actix-web/guide/src/qs_5.md

# URL Dispatch

URL dispatch provides a simple way to map URLs to `Handler` code using a simple pattern matching
language. *Regex* crate and it's
[*RegexSet*](https://doc.rust-lang.org/regex/regex/struct.RegexSet.html) is beeing used for
pattern matching. If one of the patterns matches the path information associated with a request,
a particular handler object is invoked. A handler is a specific object that implements
`Handler` trait, defined in your application, that receives the request and returns 
a response object. More informatin is available in [handler section](../qs_4.html).

## Resource configuration

Resource configuraiton is the act of adding a new resource to an application.
A resource has a name, which acts as an identifier to be used for URL generation. 
The name also allows developers to add routes to existing resources. 
A resource also has a pattern, meant to match against the *PATH* portion of a *URL* 
(the portion following the scheme and port, e.g., */foo/bar* in the 
*URL* *http://localhost:8080/foo/bar*).

The [Application::resource](../actix_web/struct.Application.html#method.resource) methods
add a single resource to application routing table. This method accepts *path pattern*
and resource configuration funnction. 

```rust
# extern crate actix_web;
# use actix_web::*;
# use actix_web::httpcodes::*;
# 
# fn index(req: HttpRequest) -> HttpResponse {
#   unimplemented!()
# }
# 
fn main() {
    Application::new()
        .resource("/prefix", |r| r.f(index))
        .resource("/user/{name}", 
             |r| r.method(Method::GET).f(|req| HTTPOk))
        .finish();
}
```

*Configuraiton function* has following type:

```rust,ignore
   FnOnce(&mut Resource<_>) -> ()
```

*Configration function* can set name and register specific routes. 
If resource does not contain any route or does not have any matching routes it
returns *NOT FOUND* http resources.

## Configuring a Route

Resource contains set of routes. Each route in turn has set of predicates and handler.
New route could be crearted with `Resource::route()` method which returns reference
to new *Route* instance. By default *route* does not contain any predicates, so matches
all requests and default handler is `HTTPNotFound`.

Application routes incoming requests based on route criteria which is defined during 
resource registration and route registration. Resource matches all routes it contains in
the order that the routes were registered via `Resource::route()`. *Route* can contain
any number of *predicates* but only one handler.

```rust
# extern crate actix_web;
# use actix_web::*;
# use actix_web::httpcodes::*;

fn main() {
    Application::new()
        .resource("/path", |resource|
            resource.route()
              .p(pred::Get())
              .p(pred::Header("content-type", "text/plain"))
              .f(|req| HTTPOk)
        )
        .finish();
}
```

In this example `index` get called for *GET* request, 
if request contains `Content-Type` header and value of this header is *text/plain*
and path equals to `/test`. Resource calls handle of the first matches route.
If resource can not match any route "NOT FOUND" response get returned.

## Route matching

The main purpose of route configuration is to match (or not match) the request's `path`
against a URL path pattern. `path` represents the path portion of the URL that was requested.

The way that *actix* does this is very simple. When a request enters the system,
for each resource configuration registration present in the system, actix checks 
the request's path against the pattern declared. *Regex* crate and it's
[*RegexSet*](https://doc.rust-lang.org/regex/regex/struct.RegexSet.html) is beeing used for
pattern matching. If resource could not be found, *default resource* get used as matched
resource.

When a route configuration is declared, it may contain route predicate arguments. All route
predicates associated with a route declaration must be `true` for the route configuration to 
be used for a given request during a check. If any predicate in the set of route predicate
arguments provided to a route configuration returns `false` during a check, that route is
skipped and route matching continues through the ordered set of routes.

If any route matches, the route matching process stops and the handler associated with
route get invoked. 

If no route matches after all route patterns are exhausted, *NOT FOUND* response get returned.

## Resource pattern syntax

The syntax of the pattern matching language used by the actix in the pattern
argument is straightforward. 

The pattern used in route configuration may start with a slash character. If the pattern
does not start with a slash character, an implicit slash will be prepended 
to it at matching time. For example, the following patterns are equivalent:

```
{foo}/bar/baz
```

and:

```
/{foo}/bar/baz
```

A *variable part*(replacement marker) is specified in the form *{identifier}*, 
where this means "accept any characters up to the next slash character and use this 
as the name in the `HttpRequest.match_info` object". 

A replacement marker in a pattern matches the regular expression `[^{}/]+`.

A match_info is the `Params` object representing the dynamic parts extracted from a 
*URL* based on the routing pattern. It is available as *request.match_info*. For example, the
following pattern defines one literal segment (foo) and two replacement markers (baz, and bar):

```
foo/{baz}/{bar}
```

The above pattern will match these URLs, generating the following match information:

```
foo/1/2        -> Params {'baz':'1', 'bar':'2'}
foo/abc/def    -> Params {'baz':'abc', 'bar':'def'}
```

It will not match the following patterns however:

```
foo/1/2/        -> No match (trailing slash)
bar/abc/def     -> First segment literal mismatch
```

The match for a segment replacement marker in a segment will be done only up to 
the first non-alphanumeric character in the segment in the pattern. So, for instance, 
if this route pattern was used:

```
foo/{name}.html
```

The literal path */foo/biz.html* will match the above route pattern, and the match result
will be `Params{'name': 'biz'}`. However, the literal path */foo/biz* will not match, 
because it does not contain a literal *.html* at the end of the segment represented 
by *{name}.html* (it only contains biz, not biz.html).

To capture both segments, two replacement markers can be used:

```
foo/{name}.{ext}
```

The literal path */foo/biz.html* will match the above route pattern, and the match 
result will be *Params{'name': 'biz', 'ext': 'html'}*. This occurs because there is a 
literal part of *.* (period) between the two replacement markers *{name}* and *{ext}*.

Replacement markers can optionally specify a regular expression which will be used to decide
whether a path segment should match the marker. To specify that a replacement marker should
match only a specific set of characters as defined by a regular expression, you must use a
slightly extended form of replacement marker syntax. Within braces, the replacement marker
name must be followed by a colon, then directly thereafter, the regular expression. The default 
regular expression associated with a replacement marker *[^/]+* matches one or more characters
which are not a slash. For example, under the hood, the replacement marker *{foo}* can more
verbosely be spelled as *{foo:[^/]+}*. You can change this to be an arbitrary regular expression
to match an arbitrary sequence of characters, such as *{foo:\d+}* to match only digits.

Segments must contain at least one character in order to match a segment replacement marker. 
For example, for the URL */abc/*:

* */abc/{foo}* will not match.
* */{foo}/* will match.

Note that path will be URL-unquoted and decoded into valid unicode string before
matching pattern and values representing matched path segments will be URL-unquoted too.
So for instance, the following pattern:

```
foo/{bar}
```

When matching the following URL:

```
http://example.com/foo/La%20Pe%C3%B1a
```

The matchdict will look like so (the value is URL-decoded):

```
Params{'bar': 'La Pe\xf1a'}
```

Literal strings in the path segment should represent the decoded value of the 
path provided to actix. You don't want to use a URL-encoded value in the pattern.
For example, rather than this:

```
/Foo%20Bar/{baz}
```

You'll want to use something like this:

```
/Foo Bar/{baz}
```

It is possible to get "tail match". For this purpose custom regex has to be used.

```
foo/{bar}/{tail:.*}
```

The above pattern will match these URLs, generating the following match information:

```
foo/1/2/           -> Params{'bar':'1', 'tail': '2/'}
foo/abc/def/a/b/c  -> Params{'bar':u'abc', 'tail': 'def/a/b/c'}
```

## Match information

All values representing matched path segments are available in 
[`HttpRequest::match_info`](../actix_web/struct.HttpRequest.html#method.match_info).
Specific value can be received with 
[`Params::get()`](../actix_web/dev/struct.Params.html#method.get) method.

Any matched parameter can be deserialized into specific type if this type 
implements `FromParam` trait. For example most of standard integer types
implements `FromParam` trait. i.e.:

```rust
# extern crate actix_web;
use actix_web::*;

fn index(req: HttpRequest) -> Result<String> {
    let v1: u8 = req.match_info().query("v1")?;
    let v2: u8 = req.match_info().query("v2")?;
    Ok(format!("Values {} {}", v1, v2))
}

fn main() {
    Application::new()
        .resource(r"/a/{v1}/{v2}/", |r| r.f(index))
        .finish();
}
```

For this example for path '/a/1/2/', values v1 and v2 will resolve to "1" and "2".

It is possible to create a `PathBuf` from a tail path parameter. The returned `PathBuf` is
percent-decoded. If a segment is equal to "..", the previous segment (if
any) is skipped.

For security purposes, if a segment meets any of the following conditions,
an `Err` is returned indicating the condition met:

  * Decoded segment starts with any of: `.` (except `..`), `*`
  * Decoded segment ends with any of: `:`, `>`, `<`
  * Decoded segment contains any of: `/`
  * On Windows, decoded segment contains any of: '\'
  * Percent-encoding results in invalid UTF8.

As a result of these conditions, a `PathBuf` parsed from request path parameter is
safe to interpolate within, or use as a suffix of, a path without additional checks.

```rust
# extern crate actix_web;
use actix_web::*;
use std::path::PathBuf;

fn index(req: HttpRequest) -> Result<String> {
    let path: PathBuf = req.match_info().query("tail")?;
    Ok(format!("Path {:?}", path))
}

fn main() {
    Application::new()
        .resource(r"/a/{tail:.*}", |r| r.method(Method::GET).f(index))
        .finish();
}
```

List of `FromParam` implementation could be found in 
[api docs](../actix_web/dev/trait.FromParam.html#foreign-impls)

## Generating resource URLs

Use the [HttpRequest.url_for()](../actix_web/struct.HttpRequest.html#method.url_for) 
method to generate URLs based on resource patterns. For example, if you've configured a 
resource with the name "foo" and the pattern "{a}/{b}/{c}", you might do this.

```rust
# extern crate actix_web;
# use actix_web::*;
# use actix_web::httpcodes::*;
# 
fn index(req: HttpRequest) -> HttpResponse {
     let url = req.url_for("foo", &["1", "2", "3"]);
     HTTPOk.into()
}
# fn main() {}
```

This would return something like the string *http://example.com/1/2/3* (at least if
the current protocol and hostname implied http://example.com).
`url_for()` method return [*Url object*](https://docs.rs/url/1.6.0/url/struct.Url.html) so you 
can modify this url (add query parameters, anchor, etc).
`url_for()` could be called only for *named* resources otherwise error get returned.

## External resources

Resources that are valid URLs, could be registered as external resources. They are useful
for URL generation purposes only and are never considered for matching at request time.

```rust
# extern crate actix_web;
use actix_web::*;

fn index(mut req: HttpRequest) -> Result<HttpResponse> {
    let url = req.url_for("youtube", &["oHg5SJYRHA0"])?;
    assert_eq!(url.as_str(), "https://youtube.com/watch/oHg5SJYRHA0");
    Ok(httpcodes::HTTPOk.into())
}

fn main() {
    let app = Application::new()
        .resource("/index.html", |r| r.f(index))
        .external_resource("youtube", "https://youtube.com/watch/{video_id}")
        .finish();
}
```

## Path normalization and redirecting to slash-appended routes

By normalizing it means:

 - Add a trailing slash to the path.
 - Double slashes are replaced by one.

The handler returns as soon as it finds a path that resolves
correctly. The order if all enable is 1) merge, 3) both merge and append
and 3) append. If the path resolves with
at least one of those conditions, it will redirect to the new path.

If *append* is *true* append slash when needed. If a resource is
defined with trailing slash and the request comes without it, it will
append it automatically.

If *merge* is *true*, merge multiple consecutive slashes in the path into one.

This handler designed to be use as a handler for application's *default resource*.

```rust
# extern crate actix_web;
# #[macro_use] extern crate serde_derive;
# use actix_web::*;
#
# fn index(req: HttpRequest) -> httpcodes::StaticResponse {
#    httpcodes::HTTPOk
# }
fn main() {
    let app = Application::new()
        .resource("/resource/", |r| r.f(index))
        .default_resource(|r| r.h(NormalizePath::default()))
        .finish();
}
```

In this example `/resource`, `//resource///` will be redirected to `/resource/` url.

In this example path normalization handler get registered for all method,
but you should not rely on this mechanism to redirect *POST* requests. The redirect of the
slash-appending *Not Found* will turn a *POST* request into a GET, losing any 
*POST* data in the original request.

It is possible to register path normalization only for *GET* requests only

```rust
# extern crate actix_web;
# #[macro_use] extern crate serde_derive;
# use actix_web::*;
#
# fn index(req: HttpRequest) -> httpcodes::StaticResponse {
#    httpcodes::HTTPOk
# }
fn main() {
    let app = Application::new()
        .resource("/resource/", |r| r.f(index))
        .default_resource(|r| r.method(Method::GET).h(NormalizePath::default()))
        .finish();
}
```

## Using a Application Prefix to Compose Applications

The `Applicaiton::prefix()`" method allows to set specific application prefix.
If route_prefix is supplied to the include method, it must be a string.
This prefix represents a resource prefix that will be prepended to all resource patterns added 
by the resource configuration. This can be used to help mount a set of routes at a different
location than the included callable's author intended while still maintaining the same
resource names. 

For example:

```rust
# extern crate actix_web;
# use actix_web::*;
# 
fn show_users(req: HttpRequest) -> HttpResponse {
   unimplemented!()
}

fn main() {
    Application::new()
        .prefix("/users")
        .resource("/show", |r| r.f(show_users))
        .finish();
}
```

In the above example, the *show_users* route will have an effective route pattern of 
*/users/show* instead of */show* because the application's prefix argument will be prepended 
to the pattern. The route will then only match if the URL path is /users/show, 
and when the `HttpRequest.url_for()` function is called with the route name show_users, 
it will generate a URL with that same path.

## Custom route predicates

You can think of predicate as simple function that accept *request* object reference
and returns *true* or *false*. Formally predicate is any object that implements 
[`Predicate`](../actix_web/pred/trait.Predicate.html) trait. Actix provides
several predicates, you can check [functions section](../actix_web/pred/index.html#functions) 
of api docs.

Here is simple predicates that check that request contains specific *header* and predicate
usage:

```rust
# extern crate actix_web;
# extern crate http;
# use actix_web::*;
# use actix_web::httpcodes::*;
use http::header::CONTENT_TYPE;
use actix_web::pred::Predicate;

struct ContentTypeHeader;

impl<S: 'static> Predicate<S> for ContentTypeHeader {

    fn check(&self, req: &mut HttpRequest<S>) -> bool {
       req.headers().contains_key(CONTENT_TYPE)
    }
}

fn main() {
    Application::new()
        .resource("/index.html", |r|
           r.route()
              .p(Box::new(ContentTypeHeader))
              .f(|req| HTTPOk))
        .finish();
}
```

In this example *index* handler will be called only if request contains *CONTENT-TYPE* header.

Predicates can have access to application's state via `HttpRequest::state()` method.
Also predicates can store extra information in
[requests`s extensions](../actix_web/struct.HttpRequest.html#method.extensions).

### Modifing predicate values

You can invert the meaning of any predicate value by wrapping it in a `Not` predicate.
For example if you want to return "METHOD NOT ALLOWED" response for all methods
except "GET":

```rust
# extern crate actix_web;
# extern crate http;
# use actix_web::*;
# use actix_web::httpcodes::*;
use actix_web::pred;

fn main() {
    Application::new()
        .resource("/index.html", |r|
           r.route()
              .p(pred::Not(pred::Get()))
              .f(|req| HTTPMethodNotAllowed))
        .finish();
}
```

`Any` predicate accept list of predicates and matches if any of the supplied
predicates match. i.e:

```rust,ignore
    pred::Any(vec![pred::Get(), pred::Post()])
```

`All` predicate accept list of predicates and matches if all of the supplied
predicates match. i.e:

```rust,ignore
    pred::All(vec![pred::Get(), pred::Header("content-type", "plain/text")])
```