roa-core 0.5.0-beta

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### Introduction

Core components of Roa framework.

If you are new to roa, please go to the documentation of roa framework.

### Application

A Roa application is a structure containing a middleware group which composes and executes middleware functions in a stack-like manner.

The obligatory hello world application:

```rust
use roa_core::App;

let mut app = App::new(());
app.end(|mut ctx| async move {
    ctx.resp_mut().write("Hello, World");
    Ok(())
});
```

#### Cascading

The following example responds with "Hello World", however, the request flows through
the `logging` middleware to mark when the request started, then continue
to yield control through the response middleware. When a middleware invokes `next.await`
the function suspends and passes control to the next middleware defined. After there are no more
middleware to execute downstream, the stack will unwind and each middleware is resumed to perform
its upstream behaviour.

```rust
use roa_core::App;
use std::time::Instant;
use log::info;

let mut app = App::new(());
app.gate_fn(|_ctx, next| async move {
    let inbound = Instant::now();
    next.await?;
    info!("time elapsed: {} ms", inbound.elapsed().as_millis());
    Ok(())
});

app.end(|mut ctx| async move {
    ctx.resp_mut().write("Hello, World");
    Ok(())
});
```

### Error Handling

You can catch or straightly throw an Error returned by next.

```rust
use roa_core::{App, throw};
use roa_core::http::StatusCode;
        
let mut app = App::new(());
app.gate_fn(|ctx, next| async move {
    // catch
    if let Err(err) = next.await {
        // teapot is ok
        if err.status_code != StatusCode::IM_A_TEAPOT {
            return Err(err)
        }
    }
    Ok(())
});
app.gate_fn(|ctx, next| async move {
    next.await?; // just throw
    unreachable!()
});
app.end(|_ctx| async move {
    throw!(StatusCode::IM_A_TEAPOT, "I'm a teapot!")
});
```

#### error_handler
App has an error_handler to handle `Error` thrown by the top middleware.
This is the error_handler:

```rust
use roa_core::{Context, Error, Result, ErrorKind, State};
pub async fn error_handler<S: State>(mut context: Context<S>, err: Error) -> Result {
    context.resp_mut().status = err.status_code;
    if err.expose {
        context.resp_mut().write(err.message.clone());
    }
    if err.kind == ErrorKind::ServerError {
        Err(err)
    } else {
        Ok(())
    }
}
```

The Error thrown by this error_handler will be handled by hyper.

### HTTP Server.

Use `roa_core::accept` to construct a http server.
Please refer to crate [![Crate version](https://img.shields.io/crates/v/roa-tcp.svg)](https://crates.io/crates/roa-tcp) for more information.