not the UI thread
  {
    co_await ExpensiveWork();
    co_await this->ui_thread;
    this->MyEvent(this, null); // Completes on UI thread
  }
}

You can see simple usage of this in the app that is included here. Take a look, click both buttons, and hopefully that will help clarify whats going on.

Background

Normally, when working with the PPL, one would provide a continuation context to the continuation specifying where that block would complete.

When working with co_routines & co_await, the implementation of the PPL to co_await adapter defined in pplawait.h, will provide you behaviour similar to use_default. e.g. if you start on the UI thread, it’ll end on the UI thread. Great! But what if you need to ensure that while your work is initiated from any thread, you need it to get back to the UI thread? Turns out that is a pain in the rear.

Example 1

task<bool> GetAnAnswer();
void MyWork()
{
    GetAnswer().then([] {
        // Will execute in the apartment where .then was invoked
    )); // Default it tast_continuation_context::use_default
}

Example 2

task<bool> GetAnAnswer();
void MyWork()
{
    GetAnswer().then([] {
        // Will execute wherever the PPL decides
    ), task_continuation_context::use_arbitary);
}

Example 3

task<bool> GetAnAnswer();
void MyWork()
{
    GetAnswer().then([] {
        // Will execute on the WinRT context, irrespective of where the prior task came from
    ), task_continuation_context::use_current);
}

co_await, co_routines

At the time of writing, I don’t think I could claim to be an expert in the wonders of co_await & friends. What I do know is that writing code using co_await is significantly cleaner & easier to read than the PPL & it’s promise-like syntax. Because of that I was using co_await in my code, and struggling understand where & how the “continuation” of the co_routines were going to execute.

After some diving, and consultation on twitter, I think I understand enough to leverage some code from C++/WinRT, to allow me to create the helper here.

Resources

I recommend watching these videos:
Putting Coroutines to work with the windows runtime
C++ Coroutines: Under the covers

Reading these blog posts:
C++/WinRT: Fun With Agility
coroutines in Visual Studio 2015 Update 1

If you’re feeling particularly inclined to go deep:
Wording for Coroutines (Or, the actual spec)
Coroutines: Reports from the field

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