Add async event handler support

[SteveSandersonMS]

Now we support oneventname="@MyMethod", we should create some built-in way to deal with MyMethod returning Task and having the renderer auto-rerender after that task completes.

Not sure whether this requires a syntax like oneventname-async or whether we can do it with oneventname dynamically based on the assigned value type.

[SteveSandersonMS]

@rynowak Looking for your opinion on how this would fit best with dotnet/blazor#516.

[rynowak]

My guess is that we could handle this with overloading in the runtime, which means that we wouldn't require different event names.

We could do different event names of course, if we wanted to.

We could test this theory by adding and additional overload to RenderTreeBuilder that accepts Func<UIEventArgs, Task> and see if this 'just works' (compiles) for some arbitrary event.

[rynowak]

I suspect also that we'll want to require C# 7.3 for this. dotnet/csharplang#98
is really valuable for us because method group->delegate conversion is so idiomatic for Blazor.

The difference is that in C# 7.2:

public void AddAttribute(..., Action) { .. }
public void AddAttribute(..., Func<Task>) { .. }

public void Foo() { .. }
builder.AddAttribute(Foo);

This is considered an ambiguity in C#7.2 but not in C# 7.3.

Any concerns with this plan?

[chassq]

Please keep the eventname the same if possible is my $.02
oneventname="@Mymethod" --calling sync
-OR-
oneventname="@await MyMethod" --calling async

[Daddoon]

Hello !

Any news on this subject ?
My current "workaround" for the moment is to do something like this:

@using Allogaia.AppResource.Properties
@using Allogaia.Web.Client.Helpers
@using Daddoon.Blazor.Helpers
@if (SignInManager.IsSignedIn())
{
    <ul class="nav navbar-nav navbar-top-links navbar-right">
        <li>
            <a>
                <!-- TODO: show static svg or profile image of current user -->
                @*@Html.RenderSVG("~/Images/resources/svg/user-circle.svg", "user-profile-image")*@
                @UserManager.GetUserFirstName()
            </a>
        </li>
        <li>
            <a onclick="@LogoutUser">@ResourceHelper.Get(nameof(Resources.LogOut))</a>
        </li>
    </ul>
}
else
{
    <ul class="nav navbar-top-links navbar-right">
        <li><a>@ResourceHelper.Get(nameof(@Resources.SignUp))</a></li>
        <li><a>@ResourceHelper.Get(nameof(@Resources.LogIn))</a></li>
    </ul>
}

@functions {

    private void LogoutUser(UIMouseEventArgs e)
    {
        Browser.SetTimeout(async delegate ()
        {
            bool success = await SignInManager.LogoutAsync();
            if (success == false)
            {
                JConsole.AlertDangerAnErrorOccured();
            }
        }, 10);
    }
} 

In order to have an async delegate called from my synchronous event, as my dispatched task with my SetTimeout helper is only waiting an Action type as delegate.

Otherwise do i missing something on the attribute binding for an async event ?

[rynowak]

We're planning to do this for 0.3.0.

If you want a slightly less gross workaround, you can use async void.

private async void LogoutUser(UIMouseEventArgs e)
{
    await Task.Delay(10);
    bool success = await SignInManager.LogoutAsync();

    ...
    StateHasChanged();
}

The only thing that's missing is that you have to call StateHasChanged() yourself when you want to rerender - usually at the end

[Daddoon]

Perfect !

Thanks a lot @rynowak !

[rynowak]

This feature has been merged for 0.3.0 to the dev branch e5fd24b

Important - as of 0.3.0 we will require the use of C# 7.3. Using async event handlers smoothly will require some changes that have been made to overload resolution in C# 7.3. You can find their instructions for setting the C# language version in their documentation here. Note that you may need to restart visual studio if you change the language version for a project.

---

Async event handlers work a lot like regular event handlers except... well they are async.

Updating @Daddoon 's example:

...
        <li>
            <a onclick="@LogoutUser">@ResourceHelper.Get(nameof(Resources.LogOut))</a>
        </li>
...
@functions {

    private async Task LogoutUser(UIMouseEventArgs e)
    {
        await Task.Delay(10);

        // The runtime will call StateHasChanged the first time you 'await'

         bool success = await SignInManager.LogoutAsync();
         if (success == false)
         {
             JConsole.AlertDangerAnErrorOccured();
         }

        // The runtime will call StateHasChanged again when your task completes
    }
}