如何在OWIN中删除中间件？

Question

I have registered an OWIN middleware in Configuration method. How can I then change remove the middleware.

Particularly, I want to modify (remove/add) StaticFiles middleware after some files are changed (which I would like detect by FileSystemWatcher).

NOTE: There is a similar question regarding Connect of Node.js (which I consider where idea of OWIN come from).

Answer 1

if you're using the microsoft implementation of AppBuilder ( Microsoft.Owin.Builder.AppBuilder ), you can't.

In this implementation, The middlewares are stored in a private IList<T> , and there is no public access provided for this instance.

You could create your own AppBuilder class, which had to implement the Owin.IAppBuilder interface.

I did not think too much about it, but allowing the removal of middlewares feel like quite the ordeal:

Middlewares can be linked one to another. So if you delete one, you have to manage the relations of the other middlewares referring to it.

So you can't simply remove a middleware. But you could wrap the execution of the middleware with a condition. There may be a better solution.

Answer 2

IAppBuilder doesn't do it but you can add your own stage in the pipeline that lets you.

A simple stage for this would use existing List<Action<OwinContext>> befores; and List<Action<OwinContext>> afters; lists. The stage gets defined and added like:

appBuilder.Use(
    new Func<Func<IDictionary<string, object>, Task>, Func<IDictionary<string, object>, Task>>(
        next => async environment =>
            {
                var ctx = new OwinContext(environment);
                lock(befores)
                {
                    foreach(before in befores)
                    {
                        before(ctx);
                    }
                }
                await next.Invoke(environment);
                lock(afters)
                {
                    foreach(after in afters)
                    {
                        after(ctx);
                    }
                }
        }));

Then, you manage your befores and afters lists any way you wish (with appropriate locking, of course). Caveat: I got this by stripping down code I use but I haven't even tried to compile it .