Add a new filter type for pages

[rynowak]

Something like:

public delegate Task<PageHandlerExecutedContext> PageExecutionDelegate();

public class PageHandlerSelectedContext : FilterContext
{
    public virtual new CompiledPageActionDescriptor ActionDescriptor { get; }

    public virtual HandlerMethodDescriptor HandlerMethod { get; set; }

    public virtual IActionResult Result { get; set; }

    public virtual object HandlerInstance { get; }
}


public class PageHandlerExecutingContext : FilterContext
{
    public virtual new CompiledPageActionDescriptor ActionDescriptor { get; }

    public virtual HandlerMethodDescriptor HandlerMethod { get; }

    public virtual IActionResult Result { get; set; }

    public virtual IDictionary<string, object> HandlerArguments { get; }

    public virtual object HandlerInstance { get; }
}


public class PageHandlerExecutedContext : FilterContext
{
    public virtual new CompiledPageActionDescriptor ActionDescriptor { get; }

    public virtual HandlerMethodDescriptor HandlerMethod { get; }

    public virtual IActionResult Result { get; set; }

    public virtual object HandlerInstance { get; }

    public virtual bool Canceled { get; set; }

    public virtual Exception Exception { get; set; }

    public virtual ExceptionDispatchInfo ExceptionDispatchInfo { get; set; }

    public virtual bool ExceptionHandled { get; set; }
}

public interface IPageFilter : IFilterMetadata
{
    void OnPageHandlerSelected(PageHandlerSelectedContext context);

    void OnPageHandlerExecuting(PageHandlerExecutingContext context);

    void OnPageHandlerExecuted(PageHandlerExecutedContext context);
}

public interface IAsyncPageFilter : IFilterMetadata
{
    void OnPageHandlerSelection(PageHandlerSelectedContext context);

    void OnPageHandlerExecution(PageHandlerExecutingContext context, PageHandlerExecutionDelegate next);
}

[rynowak]

This is a fun new thing!

You can use the OnPageHandlerSelection event to change what handler is selected, or even just to run arbitrary init code that takes place before model binding.

You can use the OnPageHandlerExection event to do all of the traditional action filter kinds of things

[rynowak]

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